Shape anisotropic colloids at interfaces

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Invited Feature Article

Shape anisotropic colloids at interfaces Thriveni G Anjali, and Madivala G Basavaraj Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01139 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Shape anisotropic colloids at interfaces Thriveni G. Anjali and Madivala G. Basavaraj* Polymer Engineering and Colloid Science (PECS) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai - 600 036, India Abstract

Research in the 1980’s demonstrated the formation of monolayer of particles achieved by interfacial particle trapping as a model system to investigate colloids in two dimensions. Since then, microscopy visualization of two-dimensional particle monolayers and quantification of the microstructure have led to significant fundamental understanding of a number of phenomenon such as crystallization, freezing and melting transition, dislocation dynamics, aggregation kinetics and others. On the application front, particles at curved interfaces – as often the case in particle stabilized emulsions and foams – have received considerable attention in the last few decades. The growing interest in the search of novel particles and new strategies to effect emulsion stabilization stems from their application in several disciplines. Moreover, particle stabilized Pickering emulsions and foams can also be used for deriving a number of advanced functional materials. Compared to large account of research on spherical colloids at fluid-fluid interfaces, investigation of the behaviour of shape anisotropic particles at interfaces, albeit receiving considerable attention in recent years is still in a nascent stage. The objective of this feature article is to highlight our recent work in this area. In particular, the adsorption of shape

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anisotropic particles to interface, wetting behaviour, interfacial self-assembly, response of nonspherical particle coated interfaces to compression and shear and their ability to stabilize emulsions are discussed.

Introduction The enormous interest in study of colloids emanates from their use as a model system to investigate several fundamental phenomena that are exhibited by atoms and molecules as well as due to their technological applications.1 Since the discovery of metal hydrosols – aqueous dispersions containing finely divided metal particles – by Michael Faraday in 1857,2-3 various aspects of colloidal dispersions have been investigated in great detail by manipulating effective interaction between particles with the help of an external field (electric, magnetic, optical, thermal, etc.) or additives (electrolytes, polymers, surfactants, etc.). Colloids in the bulk are typically characterized by size, shape, mass density, surface charge density, grafting density, surface roughness and other important parameters essential to understand the colloidal scale phenomena under investigation. Compared to colloids dispersed in a fluid, the particles trapped at the interface of two immiscible fluids exhibit several features that are in marked contrast to their behavior in the bulk and are characterized by additional parameters which otherwise in the bulk are not important. When dealing with colloidal particles at interfaces, typical scientific problems to which the answers are sought-after include (but not limited to): a) adsorption of particles to interfaces b) equilibrium position of particles at the interface c) colloidal interactions at interfaces d) self-assembly of particles at interfaces e) Shear and compression of particle monolayers f) applications of particle coated interfaces.


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a) Adsorption of particles to interfaces: The adsorption of colloidal particles to interface is the key for the stabilization of interfaces encountered in emulsions, foams, bijels and other high interface materials. To impart stability, the particle must be surface active i.e., readily adsorb to the interfaces generated during the preparation of such materials. In particle monolayers experiments – such as those used to study self-assembly of particles at interfaces or compression in a Langmuir trough, the adsorption of particles at the interface of two fluids is typically aided by using alcohol (such as ethyl alcohol or iso-propyl alcohol) as a spreading solvent. The alcohol in the aqueous dispersion helps in spreading and distribution of particles across the interface. However, several processes such as emulsification require the adsorption of particles to interface in the absence of any spreading aid. Consequently, there is considerable research on the adsorption of particles to fluid-fluid interface. The role of surface charge density of particles, the effect of additives such as electrolytes and polymers on the spontaneous adsorption of particles at interfaces has been a subject of on-going investigation.4 A scheatic of the pendant drop tensiometry experiments that are commonly used to investigate the adsorption of particles to fluid-fluid interfaces is represented in Figure 1A, which will be discussed in the results section in detail. b) Equilibrium position of a single particle at the interface: Once the particles are adsorbed at an interface, the question that one could ask is - where is the location of the particles with respect to the interface? The equilibrium position of a colloidal particle at fluid–fluid interface is characterized by the three phase contact angle (θ) defined by Young’s equation as:5-7

=

(1)

where is the surface tension of the interface between the particle (p) and the upper (lighter) fluid (f1) interface (usually oil or air), is the surface tension of the interface between the


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particle (p) and the bottom (heavy) fluid (f2) interface (usually water), and is the surface tension of the interface between the two immiscible fluids f1 and f2. Depending on the magnitude of the three phase contact angle, a colloidal particle can occupy any of the interfacial position as shown in Figure 1B. The three phase contact angle is a crucial parameter to assess the type of emulsion resulting from the use of colloidal particles as emulsion stabilizers. The estimation of the depth of the energy well in which the particles are trapped at the interface, required to define the stability of colloids at an interface also depends on the contact angle of the particles.8 The surface free energy Δ required to detach or remove a spherical particle of radius r and contact angle θ, positioned at the interface is:6 Δ = (1 − ||)

(2)

For more details, we refer readers to few recent reviews exclusively on different methods available for the measurement of the three phase contact angles of individual micro and nano particles at fluid-fluid interfaces.9-11 c) Colloidal interactions at interfaces: As colloidal particles residing at an interface are partially immersed in the two immiscible fluids used to create the interface; particle-particle interactions at the interface are significantly modified compared to colloidal interactions in the bulk.12-13 The extent to which they remain in each of the fluid is governed by the three phase contact angle. Therefore, a particle of homogeneous surface chemistry at an interface behaves more like a “patchy” particle. For example – if the particles have a homogeneous surface charge density and are deposited at oil-water interface, only the charges on the particle surface in contact with water or oil dissociate depending on the nature of dissociable groups on the particle surface as represented schematically in Figure 1C1. This brings about asymmetric distribution of


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charges and the counter ions across the interface plane. Therefore, every charged particle at an interface effectively behaves like a dipole. The dipolar repulsion between the particles leads to loosely packed hexagonally ordered arrangement of particles with inter-particle spacing of the order of few particle diameter.8 In addition to this, a charged particle experiences a repulsion from a hypothetical image charge located in the non-polar phase as shown in Figure 1C2 which will be discused later. The interactions that otherwise are not present in the bulk but appear when particles are trapped at interfaces are the capillary interactions. Such interactions arise due to the overlap of the interface deformation around two or more neighboring particles. The schematic in Figure 1C3 demonstrates the nature of interface deformation around spherical particles floating at an interface arising due to particle weight (gravity driven) and surface roughness. The interface deformation may occur solely due to gravity if the particles are large or heavy which can be neglected for polystyrene particles (density=1050 kg/m3) of 10 µm diameter. The interface can still be deformed significantly if the particles are heterogeneous, highly charged or non-spherical. A detailed discussion of the different types of colloidal interactions at interface and their origin can be found elsewhere.12-16 d) Self-assembly of particles at interfaces: When large numbers of particle are adsorbed at fluid–fluid interface, colloidal particles can spontaneously self-assemble into various structures depending on the magnitude of competing interactions. The most striking example of selfassembly of colloidal particles at interfaces,8 is the formation a homogeneous loosely packed two-dimensional colloidal crystal at the interface as shown in Figure 1D. From the observations that the distance between the charged particles at the air-water interface is several times the particle radius, Pieranski argued that the electrostatic interactions are long ranged. The formation


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of such homogeneous monolayers is the key to tune the structure as well as the surface rheological properties of particle-laden interfaces. The self-assembly of particles at interfaces can be tuned by the addition of surfactant, electrolytes, polymers or other additives to obtain open network of particles as in diffusion- limited cluster aggregation (DLCA) or compact network as observed in reaction-limited cluster aggregation (RLCA)17-18.

There are also reports of

intriguing mesoscale structures generated by exposing particles to a series of external stimuli such as addition of ions, electric current, and ultraviolet light19. The arrangement of particles in these mesoscale structures such as striations and loops are still not understood and point to complex interplay of various colloidal interactions at interfaces 20. e) Response of particle monolayer to compression and shear: A rectangular Langmuir trough provides a convenient way to characterize collective behavior of colloids at the interfaces by manipulating the area available per adsorbed species and is schematically shown in Figure 1E1. The response of particle monolayer to lateral compression gives the “surface pressure ( π ) - area isotherms (A)” which is a plot of surface pressure, the difference in the interfacial tension of the particle laden interface and the bare surface, versus the area available per particle at a constant temperature. The π - A isotherms are analogue of the constant temperature pressure-volume diagrams in the bulk, and can be used to calculate the compression modulus and Young’s modulus of the particle monolayers. It is also possible to couple microscopy with Langmuirtrough compression experiments to visualize the re-arrangement of particles, structural and thermodynamic aspects of phase transitions during lateral compression.21 The compressed monolayers of spherical particles have revealed change in the slopes of the surface-pressure isotherms corresponding to the formation of gas-like phases, close packed monolayer and buckling and wrinkling transitions.21-23


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Alternatively, the particle-laden monolayers at the surface of pendant drops can also be compressed to assess their mechanical response as shown in Figure 1E2. Typically, a pendant drop is created with the help a syringe and allowed to equilibrate with another immiscible fluid in a rectangular cuvette. The compression of the particle-laden film formed at the surface a drop is typically carried out by drop compression experiments i.e., by withdrawing the liquid from a pendant droplet coated particle with monolayer. The drop can also be subjected to expansion by injecting the liquid into the pendant droplet. These experiments are performed using a contact angle meter (or Goniometer) by controlled addition or withdrawal of fluid using a computercontrolled mico-syringe and by recording the shape fluctuations of the drop volume using a camera. The analysis of shape and wrinkling behavior of the compressed pendant drops can be used to estimate the elastic modulus of the interface.24 It is also possible to determine the energy required to detach individual nanoparticles from oil-water interface using pendant drop compression experiments.25 Particle monolayers of well-defined microstructure – either crystalline or aggregated can be subjected to surface deformation as shown in Figure 1E3. Surface rheological measurements can be carried out using an interfacial stress rheometer in which a magnetic needle positioned at the interface is used to deform the interface26 or using Du Noüy ring, Bi-cone or Double Wall Ring geometry,27-28 attached to conventional rotational rheometers. The aggregated percolating networks of particles at the interface have been shown to exhibit an elastic response with small linearity limits. A power law dependence of the elastic surface modulus with the surface coverage (defined as the area occupied by particles divided by the total area of the interface), a behavior similar to bulk colloidal gels has been demonstrated.29 The tailoring of interactions and


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hence the microstructure is shown to impart viscoelasticy to the interface, which is crucial for their application in diverse fields.29-30 f) Applications: The concept of colloids at interfaces has been exploited in several different applications: (1) Stabilization of large interface materials: Particle stabilized emulsions or PickeringRamsden emulsions are typically obtained by mixing aqueous dispersion of particles with oil phase, each of known volumes. Ramsden31 and Pickering32 independently demonstrated this effect more than a century ago. More recent applications that exploit interfacial trapping of particles include stabilization of immiscible polymer blends33-34 and “Bijels” - Bicontinuous Interfacially Jammed Emulsion Gels.35 Bijels are typically obtained by considering binary liquid mixtures that exhibit either a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST) which completely phase separate when heated or cooled respectively. When colloidal particles are incorporated in the binary fluids and the composition is such that the phase separation occurs via spinodal decomposition, the particles can arrest phase separation leading to Bijels with two continuous fluid domains. (2) Colloidosomes and porous materials: The colloidal particles that stabilize and surround emulsion drops can be made to “bind” to each other with the help of strong coagulants resulting in the formation of larger microstructured hollow spherical supraparticles.36 These structures are called “colloidosomes” and have potential application in encapsulation and release of active ingredients.37 Particle stabilized emulsions and foams also serve as excellent starting materials for the fabrication of macroporous ceramic materials.38 The particles on the surface of the drops hinder the coalescence process during solvent extraction and porous materials can be produced by a simple approach that is versatile and scalable.39


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(3) Synthesis of novel particles: Strong pinning of the colloidal particles to interfaces has also been exploited to deform polymeric particles trapped at oil-water and air-water interface to fabricate monodisperse non-spherical particles that are both shape and surface anisotropic.40-42 These method are simple, easy and scalable and moreover, provide a route for the synthesis of unusual shaped particles that otherwise are difficult to achieve by other techniques. The focus of this feature article is to highlight our recent contributions to the investigation of shape anisotropic particles at interfaces and the dominant role of interfaces in engineering their self-assembly. In particular, recent work concerning the adsorption of shape anisotropic particles to interface, equilibrium and metastable configuration of shape anisotropic particles at interfaces, their interfacial assembly, response of shape anisotropic particle coated interfaces to compression and shear, and their applications are discussed. 2 Adsorption of shape anisotropic particles to interfaces The adsorption of a colloidal particle to the interface of any two immiscible fluids crate two new interfaces, that between the particle and the two fluids and a loss of a small fluid-fluid interfacial area, the latter depends on the size, the particle shape, the three-phase contact angle of the particles at the interface and the interface deformation profile around the particle. Similar to amphiphilic molecules, the adsorption of particles to interface decreases the surface free energy of the system. However, not all colloidal particles adsorb spontaneously to interfaces and several factors play a key role. In investigating colloidal particles at fluid-fluid interfaces, most often, water (in some cases with added solutes) is used as the sub-phase or the bottom phase. Since most colloidal particles considered in such studies are readily dispersible in water, particles have inherent “liking” for water and therefore would prefer to remain in the aqueous sub-phase. It has been argued that for the adsorption of particles to interfaces, the wettability of particles must be


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appropriate i.e., the particles must not be completely hydrophilic or hydrophobic. In a recent review, eight different strategies that have been developed over the years to enable the adsorption of particle to oil–water interface in the context of emulsion stabilization have been discussed.43 These strategies include the use of additives such as electrolytes, acid/base, charged polymers, particles, surfactants, or their mixtures. Our recent work on the adsorption of non-spherical particles to interfaces include pendant drop tensiometry investigation of the adsorption kinetics of nano-ellipsoids (major axis - 254±50 nm, minor axis - 55±12 nm, aspect ratio - 4.6±0.6) to oil-water interface44 and the forced or the external energy aided adsorption of peanut shaped particles (length - 1.89±0.04 µm and lobe diameter 0.73 ± 0.03 µm) to oil-water interface created during emulsification of oil and aqueous dispersion of particles.45 In both these studies model well characterized hematite particles are used. The dynamic surface tension measurements recorded by creating an aqueous drop with and without hematite ellipsoids (0.3 wt%) are shown in Figure 2A. The time evolution of the interfacial tension for the particle free water drop and the aqueous hematite dispersion at pH 2 in contact with a reservoir of decane is nearly the same. This suggests that the hematite particles when dispersed in water maintained at pH 2 do not adsorb to the interface. In marked contrast, the pendant drops of hematite dispersions at pH 4 and 6.5 show a decrease in interfacial tension with time indicating the adsorption of ellipsoids and the formation of a monolayer of hematite ellipsoids at the interface, which is further confirmed by scanning electron microscopy. The decrease in surface tension is significant when the pH is 6.5. We identify that the time evolution of surface tension recorded at pH 4 and 6.5 shows two distinct regions of markedly different slopes. A steep decrease in the surface tension observed at initial time indicates a higher rate of adsorption of ellipsoids to interfaces and a lower energy barrier for adsorption. As more particles


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adsorb, the concentration of particles at the interface increases, correspondingly, the rate of adsorption decreases drastically due to repulsion between the particles that approach the interface and those already adsorbed at the interface. Therefore, as particles adsorb to the interface in the latter regime, substantial re-arrangement of particles are expected to occur. An alternate method to study the adsorption of particles to interfaces and to visualize the microstructural arrangement of particles at interfaces is by carrying out emulsification. In our study, aqueous dispersions containing 1 wt% peanut shaped hematite particles maintained at different pH (2, 6.5 and 12) are emulsified with decane (2:1 volume ratio).45 The images of vials after emulsification when all the conditions are identical, except for the pH are shown in Figure 2B. The particles in the aqueous dispersions are able to stabilize the emulsion droplets only at pH 6.5, and the emulsions formed are observed to exhibit extraordinary stability. The microscopy visualizations confirmed the adsorption of particles and the formation of a dense monolayer of particles around the drop surface. While the images in Figure 2B are results of emulsification by manual mixing, similar behaviour is observed when a large amount of energy through homogenization at 20,000 rpm is used – i.e., no emulsions are formed when the hematite particles dispersed in pH 2 and 12. Similar effect of pH on the adsorption of particles to interfaces has been elucidated in emulsification experiments using hematite particles of other shapes (ellipsoids, cuboids, spherocylinders) and silica rods. The result pertaining to both pendant drop and emulsification experiments in Figure 2 point to the possible existence of an energy barrier for the adsorption of particles to the interface. The surface charge density or the zeta potential of hematite particles used in these experiments varies with the dispersion pH. The hematite particles are known to exhibit pH dependent zeta-potential with an iso-electric point close to pH 9, where the particles are uncharged46. This suggests that


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the energy barrier for the absorption of particles to the interface under highly acidic and highly basic conditions – i.e., at pH 2 and pH 12 – is due to their high surface charge density and therefore their adsorption to interfaces is hindered due to repulsive electrostatic interactions. Some of our understanding of the adsorption of non-spherical particles to interfaces is based on very nice theoretical and experimental analysis of the adsorption of spherical particles to planar and curved interfaces.47-48 When a highly charged particle dispersed in water approaches an interface created due to the presence of oil, it experiences repulsion from an image charge49 in the oil phase. Therefore the electrostatic interactions between the charged particle in water and the image charge in oil significantly influence the adsorption of charged particles to fluid interfaces. We have also shown that the change in the pH of the dispersion also alters the equilibrium position of particles such that their wetting conditions are favourable for the adsorption of particles to interfaces.45 It is possible to overcome the image charge repulsion by enhancing attractive Van der Waals interactions or by introducing hydrodynamic effects in the form of high speed homogenization to enable the adsorption of particles to interfaces. 44 We have also demonstrated that the adsorption of particles to interfaces can also be facilitated by the addition of salt50 or by the addition of oppositely charged particles.51-52 More recent works on adsorption of shape anisotropic particles report the time evolution of the dynamics of particles in the vicinity of the interface.53 The experimental investigation of approach of individual ellipsoidal particles towards an interface and associated evolution of particle orientation as they reach the interface show complex interplay of capillary forces and evolution of the three phase contact line, which needs further investigation.54-55 3 Equilibrium position and orientation of shape anisotropic particles at interfaces


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The equilibrium position of particle with respect to the interface is a measure of their wettability and is characterized by the three phase contact angle. It is an important parameter that can be used to estimate the energy required to detach (attach) a particle from (to) the interface, crucial to assess the stability of emulsions and also predict the emulsion type.6 The possibility of multiple orientations, multiple interfacial positions and the non-planar nature of the three phase contact line add additional complexity to the characterization of wetting behavior of non-spherical colloids at interfaces.56-61 The motivation to study particle wettability stems from the fact that the equilibrium position of particles at the interface and their orientations can directly influence the inter-particle interactions and hence their interfacial self-assembly. This is challenging as the direct measurement of the three phase contact angle requires the imaging of particle with the help of high-resolution microscopy at a single particle level and also this must be done in the presence of two fluid-phases. The contact angle of non-spherical particles trapped at interfaces have been measured by direct measurement methods such as phase shifting interferometry coupled with optical trapping,62 freeze fracture shadow casting cryo-scanning electron microscopy (FreSCa cryo-SEM)63-65, and gel trapping technique (GTT).57,

66-67

In these

experiments, particles are spread at the interface at very low concentrations to ensure the presence of several isolated particles at the interface. All these techniques, exploit the possibility of immobilizing colloids at the interface using a suitable method, followed by the use of an appropriate technique to visualize the equilibrium position, their orientation at interfaces and particle induced interface deformation. The investigation of the wettability of only a few types of shape anisotropic colloids have been carried out, probably due to lack of accessibility to suitable techniques to image the equilibrium position of particles at an individual particle level.9-11 The interfacial position and the nature of the interface around micron sized prolate ellipsoids are


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characterized using phase shifting interferometry technique.62 The particle sizes involved in the study varied from 11 to 45 µm and the adaptation of this technique for the case of submicron particles both in terms of trapping and imaging of subtle interface deformations are challenging. These limitations are overcome in the FreSCa cryo-SEM technique,64-65 a standard characterization method used in biology. To date, the wetting behavior of amidine and polystyrene latex particles,63 surfactant modified silica particles,68 poly methyl methacrylate (PMMA) ellipsoidal particles65 and amphiphilic dumbbell particles64 have been characterized by the this technique. While majority of the reports on the use of GTT are for the measurement of contact angle of spherical particles, recently several groups have employed this technique to visualize non-spherical particles at interfaces.67, 69-72 The contact angle values of particles above tens of nanometers have been estimated by coupling GTT with atomic force microscopy (AFM)73 and FreSca cryo-SEM.63 We have recently measured the three phase contact angle and the orientation of non-spherical particles of different shapes - dumbbells, spherocylinders and cuboids, sizes and surface properties using the GTT. The electron microscopy images that depict the equilibrium position and orientation of particles is given in Figure 3A-E.57 In quantifying the three phase contact angle, local interface deformation around the particle is not considered. In the GTT66, particles are spread at the air/oil-water interface. A small quantity (2 wt%) of gellan, a water dispersible hydrocolloid is added to the aqueous phase to aid the arresting of particles at their equilibrium position at the interface. Then the particles are transferred on to the polydimethylsiloxane (PDMS) elastomer surface (without any change in the interfacial position and orientation) and are imaged using SEM. The dumbbell and spherocylindrical particles orient with their long axis parallel to the interface (Figure 3A and B), whereas cuboidal particles exhibit multiple


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orientations – face up, edge-up and vertex up orientations (Figure 3C-E). The frequency of occurrence of three different orientations of cuboidal particles at decane-water interface is displayed in Figure 3F. The face-up configuration, which corresponds to the global energy minima is observed most frequently while the other two states are metastable.71 The edge-up orientation corresponds to the local energy minima. The vertex-up (sunken and tilted) configuration which is a kinetically rapped state show least probability of occurrence (< 5%). The contact angle values indicate partial hydrophilic nature of hematite particles, in agreement with literature reports.74-75 The effect of particle surface properties is also investigated by comparing the contact angle values of spherocylindrical silica shells and silica coated particles. The surface energies and the detachment energy of hematite particles are estimated using the measured contact angles. In addition to the contact angle measurements of particles, GTT enables the direct visualization of the particle induced interface deformations and also the capillary bridge between the particles. The nonplanar nature of the interface deformations in the vicinity of isolated particle adsorbed at an interface, a distinct feature of particle shape anisotropy is affected by their orientation. The nature and extent of this deformation, crucial for understanding the particle assembly at interfaces is discussed in the following section. 4 Particle shape induced interface deformation Contrary to the circular contact line around smooth, homogeneous spherical particles, the three phase contact line around an anisotropic particle adsorbed at the interface is non-planar.15, 56, 62 Therefore, in the immediate vicinity of anisotropic particles, the deformed interface most often shows elevations and depressions. This undulation in the contact line arises from the need to satisfy the Young’s equation at each and every point along the three phase contact line.15 The growing scientific interest in studying shape induced interface deformations comes from the


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possibility of tuning the particle interactions which in turn dictate the self-assembly of anisotropic particles at interfaces. The particle induced out of plane distortion of the interface (h) follows the Laplace equation in 2D, the solution which is typically expressed as sum of capillary multipoles viz., monopole, dipole, quadrupole, octupole, etc.56,

76-77

The Laplace equation is

given by:

∇ h = 0

(3)

and a general solution to this equation in terms of polar multipoles is,56 h(r, θ) = A" ln + A cos( + ) ) + A cos(2 + ) ) + ⋯ (4) where r, θ are the polar coordinates in a particle centered reference frame. The elevation and depression of the three phase contact line with respect to the flat interface is normally referred to as positive (+) and negative (-) capillary charge respectively. When the interface deformations of neighboring particles overlap, the interactions are attractive for like capillary charges and repulsive otherwise. The capillary interactions originating from the interface deformations dominate over all the interactions present at the interface and drive the self-assembly of nonspherical particles at interfaces. Therefore, unlike Pieranski's observation of two-dimensional colloidal crystals of polystyrene spheres, there have been no reports of the formation of loosely packed ordered monolayer of non-spherical particles at interfaces to date. The engineering of particle laden interfaces of desired structure and properties requires identification of capillary multipoles around individual particles, which can further be used to infer the nature of particle arrangement (such as tip-to-tip, side-by-side, triangular, etc.) in the 2D assembly. In addition to


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the shape anisotropy, any heterogeneity either physical or chemical can also alter the interface deformation profile and may lead to higher order capillary multipoles.77-79 The experimental realization of the multipolar nature of interface deformation is limited due to small length scale (~nm) of these undulations. The nature of interface deformation around particles of different shapes has been characterized by phase shifting interferometry technique,62 FreSCa cryo-SEM65 and GTT.58, 70 Both ellipsoids (pointed edges)62, 70, 78, 80-83 and cylindrical particles (flat edges)72,

82, 84-85

induce a quadrupolar type interface deformation. In case of

ellipsoidal and cylindrical particles of similar wettability (i.e., both hydrophilic), a capillary charge reversal is observed.62, 72, 82 The reversal of capillary charges is also observed when the wettability of the ellipsoidal and cylindrical particles changes from hydrophilic to hydrophobic. 62, 65, 70, 82, 85

Thus both shape and wettability changes can lead to capillary charge reversal. The

other particle shapes whose interface deformations are investigated include 3D printed objects with different branch curvatures86 equilateral triangular prisms of different thickness,87 and thermo-responsive soft-programmed gel particles (SPGPs).88 The interface profiles based on theoretical calculations and simulations support the experimental observations i.e., quadrupolar78, 82-83, 89

type of deformation for ellipsoids/cylinders and octupolar58,

60, 83, 90

type for cuboidal

particles. The Surface evolver simulations91 have been used extensively to investigate various aspects of particle laden interfaces - such as wetting behavior, interface deformation, capillary bridge formation and the resulting particle assemblies.56, 83, 87 Recently, we have visualized interface deformations around cuboidal particles adsorbed at airwater and oil-water interface using GTT.58 Depending on the location and orientation of the cuboids with respect to the interface, monopolar to octupolar type of deformations are observed which are presented in Figure 4. The cuboidal particles in the face-up orientation induce either


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octupolar or hexapolar interface deformation (Figure 4A1-A3), the edge-up orientation result in quadrupolar type (Figure 4B1-B3) and the vertex -up orientation induce a monopolar deformation (Figure 4C1-C3). The nature of interface deformation captured using the GTT are quantified by the surface profiler images and the interface mediated particle assembly using optical and electron microscopy. Though there is reasonable progress on characterization of particle induced deformation of the interface, these studies are still limited to particles of few shapes. Moreover, the quantification of the interface distortions especially in the immediate vicinity of the particles is limited and needs further work. 5 Directed-assembly of particles through shape-induced interface deformations Self-assembly is a process of spontaneous organization of molecular, colloidal or macroscopic objects into well-defined structures. As the dynamics and self-assembly of colloids can be visualized directly, several phenomena relevant to molecular self-assembly can be investigated in great detail. The principles of bottom-up self-organization have been exploited to create highly ordered matter with great precision, for example, the fabrication of three dimensional assemblies with distinct optical and electrical properties that find application in many fields including photonics.92-95 The interface directed self-assembly of particles can lead to a variety of structures - loosely or close packed crystals, two dimensional gels, colloidal alloys.56,

96-97

Due to the

possibility of tuning the colloidal interactions via shape induced interface deformations, the nonspherical particles are excellent building blocks to realize unique interfacial particle assemblies which otherwise cannot be achieved with spherical particles. The interfacial assembly of nonspherical particles of different types of materials such as metals, inorganic, organic and particles of biological origin - cellulose, mosquito eggs, bacteria and viruses have been experimentally studied.56 When the interface deformations of neighboring particles overlap, the particles


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assemble to minimize the excess interfacial area. Possible equilibrium assemblies of three and four particles at the interface have been predicted based on many-body interaction calculations by considering interracially trapped particles that induce octupolar deformations.98 In the case of three particles, the triangular assembly with the radial orientation of particles is shown to be most stable, whereas the orthoradial square is most stable in the case of four particles. When interfacially trapped particles experience capillary interactions, they rotate to preferred orientations to minimize the surface free energy81, 84. The reorientation of the particles before they assemble may be due to many factors such as surface charge density of the particles, difference in particle sizes, interface curvature or attractive/repulsive capillary charges. The early reports that demonstrate the effect of particle anisotropy on the self-assembly of particles at interfaces are carried out by spreading custom made millimeter sized polydimethylsiloxane (PDMS) particles of different shapes at perfluorodecalin/water interface99. The directed assembly of particles at interfaces is observed to occur due to capillary bonding which is further tuned by rendering amphiphilicity to the shape anisotropic particles. It is argued that the interactions are of long range, the aggregates formed at the interface are stable and the assembly process is reversible. The quadrupolar nature of interface deformation causes the prolate ellipsoids to attract and self-assemble in side-by side or tip-to-tip configuration, however particles repel when they approach in tip-to-side orientation.70, 83, 89 In the case of three particle assemblies, both side-by-side and triangular configurations are equally energetically favorable70, 80

Similarly, the cylindrical particles with sharp edges prefer tip-to-tip arrangement and form

long chains, which can be compressed to form "bamboo" like structures.72, 82, 84 The curvature of the interface also plays an important role in the capillary assembly of particles. The interface curvature suppresses the tip-to-tip assembly observed at planar interfaces and drives side-by-side


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aggregation.84 Although the shape induced capillary interactions dominate at interfaces, their assemblies are observed to be influenced by particle surface charge. While the particles in the 2D aggregates are connected tip-to-tip and form linear chains when the surface charge density is high, the side-by-side assemblies are favored in the case of low particle surface charge densities.70 Uncharged polystyrene ellipsoids and silica coated polystyrene ellipsoids are observed to assemble in the side-by-side manner through capillary interactions.70, 80, 89 Interfacial assemblies of different shaped particles are shown in Figure 5. Polystyrene spheres form wellordered crystalline structure (Figure 5A), highly charged polystyrene ellipsoids assemble in tipto-tip configuration (Figure 5B) and result in a percolating network at the decane-water interface (Figure 5B).70 In the case of polystyrene particles with less surface charge density, capillary interactions dominate and they assemble into complex structures as shown in Figure 5C. The directionality and reversibility of shape induced capillary assembly are investigated by employing temperature responsive shape-programmed gel particles (SPGPs).88 The interfacial assemblies of these particles characterized using fluorescence microscopy are observed to be comparable with the digital photographs of the assemblies of elastomer particles of identical shape.88 In another study, the capillary assembly of branched millimeter scale objects at air-water interface shows various mesoscopic structures that can be controlled by subtle changes in particle morphology.86 In a most recent study, the interfacial adsorption and assembly of thin equilateral triangular prisms of varying thickness at the air-water interface are investigated.87 When adsorbed at the air-water interface, the triangular prisms occupy different equilibrium positions and induce a hexapolar type deformation that directs the assembly of neighboring prisms.


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We have experimentally investigated the shape induced interface deformations, capillary bridging and the interface directed assembly of micron sized cuboidal particles.58 The particle shape induced interface deformations are observed to be affected by their orientations and varied from monopolar to octupolar types. In many isolated multi-particle assemblies, individual particles form capillary bonds with neighboring particles through capillary interactions such that the rules of capillary charges are satisfied. The optical and electron microscopy images of the percolated particle networks formed at air-water and decane-water interfaces are observed to contain hexagonal, honeycomb and square lattices that are predicted in theory and simulations60, 90

as shown in Figure 5D1 and D2. The interface deformations and the interfacial assembly

structures of hematite cuboidal particles are similar to the predictions based on numerical calculations.60,

90

In the large scale assemblies of cuboids at interfaces, every particle in the

hexagonal and honey-comb lattices is observed to satisfy all the capillary bonds. The reports on the interfacial assembly of particles to date point that the valence and capillary bonding of anisotropic particles are solely due to the capillary charges arising from the interface deformation and direct the enormous potential to mitigate complex assembly of particles of many length scales. 6 Response of monolayer of shape anisotropic particles to compression and shear Compression-driven particle assembly and surface pressure - area isotherms: The surface pressure driven organization of non-spherical particles of different shapes floating at fluid-fluid interfaces into two-dimensional close packed superlattices have been well studied.100 Compared to large account of research on use of LB technique to create assemblies of non-spherical nanoparticles, work on compression of monolayers of micron sized particles that allow in situ visualization of microstructural changes are limited.


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The surface-pressure isotherms recorded during the compression of monolayers containing micron sized spherical particles and ellipsoids of different aspect ratios are shown in Figure 6A, where the surface pressure ( π ) vs. rescaled Langmuir through area is plotted (ARS). Typically the concentration of particles deposited at the interface is such that the particle monolayer exhibits a gas-like state i.e., the particles in the monolayer are far away from each other and the inter-particle interactions are not important. The inset shows a representative

π -A plot, where

the inflection points – IP1, IP2, and IP3 identified by drawing the tangents as indicated, point to regions in the isotherm where significant microstructural changes and a corresponding increase in the resistance to compression is observed. It must be noted that traditionally, surface pressure is plotted against area per particle or area per molecule. The use of rescaled area provides a good measure of area per particle in the absence of information on the total number of particles at the interface, which is difficult to estimate if (a) large numbers of particles are lost to the sub-phase during the deposition of particles at the interface. This can happen when the concentration of spreading solvent (such as isopropyl alcohol) used is low or when the dispersion contains elongated particles (b) the surface area or the surface area occupied by the particles varies significantly across the monolayer, which is the case when uncharged particles are spread at the interface. In such cases, due to van der Waals and lateral capillary forces, the deposited particles from aggregates, and the microstructure of the interface is not homogeneous throughout. However, this is not the case when the monolayer structure is homogeneous – for example – when charged polystyrene or silica particles deposited at oil-water interface, the particles form hexagonally ordered loosely packed structures and the calculation of surface coverage by image analysis can be used to calculate the number of particles at the interface and hence the area per particle accurately. (c) The possibility of multiple orientations of non-spherical particles at


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interfaces adds further complexity to the calculation of area per particle. The rescaled area (ARS) is calculated form the trough area (AT) and the surface pressure at the inflection point (IP1) at which the surface pressure starts to increase and the surface coverage at this infection point. The use of micron size particles and the air-liquid microscopy Langmuir trough allows direct visualization of the arrangement of particles in the monolayer and the calculation of the surface coverage. Therefore raw data from Langmuir trough experiments, i.e., the surface pressure−trough area is converted to surface pressure - rescaled through area isotherms as shown in Figure 6. The analysis of the surface pressure – area isotherms and the microstructure recorded during the compression of monolayer of ellipsoids reveal several salient features, some of which are unique and arise due to particle shape anisotropy: (1) Immediately after the deposition of particles at air-water interface, the microstructure of the monolayer of long aspect ratio ellipsoids revealed the existence of aggregates in which the particles show tip-to-tip as well as side-by-side arrangements. Formation of such aggregates is due to the shaped induced lateral capillary attraction. (2) The changes in surface pressure between IP1 - IP2 and beyond IP3 are more gradual for monolayers of higher aspect ratio ellipsoids. (3) In the vicinity of IP1, the surface pressure is observed to increase (note that the initial surface pressure is zero) when the particles in the monolayer come into contact and form an inter-connected network indicating percolation. With increase in the aspect ratio of particles, the percolation is observed to occur at lower surface coverage. (4) When compressed beyond IP1, significant rearrangement of particles and densification of the monolayer is observed. Due to compression, few particles in the network, which are initially oriented with major axis parallel to the interface, are observed to take-up an upright position i.e., their major axis becomes perpendicular to the interface. Such particles that appear as circles in the microscopy image in in Figure 6C are termed “flippers” and observed to


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first appear in locally dense regions of the monolayer where the surface coverage is high. Such a transitions observed for high aspect ratio are associated with release of compressive stresses (5) the slope of the isotherm between IP2 and IP3 for monolayers of ellipsoids of aspect ratio 2.3 is steepest compared to other aspect ratio particles. Thus, in this zone, the monolayer of low aspect ratio ellipsoids offer highest resistance due to efficient packing and the packing density is observed to be the highest. (6) The 2D microstructure at IP2 and IP3 is quantitatively analyzed to reveal non-monotonic variation of packing fraction with aspect ratio. It is shown that the surface coverage or packing fraction is highest when the aspect ratio is 2.3 and decreases on either side of this aspect ratio. Albeit the arrangement of particles being not truly random (due to long range attractive forces) and the presence of flippers, this trend is similar to that observed in variation of packing density of ellipsoids in the bulk.101 However, the co-ordination at IPI and IP2 is observed to decrease monotonically with increase in aspect ratio. The decrease in co-ordination number and the 2D packing density at higher aspect ratio is attributed to the formation of open network of particles (with large voids) probably due to enhanced capillary attraction. (7) The compression modulus calculated by considering the linear variation of surface pressure with area in the region from IP2 to IP3 is also observed to display a nonmonotonic evolution with particle aspect ratio. The magnitude of compression modulus is calculated to be highest for 2.3 aspect ratio particles, which is expected, as the packing density is the highest. (8) Towards the end of compression cycle when the compression ratio is high, the monolayer of ellipsoids shows buckling. The darker regions in Figure 6D correspond to folds in the monolayer, which are observed in the direction perpendicular to compression as indicated by the arrows. While the flippers occur in locally dense regions initially, at high compression ratios, long trains of flippers


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span the entire width of the microscopy image. During the final stages of compression, the monolayer is not planar anymore as some regions in the microscopy image appear out of focus. Recently, the surface pressure – area isotherms for monolayers containing spherical particles and spherocylinders of different aspect ratios have been reported102, which show a similar trend as shown in Figure 6, however add to the understanding of the dynamics of some of the structural transitions observed earlier. In situ microscopy visualization of flipping events shows that flippers form in regions of the monolayer where the nematic ordering is rather low i.e., in the regions between the nematic domains. Since the monolayer is already dense when flipping events occur, local rearrangements of particles that exhibit conflicting orientations are not possible and thus they flip. It is also shown that when the aspect ratio of spherocylinders is sufficiently high, particle monolayers form colloidal multilayers at high compression ratio. There have also been several reports of compression of monolayers of carbon-based materials such as graphene oxide (GO) sheets103 and carbon nanotubes (CNT).104 The response of monolayer of GO sheets to compression is argued to be influenced by the lateral capillary attraction due to chemical heterogeneity of GO sheets103 and also by the wetting properties of GO sheets which can be tuned by changing the pH of the aqueous phase used to create air–water interface.105 Surface pressure-area isotherms have been used to demonstrate the high surface activity and stability of nanometer thick monolayers of GO sheets. The monolayers of GO sheets show wrinkles upon compression and which completely relax upon expansion, which is reminiscent of elastic-like behavior of membrane.103 Pendant drop compression: Unlike Langmuir trough experiments, which need elaborate preparations including exhaustive cleaning, pendant drop compression experiments are relatively easy to set-up and have been extensively used to understand the response of complex fluid


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interfaces to compression and expansion. Figure 7 shows a series of images of particle laden pendant drop corresponding to V/V0=1 (where V0 is the initial drop volume and V is the drop volume at a particular time during compression) subjected to a compression cycle. The aqueous drop at pH=2 contains nano-ellipsoids (major axis - 254±50 nm, minor axis - 55±12 nm, aspect ratio - 4.6±0.6) at a particle concentration of 0.3 wt%.44 The compression is achieved by withdrawing the dispersion of ellipsoids such that the drop volume and hence the interfacial area available for the particles decreases. In these experiments, the drop volume is decreased at a rate of 0.8 µl/s. The drop image corresponding to V/V0=1 shown in Figure 7A is suspended in pendant configuration in the oil phase (decane) in a cuvette and left to equilibrate for 60 min. This wait period is much larger than the diffusion time scale, sufficient for the particles to reach the interface and form a particle coated drop. As V/V0 decreases, the interface is compressed and the drop sizes decrease. However, even at high compression ratio, drop shape does not change and the drop continues to be axi-symmetric. However, all conditions remaining the same, except the change in pH from 2 to 6.5, the droplet shapes become non-symmetric, show bucking and wrinkling instabilities at V/V0=0.16, which further grow with increase in compression ratio. These shape changes point to fluid–solid transition, typical behavior of space spanning interconnected network of particles on the drop surface. The behavior of ellipsoids coated drops subjected to compression and expansion cycles in Figure 7B shows that the drops that exhibit wrinkles if expanded relax by smoothening out the wrinkles. The wrinkles disappear completely and the drops eventually become axi-symmetic. The drop shape changes are reversible and observed repeatedly over many compression-expansion cycles. These experiments demonstrate irreversible adsorption of ellipsoids to the interface and that the ellipsoids are not expelled from the interface even when subjected to multiple compression/expansion cycles. The scanning


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electron microscopy images in Figure 7C and D show the microstructure of the monolayer of ellipsoids formed on the drop surface. The images reveal features such as wrinkles, buckled regions and locally nematic like ordering observed during compression of planar monolayers of elongated particles.21, 102 The contrasting behavior of drops shown in Figure 7A and B is due to fluid-like and solid-like response of the drop surface to compression when the drops contain ellipsoids dispersed in water at pH 2.0 and 6.5 respectively. As discussed in earlier section, the adsorption of ellipsoids to interfaces is dictated by the surface charge density of the particles and the network of ellipsoids formed due to lateral capillary attraction leads to fluid-solid transition observed at pH 6.5. We have not seen such shape buckling and wrinkling instabilities when drop surfaces coated with spherical particles are subjected to compression.50 Therefore, the particle shape does play a major role. The compression of pendant drops decorated with graphene oxide (GO) sheets and block copolymer are shown to be mechanically robust and show buckling and wrinkling behavior106 similar to that shown in Figure 7B. This is due to elastic nature of the composite monolayer formed due to attractive interaction between graphene oxide particles and the block copolymer chains. Interface rheology of shape anisotropic particles adsorbed at fluid interface: Surface rheological measurements need the development of stringent measurement protocols and are much more challenging compared to bulk rheological measurements.28,

107

For example,

conditioning steps such as pre-shear typically carried out prior to bulk rheological measurements to erase sample history cannot be performed as this will lead to significant re-structuring of the particles or any other species at the interface or loss of material to the bulk. Most often, multiple measurements cannot be performed, especially for monolayers of aggregated particles. Prior to surface rheological measurements, it is also necessary to verify that recorded rheological data is


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not affected by the sub-phase drag, which depends on the nature of interface and the measuring geometry used. It is always a good practice to calculate the Boussinesq number, defined as the ratio of surface drag to sub-phase drag.103 For reliable surface rheological measurements, the sub-phase drag should be minimal or the Boussinesq number should be higher.29, 103 The influence of aspect ratio and surface coverage on the surface rheology of monolayers of hematite ellipsoids deposited at decane-water interface and polystyrene ellipsoids deposited at air-water and decane-water interface has been investigated.70 The Surface rheological measurements have been carried out with magnetic rod interfacial stress rheometer when the surface coverage of particles at the interface is low and with a bi-cone attached to a conventional stress rheometer when the surface coverage is moderate and high. As mentioned earlier, to obtain reliable and reproducible data, the measurements are always carried by creating a fresh particle monolayer. Figure 8A shows the surface elastic and surface viscous modulus measured as a function of strain at a fixed 1 Hz frequency for the network of ellipsoids at the air-water interface at a surface coverage of 0.65 and 0.83. With increase in applied strain amplitude, the surface storage modulus decreases monotonically, however, the surface loss modulus shows slight increase at intermediate strain amplitude, a rheological signature of the break-up of flocculated network of particles. The surface storage and loss modulus measured at strain amplitude of 0.01%, which is well below the critical strain are observed to be frequency independent, which again is a typical signature of colloidal gels. A comparison of surface storage modulus of monolayer of ellipsoids and spheres spread at air-water interface is shown in Figure 8B. These data are obtained from strain sweep measurements at low frequency and the moduli are in the linear viscoelastic regime. For the monolayers of both types of particles, the surface storage modulus increases with increase in particle surface coverage. However, the monolayer of


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ellipsoids show significantly higher moduli compared to monolayer of spherical particles at a given surface coverage. This is due to interface mediated self-assembly of ellipsoids that form mechanically robust particle network due to the particle shape induced capillary attraction. 7 Pickering emulsions stabilized by non-spherical particles The possibility of engineering robust assemblies of shape anisotropic particles through interface deformation mediated capillary interactions makes them ideal candidates for emulsion/foam stabilization.108-111 Pickering foams/emulsions stabilized by particles of different shapes and aspect ratios can be used to fabricate hollow shells (colloidosome capsules) of modular permeability which may find applications in the controlled release of active materials, be it nutrients or medicine. As majority of the biocompatible particle emulsifiers are shape anisotropic, a fundamental study of Pickering emulsions realized using non-spherical particles can help in the rational understanding of the formation and stability of such emulsions and will also broaden their applicability. A large number of non-spherical particles from various sources and materials have been used as emulsifiers, which include, SU-8 polymeric rods,69,

112

polystyrene ellipsoids,74 hematite particles of ellipsoidal, cuboial, spherocylindrical and peanut shapes,45,

74-75

chitin,113-114

nanocrystals117 silica rods,

bacteria-chitin

118-120

networks,115

graphene

sheets,116

cellulose

and many other irregular shaped particles of biological

origin.108 Particle covered agarose shells69 and foams112 of superior stability are prepared using SU-8 microrods. Colloidosomes are prepared from submicrometer sized monodispersed Ga-socMOF or Fe-soc-MOF using a single step solvothermal process.121-122 This synthesis-cumorganization route provides a platform to tune the droplet size, shell thickness and porosity of the particle network. There is a growing interest in the preparation of 'oil free' water-in-water emulsions for applications in cosmetics, pharmaceuticals and food industries, which pose a real


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challenge in terms of the identification of suitable emulsifiers for such systems.123 Here the two immiscible phases are prepared from aqueous dispersions of two polymers with more than 90 % water and the shape anisotropic particles are proved to be better candidates for their emulsification. In another class of particle stabilized systems, 'bijels', a decrease in the domain size and an increase in packing fraction with increase in particle concentration is experimentally observed when rod-shaped particle are used as stabilizers compared to spherical particles.124 Natural emulsifiers of biological origin are very potential systems that have been explored for the development of novel materials capable of encapsulation for use in pharmaceutical and food industries. Naturally occurring spore particles stabilize O/W emulsions as demonstrated with oils of different polarities and the droplets are observed to be stable even at very low surface coverages.125 Cellulose nanocrystals (CN) from different sources - Cotton (CCN), Bacterial (BCN), Cladophora (CaCN), at low particle concentrations can form ultrastable O/W emulsion drops of identical sizes irrespective of their origin.117,

126

Bacteria based O/W Pickering

emulsions are prepared by mixing E.coli, chitosan and n-tetradecane as the organic solvent.115 The self-assembled bacteria-chitosan networks trapped at the surface of drops act as environmental friendly interface and as a bacteria based microreactor. Regenerated chitin nanofibers are reported to be effective O/W emulsifiers.114 W/O emulsions that can withstand high temperature are prepared from high salinity water and paraffin oil using hydrophobically modified rodlike sepiolite particles as the emulsifier.127 The effect of particle wettability on the stability and type of Pickering emulsion is studied by using multi-walled carbon nanotubes (MWNT).128 The wettability of MWNT is tuned by controlled surface functionalization and observed to influence the type of emulsions and the droplet size distribution - while amphiphilic MWNT stabilize smaller droplets, at higher hydrophilicity/hydrophobicity larger droplets are


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observed. The amphiphilic MWNT orient with their long axis parallel to the interface and occupy more interfacial area whereas extremely hydrophilic/hydrophobic MWNT may re-orient on droplet during coalescence due to the weak energy of attachment resulting in an increase in drop size. The role of particle shape anisotropy on the formation and stability of emulsions, the microscopy images of emulsion droplets and the arrangement of particles around the drop surface are shown in Figure 9. The emulsification carried out using partially hydrophilic spindle hematite particles proved that stable emulsions are formed ony above a critical aspect ratio (i.e., at AR = 4.6 ±0.9. AR = 5.3 ± 0.8 and AR = 6 ± 1.) as demonstrated in Figure 9A.74 The formation of inverse emulsions using partially hydrophobic polystyrene ellipsoids of different aspect ratios shows that emulsion formation above a critical aspect ratio is more general. The cryo-SEM (Figure 9B) observations of water droplets covered with polystyrene ellipsoids confirm the dense packing of particles on the droplet surfaces. The rheological measurements have been sued to show that the viscoelastic nature of the assembled particle networks at the interface is responsible for the super stability of ellipsoid stabilized emulsions. Silica rod stabilized W/O emulsions are prepared by arresting the temperature induced phase separation of lutidine-water mixture.118 The lutidine phase completely wets the silica rods and partially wetted by water resulting in the formation of W/O emulsion droplets whose size can be tuned with the particle concentrations. The particle covered water droplet retain their shape on drying and are imaged through SEM is displayed in Figure 9C. The effect of particle shape in the formation of Pickering emulsions is studied by using micron sized hematite particle of cuboidal, spherocylindrical and peanut shapes.45, 75 These partially hydrophilic particles form O/W emulsions that exhibit long term stability for more than a year. The particles adsorbed on to the droplet surfaces do not respond to an external magnetic


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field display the extraordinary stability, only the free particles in the suspension are influenced by the magnetic field.75 Optical microscopy images of the Pickering emulsion droplets stabilized by particles of different shapes – Polystyrene ellipsoids (Figure 9D), peanut shaped (Figure 9E) and ellipsoidal (Figure8F) hematite particles and silica rods (Figure 9G) are displayed in Figure 9D-G. The polystyrene74 and hematite particles45, 75 in the micron size range enable the direct visualization of closely packed particle assembly on the liquid droplet surface through optical microscopy at higher magnifications, as shown in Figure 9H-K. The close packing of particles at the interfaces resulting from the shape induced capillary interactions impart high stability to the emulsion droplets. pH responsive Pickering emulsions: Pickering emulsions that can be stabilized and destabilized in a controlled manner by the application of an external field or change in solution conditions, also called stimuli-responsive Pickering emulsions are relevant in biomedicine, oil recovery, catalysis, cosmetics etc.110 External stimuli to which the particle stabilized emulsions/foams respond are – pH,110, 113, 116 temperature,129 magnetic field,130 light intensity,131 CO2,132 etc. The stimuli-responsive emulsions have been prepared using graphene oxide,116 chitosan113 and hematite and silica particles45 as emulsifiers without any other additives. There have also been reports, wherein the particulate emulsifiers are surface modified using suitable methods to achieve stimuli-responsiveness of Pickering emulsions. Graphene oxide sheets (GO) which are amphiphilic (with hydrophilic edges and hydrophobic basal plane) stabilize O/W emulsions and moreover, their amphiphilicity can be tuned by changing the pH of the dispersion. Stable emulsions are formed only at acidic pH and the emulsion droplets are destabilized upon adjusting the pH to 10.116 The pH induced emulsification/demulsification process is observed to be reversible and with the change in pH, the GO particles shuttle between the aqueous phase and the


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O/W interface. Reversible pH-responsive Pickering emulsions are prepared by using chitosan as an emusifier. At pH > 6, the chitosan aggregates formed adsorb at the interface and stabilize O/W emulsion droplets.113 At acidic pH, the chitosan is soluble in the aqueous phase resulting in demulsification. It is possible re-create emulsion by adjusting the pH to the basic range. The hematite particles of different shapes and sizes have been shown to stabilize O/W emulsions of exceptional long term stability.44-46 Recently, we have exploited the possibility of turning the surface charge of hematite particles as a versatile method to prepare stimuli responsive emulsions. The strategy is demonstrated to be general as shown by the pH responsiveness of emulsions stabilized by silica and polystyrene spheres, silica rods, hematite particles of different shapes (ellipsoids, cuboids, spherocylinders, and peanuts). The pH dependent formation, stabilization and destabilization of O/W emulsions stabilized by peanut shaped hematite particles and silica rods are demonstrated in Figure 10. The hematite particles stabilize O/W emulsions when they are weakly charged i.e., when dispersed in water at pH near to isoelectric point (pH 8.5). At low surface charge, the particle stabilized emulsions are completely covered with the particles. The self-assembled close packed particle network at the drop surface formed during emulsification is directly visualized through optical microscopy. No emulsions are formed when the particles are highly charged (at pH 2 and 12). When highly charged particles approach the oil-water interface during emulsification, the repulsive interaction between the particle and the image charge in the oil phase prevents their interfacial adsorption and therefore no emulsions are obtained. Moreover, the particles adsorbed on the droplet surfaces do respond to the variations in the pH of the continuous medium. Therefore, the pH can be used to increase in the surface charge density of particles to enable their detachment from the droplet surface resulting in demulsification. During destabilization, a gradual decrease in the surface coverage of particles on


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the drop surface is observed with time. When the surface coverage is sufficiently low, the coalescence of sparsely covered drops occur leads to complete destabilization of emulsions as shown in Figure 10A. The pH induced wettability of particles is demonstrated to cause the detachment of adsorbed particles from the interface and has been characterized by the modified gel trapping technique. The direct visualization of the shift in the equilibrium position of the particles is used to characterize the change in particle wettability with change in the pH (particle surface charge). In the case of silica rods, O/W emulsions are formed at pH 2 and demulsify upon adjusting the pH to 12 as shown in Figure 10B. Moreover, in both the cases, the pH responsiveness is observed to be reversible. i.e., stable emulsions are formed upon adjusting the dispersion pH back to the pH close to the isoelectric point. Therefore, a control over surface charge of the particles which leads to corresponding change in the wettability of the particles at interfaces dictate the response of Pickering emulsions to pH which may be desirable for many applications. 8 Interface mediated conversion of spheres to anisotropic particles Fluid-fluid interfaces have been used as a template for the synthesis of novel particles that are anisotropic in terms of either shape or surface properties or both. The first step towards realizing the fabrication new types of particles involves the formation of particle monolayer at a suitable fluid-fluid interface. Then the surface of the particle that is in contact with either the sub-phase (typically water) or the upper phase (typically air or oil) can be selectively modified. The surface modification can be achieved by dispersing certain species (nanoparticles, surfactants, polymers, etc.) in the bulk fluid phase such that they deposit on the particle surface in contact with the fluid either by physical adsorption or electrostatic interactions. As the equilibrium position of particles at the interface can be tuned either by using different water-oil systems or by using additives, the


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fraction of area over which surface modification can be achieved is tunable. We have recently reported a versatile method to transform spherical polystyrene particles into patchy particles by depositing gold nanoparticles on the polystyrene particle surface that exposed to the aqueous phase.133 The patch area i.e., area over which gold is deposited is tuned by using sodium dodecyl sulfate (SDS) to vary the wettability of the particle or by spreading the particles at different fluidfluid interfaces such as octanol–water, decane–water and air–water. In a study by Paunov and coworkers, the monolayer of particles originally trapped at fluid-fluid interface has been transferred on to PDMS and then sputter coated with gold to obtain Janus particles (Latex/gold) using the gel trapping technique.134 These techniques can be easily adaptable to metallic, organic, inorganic or polymeric particles as long as the particles retain their shape during the experimental conditions. Lately, innovative interface mediated particle synthesis route to transform spherical particles into non-spherical particles have been developed. However, this is possible only when the particles deposited at the interface are easily deformable. The fact that polymeric particles can be deformed using various stimuli – such as temperature, solvents, or vapors – has been exploited to fabricate particles of fascinating shapes. The 'flying saucer' like supraparticles are obtained by drying the particle laden PDMS film prepared by gel-trapping procedure followed by film stretching.134 Using a similar approach, asymmetric polystyrene particles of different shapes are prepared by trapping them at the air-water interface followed by incubation in hot decane.40 The particles are collected after subsequent removal of the top oil phase followed by sonication and centrifugation. The parameters that control the morphology of the particles are the temperature of the aqueous gellan phase and the incubation time. The spreading of particles at air-water or oilwater interface provides a provision to add surface active additives such that shape anisotropic


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amphiphilic particles can be prepared in a single step process. Polystyrene particles of different shapes ('acron' and 'idly' like) and surface properties are prepared through the non-uniform swelling of interfacially adsorbed spherical particles.41 This is achieved by simultaneous heating and swelling of particles in the presence of decane.4 Selective modification of the particle surface exposed to the water phase is accomplished by adsorption of silica particles or cetyltrimethylammoniumbromide (CTAB) (initially dispersed in the aqueous phase) through electrostatic interactions. A low ambient temperature synthesis of shape anisotropic colloidal particles involves the exposure of monolayer of polystyrene particles of sizes ranging from nanometer to micrometer length scales trapped at air-water interface to a solvent (toluene) vapor for varying incubation time.42 Similar to the planar particle-laden interfaces, the curved interfaces act as templates for the synthesis of anisotropic particles with an additional advantage of high throughput. Janus silica particles of varying amphiphilicity are synthesized through the emulsification of molten wax and water and an oppositely charged surfactant, the concentration of which dictates the equilibrium position of particles on the wax droplet surface.135 After solidifying the wax, the coloidosomes are washed, dried and then treated with the surfactant solution for surface modification. Finally, the particles are collected after dissolving the wax in chloroform and subsequent washing. The similar emulsion based approach has also been demonstrated for high throughput synthesis of shape anisotropic particles.41 Anisotropic particles of different morphologies – crescent to moon shapes – are prepared by using Janus emulsion droplets as templates136. The emulsification is carried out by considering a mixture of oils (photopolymerizable monomer, ethoxylated trimethylolpropane triacrylate (ETPTA), the nonpolymerizable phase is an immiscible fluorocarbon oil - HFE 7200) and an aqueous solution containing Pluronic copolymer (F127).


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The oil-in-water emulsions are exposed to UV light, the non-cured oil is removed to obtain particles of different sizes and shapes. The morphology and size of the particles can be tuned by controlling the energy input during emulsification and the ratio of the two components that form the oil phase. 8. Conclusions and Outlook In this article, our contribution on the influence of particle shape in the area of colloidal particles at interfaces is outlined. The contribution of chemists and various researchers who have invented and documented procedures for the synthesis of organic, inorganic, metallic and polymeric particles of various sizes and shapes has played a significant role in the exciting developments in this field. Various fundamental aspects such as adsorption of shape anisotropic particles to fluid-fluid interfaces, equilibrium position and orientation of particles at the interface, shape induced interface deformations, self-assembly and responses to compression/shear are discussed. In the last section, the use of non-spherical particles in emulsion and foam stabilization and novel applications of colloids at interfaces for the fabrication of asymmetric particles that exhibit shape and surface anisotropy are highlighted. While aspects such as adsorption of particles to interface are governed by surface charge irrespective of particle shape, non-spherical particles at interfaces do exhibit several unique features. The distinct characteristics include 1) the possibility of multiple orientations and one or more wetting states for a particular orientation 2) out of plane particle orientation during compression of monolayer of elongated particles 3) appearance of capillary attractions even at small deviations from the spherical shape 4) mechanically robust interfaces with superior surface moduli at relatively low surface concentration 5) ability to stabilize interfaces purely due to shape induced interface mediated interactions.


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Considering ever growing interest in the design and synthesis of regular shape anisotropic particles and particles of other complex geometric shapes, we look forward to exciting developments in the field of non-spherical colloids at interfaces. Of all the developments to date, the use of shape anisotropic particles for stabilization of emulsions appears to be the most researched followed by capillary interactions due to non-spherical particles though limited to elongated particles such as rods and ellipsoids, more recently cuboids, triangular shaped particles and other complex geometries. While the interface deformations around large particles trapped at interface have been measured, there are no such measurements when the particles are sub-micron in size. Therefore, there is need for the development of suitable etiquettes for the investigation of near field interface deformations close to the particle i.e., in the immediate neighbourhood. A detailed investigation of the large scale self-assembly of non-spherical particles and their interfacial rheology is limited to few fluid-fluid-particle systems such as ellipsoids at air-water and oil-water interfaces. The role of particle shape, surface charge, surface coverage on the structure and surface rheology of different types of non-spherical particles remains to be explored. Measurement of surface pressure-area isotherms and dynamics of compression driven assembly of particles of different shapes at fluid-fluid interfaces is also an area open for exiting developments of which there are limited studies. Another area that appears promising is the use of interfacial route for the synthesis of non-spherical particles. It is worth to exploit the interface mediated particle deformation combined with aqueous/oil phase chemical synthesis or physical adsorption to create modular novel functional particles. If large interface areas such as those in Pickering emulsions are used for synthesis of particles, the yield will be significantly high and


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provides a platform for the investigation of structure, self-assembly and dynamics of patchy and other novel particle systems.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Funding Sources Notes The authors declare no competing financial interest. ACKNOWLEDGMENT M.G.B gratefully acknowledges stimulating research experience and mentoring at IISc (Prof. Govind S Gupta), KU Leuven (Prof. Jan Vermant currently at ETH Zurich), University of Delaware (Prof. Norman J Wagner) and the contribution of many research students over the years. We are thankful for collaborations with several groups conducive for exciting research at IIT Madras. We are also thankful for the financial support from the Department of Science and Technology (SR/S3/CE/047/2011, SB/S3/CE/053/2015), Government of India and IIT Madras. REFERENCES 1. Poon, W., Colloids as Big Atoms. Science 2004, 304, 830-831.


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24. Dupas, J.; Verneuil, E.; Ramaioli, M.; Forny, L.; Talini, L.; Lequeux, F., Dynamic Wetting on a Thin Film of Soluble Polymer: Effects of Nonlinearities in the Sorption Isotherm. Langmuir 2013, 29, 12572-12578. 25. Garbin, V.; Crocker, J. C.; Stebe, K. J., Forced Desorption of Nanoparticles from an Oil– Water Interface. Langmuir 2012, 28, 1663-1667. 26. Brooks, C. F.; Fuller, G. G.; Frank, C. W.; Robertson, C. R., An Interfacial Stress Rheometer to Study Rheological Transitions in Monolayers at the Air− Water Interface. Langmuir 1999, 15, 2450-2459. 27. S.; Franck, A.; Fuller, G. G.; Moldenaers, P.; Vermant, J., A Double Wall-Ring Geometry for Interfacial Shear Rheometry. Rheol. Acta 2010, 49, 131-144. 28. Fuller, G. G.; Vermant, J., Complex Fluid-Fluid Interfaces: Rheology and Structure. Annu. Rev. Chem. Biomol. Eng. 2012, 3, 519-543. 29. Reynaert, S.; Moldenaers, P.; Vermant, J., Interfacial Rheology of Stable and Weakly Aggregated Two-Dimensional Suspensions. Phys. Chem. Chem. Phys. 2007, 9, 6463-6475. 30. Barman, S.; Christopher, G. F., Role of Capillarity and Microstructure on Interfacial Viscoelasticity of Particle Laden Interfaces. J. Rheol. 2016, 60, 35-45. 31. Ramsden, W., Separation of Solids in the Surface-Layers of Solutions and 'Suspensions' (Observations on Surface-Membranes, Bubbles, Emulsions, and Mechanical Coagulation). -Preliminary Account. Proc. Royal Soc. Lond. 1903, 72, 156-164. 32. Pickering, S. U., CXCVI.-Emulsions. J. Chem. Soc. 1907, 91, 2001-2021. 33. Vermant, J.; Cioccolo, G.; Nair, K. G.; Moldenaers, P., Coalescence Suppression in Model Immiscible Polymer Blends by Nano-Sized Colloidal Particles. Rheol. Acta 2004, 43, 529-538. 34. Nagarkar, S. P.; Velankar, S. S., Morphology and Rheology of Ternary Fluid–Fluid–Solid Systems. Soft Matter 2012, 8, 8464-8477. 35. Herzig, E.; White, K.; Schofield, A.; Poon, W.; Clegg, P., Bicontinuous Emulsions Stabilized Solely by Colloidal Particles. Nat. Mater. 2007, 6, 966.


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36. Velev, O.; Furusawa, K.; Nagayama, K., Assembly of Latex Particles by Using Emulsion Droplets as Templates. 1. Microstructured Hollow Spheres. Langmuir 1996, 12, 2374-2384. 37. Dinsmore, A.; Hsu, M. F.; Nikolaides, M.; Marquez, M.; Bausch, A.; Weitz, D., Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles. Science 2002, 298, 1006-1009. 38. Studart, A. R.; Gonzenbach, U. T.; Tervoort, E.; Gauckler, L. J., Processing Routes to Macroporous Ceramics: a Review. J. Am. Ceram. Soc. 2006, 89, 1771-1789. 39. Akartuna, I.; Studart, A. R.; Tervoort, E.; Gauckler, L. J., Macroporous Ceramics from Particle Stabilized Emulsions. Adv. Mater. 2008, 20, 4714-4718. 40. Park, B. J.; Furst, E. M., Fabrication of Unusual Asymmetric Colloids at an Oil− Water Interface. Langmuir 2010, 26, 10406-10410. 41. Sabapathy, M.; Shelke, Y.; Basavaraj, M. G.; Mani, E., Synthesis of Non-Spherical Patchy Particles at Fluid–Fluid Interfaces via Differential Deformation and Their Self-Assembly. Soft Matter 2016, 12, 5950-5958. 42. Zheng, L.; Huang, P.; Zhang, L.; Guo, D.; Yan, Q., Facile Fabrication of Anisotropic Colloidal Particles with Controlled Shapes and Shape Dependence of Their Elastic Properties. Part. Part. Syst. Charact. 2016, 33), 842-850. 43. Binks, B. P., Colloidal Particles at a Range of Fluid–Fluid Interfaces. Langmuir 2017, 33, 6947-6963. 44. Dugyala, V. R.; Anjali, T. G.; Upendar, S.; Mani, E.; Basavaraj, M. G., Nano Ellipsoids at the Fluid–Fluid Interface: Effect of Surface Charge on Adsorption, Buckling and Emulsification. Faraday Discuss. 2016, 186, 419-434. 45. Anjali, T. G.; Basavaraj, M. G., General Destabilization Mechanism of pH-Responsive Pickering Emulsions. Phys. Chem. Chem. Phys. 2017, 19, 30790-30797. 46. Dugyala, V. R.; Basavaraj, M. G., Control Over Coffee-Ring Formation in Evaporating Liquid Drops Containing Ellipsoids. Langmuir 2014, 30, 8680-8686.


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47. Reincke, F.; Kegel, W. K.; Zhang, H.; Nolte, M.; Wang, D.; Vanmaekelbergh, D.; Möhwald, H., Understanding the Self-Assembly of Charged Nanoparticles at the Water/Oil Interface. Phys. Chem. Chem. Phys. 2006, 8, 3828-3835. 48. Wang, H.; Singh, V.; Behrens, S. H., Image Charge Effects on the Formation of Pickering Emulsions. J. Phys. Chem. Lett. 2012, 3, 2986-2990. 49. Kelleher, C. P.; Wang, A.; Guerrero-García, G. I.; Hollingsworth, A. D.; Guerra, R. E.; Krishnatreya, B. J.; Grier, D. G.; Manoharan, V. N.; Chaikin, P. M., Charged Hydrophobic Colloids at an Oil–Aqueous Phase Interface. Phys. Rev. E 2015, 92, 062306. 50. Dugyala, V. R.; Muthukuru, J. S.; Mani, E.; Basavaraj, M. G., Role of Electrostatic Interactions in the Adsorption Kinetics of Nanoparticles at Fluid–Fluid Interfaces. Phys. Chem. Chem. Phys. 2016, 18, 5499-5508. 51. Nallamilli, T.; Binks, B. P.; Mani, E.; Basavaraj, M. G., Stabilization of Pickering Emulsions with Oppositely Charged Latex Particles: Influence of Various Parameters and Particle Arrangement Around Droplets. Langmuir 2015, 31, 11200-11208. 52. Nallamilli, T.; Mani, E.; Basavaraj, M. G., A Model for the Prediction of Droplet Size in Pickering Emulsions Stabilized by Oppositely Charged Particles. Langmuir 2014, 30, 93369345. 53. de Graaf, J.; Dijkstra, M.; van Roij, R., Adsorption Trajectories and Free-Energy Separatrices for Colloidal Particles in Contact with a Liquid-Liquid Interface. J. Chem. Phys. 2010, 132, 164902. 54. Coertjens, S.; De Dier, R.; Moldenaers, P.; Isa, L.; Vermant, J., Adsorption of Ellipsoidal Particles at Liquid–Liquid Interfaces. Langmuir 2017, 33, 2689-2697. 55. Wang, A.; Rogers, W. B.; Manoharan, V. N., Effects of Contact-Line Pinning on the Adsorption of Nonspherical Colloids at Liquid Interfaces. Phys. Rev. Lett. 2017, 119, 108004. 56. Botto, L.; Lewandowski, E. P.; Cavallaro, M.; Stebe, K. J., Capillary Interactions Between Anisotropic Particles. Soft Matter 2012, 8, 9957-9971.


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57. Anjali, T. G.; Basavaraj, M. G., Contact Angle and Detachment Energy of Shape Anisotropic Particles at Fluid-Fluid Interfaces. J. Colloid Interface Sci. 2016, 478, 63-71. 58. Anjali, T. G.; Basavaraj, M. G., Shape-Induced Deformation, Capillary Bridging, and SelfAssembly of Cuboids at the Fluid–Fluid Interface. Langmuir 2017, 33, 791-801. 59. Morris, G.; Neethling, S.; Cilliers, J., An Investigation of the Stable Orientations of Orthorhombic Particles in a Thin Film and Their Effect on its Critical Failure Pressure. J. Colloid Interface Sci. 2011, 361, 370-380. 60. Soligno, G.; Dijkstra, M.; van Roij, R., Self-Assembly of Cubes into 2D Hexagonal and Honeycomb Lattices by Hexapolar Capillary Interactions. Phys. Rev. Lett. 2016, 116, 258001. 61. Shi, W.; Zhang, Z.; Li, S., Quantitative Prediction of Position and Orientation for Platonic Nanoparticles at Liquid/Liquid Interfaces. J. Phys. Chem. Lett. 2018, 9, 373-382. 62. Loudet, J.-C.; Yodh, A. G.; Pouligny, B., Wetting and Contact Lines of Micrometer-Sized Ellipsoids. Phys. Rev. Lett. 2006, 97, 018304. 63. Isa, L.; Lucas, F.; Wepf, R.; Reimhult, E., Measuring Single-Nanoparticle Wetting Properties by Freeze-Fracture Shadow-Casting Cryo-Scanning Electron Microscopy. Nat. Commun. 2011, 2, 438. 64. Isa, L.; Samudrala, N.; Dufresne, E. R., Adsorption of Sub-Micron Amphiphilic Dumbbells to Fluid Interfaces. Langmuir 2014, 30, 5057-5063. 65. Coertjens, S.; Moldenaers, P.; Vermant, J.; Isa, L., Contact Angles of Microellipsoids at Fluid Interfaces. Langmuir 2014, 30, 4289-4300. 66. Paunov, V. N., Novel Method for Determining the Three-Phase Contact Angle of Colloid Particles Adsorbed at Air−Water and Oil−Water Interfaces. Langmuir 2003, 19, 7970-7976. 67. Sharp, E. L.; Al-Shehri, H.; Horozov, T. S.; Stoyanov, S. D.; Paunov, V. N., Adsorption of Shape-Anisotropic and Porous Particles at the Air-Water and the Decane-Water Interface Studied by the Gel Trapping Technique. RSC Adv. 2014, 4, 2177-2185.


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68. Binks, B. P.; Isa, L.; Tyowua, A. T., Direct Measurement of Contact Angles of Silica Particles in Relation to Double Inversion of Pickering Emulsions. Langmuir 2013, 29, 49234927. 69. Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N., Fabrication of “Hairy” Colloidosomes with Shells of Polymeric Microrods. J. Am. Chem. Soc. 2004, 126, 80928093. 70. Madivala, B.; Fransaer, J.; Vermant, J., Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces. Langmuir 2009, 25, 2718-2728. 71. Morgan, A. R.; Ballard, N.; Rochford, L. A.; Nurumbetov, G.; Skelhon, T. S.; Bon, S. A. F., Understanding the Multiple Orientations of Isolated Superellipsoidal Hematite Particles at the Oil-Water Interface. Soft Matter 2013, 9, 487-491. 72. Lewandowski, E. P.; Cavallaro Jr, M.; Botto, L.; Bernate, J. C.; Garbin, V.; Stebe, K. J., Orientation and Self-Assembly of Cylindrical Particles by Anisotropic Capillary Interactions. Langmuir 2010, 26, 15142-15154. 73. Arnaudov, L. N.; Cayre, O. J.; Cohen Stuart, M. A.; Stoyanov, S. D.; Paunov, V. N., Measuring the Three-Phase Contact Angle of Nanoparticles at Fluid Interfaces. Phys. Chem. Chem. Phys. 2010, 12, 328-331. 74. Madivala, B.; Vandebril, S.; Fransaer, J.; Vermant, J., Exploiting Particle Shape in Solid Stabilized Emulsions. Soft Matter 2009, 5, 1717-1727. 75. de Folter, J. W.; Hutter, E. M.; Castillo, S. I.; Klop, K. E.; Philipse, A. P.; Kegel, W. K., Particle Shape Anisotropy in Pickering Emulsions: Cubes and Peanuts. Langmuir 2013, 30, 955964. 76. Danov, K. D.; Kralchevsky, P. A., Capillary Forces Between Particles at a Liquid Interface: General Theoretical Approach and Interactions Between Capillary Multipoles. Adv. Colloid Interface Sci. 2010, 154, 91-103.


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77. Stamou, D.; Duschl, C.; Johannsmann, D., Long-Range Attraction Between Colloidal Spheres at the Air-Water Interface: The Consequence of an Irregular Meniscus. Phys. Rev. E 2000, 62, 5263. 78. Rezvantalab, H.; Shojaei-Zadeh, S., Role of Geometry and Amphiphilicity on CapillaryInduced Interactions Between Anisotropic Janus Particles. Langmuir 2013, 29, 14962-14970. 79. Wolfe, D. B.; Snead, A.; Mao, C.; Bowden, N. B.; Whitesides, G. M., Mesoscale SelfAssembly: Capillary Interactions When Positive and Negative Menisci Have Similar Amplitudes. Langmuir 2003, 19, 2206-2214. 80. Loudet, J.; Pouligny, B., How Do Mosquito Eggs Self-Assemble on the Water Surface? Eur. Phys. J. E 2011, 34, 1-17. 81. Loudet, J.-C.; Pouligny, B., Self-Assembled Capillary Arrows. Europhys. Lett. 2009, 85, 28003. 82. Botto, L.; Yao, L.; Leheny, R.; Stebe, K., Capillary Bond Between Rod-Like Particles and the Micromechanics of Particle-Laden Interfaces. Soft Matter 2012, 8, 4971-4979. 83. Dasgupta, S.; Katava, M.; Faraj, M.; Auth, T.; Gompper, G., Capillary Assembly of Microscale Ellipsoidal, Cuboidal, and Spherical Particles at Interfaces. Langmuir 2014, 30, 11873-11882. 84. Lewandowski, E.; Bernate, J.; Searson, P.; Stebe, K., Rotation and Alignment of Anisotropic Particles on Nonplanar Interfaces. Langmuir 2008, 24, 9302-9307. 85. Cavallaro, M.; Botto, L.; Lewandowski, E. P.; Wang, M.; Stebe, K. J., Curvature-Driven Capillary Migration and Assembly of Rod-Like Particles. Proc. Natl. Acad. Sci. USA. 2011, 108, 20923-20928. 86. Poty, M.; Lumay, G.; Vandewalle, N., Customizing Mesoscale Self-Assembly with ThreeDimensional Printing. New J. Phys. 2014, 16, 023013. 87. Ferrar, J. A.; Bedi, D. S.; Zhou, S.; Zhu, P.; Mao, X.; Solomon, M. J., Capillary-Driven Binding of Thin Triangular Prisms at Fluid Interfaces. Soft Matter 2018, 14, 3902-3918.


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88. Bae, J.; Bende, N. P.; Evans, A. A.; Na, J.-H.; Santangelo, C. D.; Hayward, R. C., Programmable and Reversible Assembly of Soft Capillary Multipoles. Mater. Horizons 2017, 4, 228-235. 89. Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G., Capillary Interactions Between Anisotropic Colloidal Particles. Phys. Rev. Lett. 2005, 94, 018301. 90. Soligno, G.; Dijkstra, M.; van Roij, R., Self-Assembly of Cubic Colloidal Particles at Fluid–Fluid Interfaces by Hexapolar Capillary Interactions. Soft Matter 2018, 14, 42-60. 91. Brakke, K. A., The Surface Evolver. Exp. Math. 1992, 1, 141-165. 92. Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzán, L. M., Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591-3605. 93. von Freymann, G.; Kitaev, V.; Lotsch, B. V.; Ozin, G. A., Bottom-up Assembly of Photonic Crystals. Chem. Soc. Rev. 2013, 42, 2528-2554. 94. Whitesides, G. M.; Grzybowski, B., Self-Assembly at all Scales. Science 2002, 295, 24182421. 95. Vogel, N.; Retsch, M.; Fustin, C.-A.; del Campo, A.; Jonas, U., Advances in Colloidal Assembly: The Design of Structure and Hierarchy in Two and Three Dimensions. Chem. Rev. 2015, 115, 6265-6311. 96. Furst, E. M., Directed Self-Assembly. Soft Matter 2013, 9, 9039-9045. 97. Law, A. D.; Buzza, D. M. A.; Horozov, T. S., Two-Dimensional Colloidal Alloys. Phys. Rev. Lett. 2011, 106, 128302. 98. Fournier, J. B.; Galatola, P., Anisotropic Capillary Interactions and Jamming of Colloidal Particles Trapped at a Liquid-Fluid Interface. Phys. Rev. E 2002, 65, 031601. 99. Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M., Self-Assembly of Mesoscale Objects into Ordered Two-Dimensional Arrays. Science 1997, 276, 233-235.


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100. Tao, A. R.; Huang, J.; Yang, P., Langmuir− Blodgettry of Nanocrystals and Nanowires. Accounts of Chemical Research 2008, 41, 1662-1673. 101. Sacanna, S.; Rossi, L.; Wouterse, A.; Philipse, A., Observation of a Shape-Dependent Density Maximum in Random Packings and Glasses of Colloidal Silica Ellipsoids. J. Phys. Condens. Matter. 2007, 19, 376108. 102. Li, T.; Brandani, G.; Marenduzzo, D.; Clegg, P., Colloidal Spherocylinders at an Interface: Flipper Dynamics and Bilayer Formation. Phys. Rev. Lett. 2017, 119, 018001. 103. Imperiali, L.; Liao, K.-H.; Clasen, C.; Fransaer, J.; Macosko, C. W.; Vermant, J., Interfacial Rheology and Structure of Tiled Graphene Oxide Sheets. Langmuir 2012, 28, 79908000. 104. Vora, S. R.; Bognet, B.; Patanwala, H. S.; Chinesta, F.; Ma, A. W., Surface Pressure and Microstructure of Carbon Nanotubes at an Air–Water Interface. Langmuir 2015, 31, 4663-4672. 105. Botcha, V. D.; Narayanam, P. K.; Singh, G.; Talwar, S.; Srinivasa, R.; Major, S., Effect of Substrate and Subphase Conditions on the Surface Morphology of Graphene Oxide Sheets Prepared by Langmuir–Blodgett Technique. Colloids Surf. A: Physicochem. Eng. Asp. 2014, 452, 65-72. 106. Sun, Z.; Feng, T.; Russell, T. P., Assembly of Graphene Oxide at Water/Oil Interfaces: Tessellated Nanotiles. Langmuir 2013, 29, 13407-13413. 107. Thijssen, J.; Vermant, J., Interfacial Rheology of Model Particles at Liquid Interfaces and Its Relation to (Bicontinuous) Pickering Emulsions. J. Phys. Condens. Matter 2017, 30, 023002. 108. Lam, S.; Velikov, K. P.; Velev, O. D., Pickering Stabilization of Foams and Emulsions with Particles of Biological Origin. Curr. Opin. Colloid Interface Sci. 2014, 19, 490-500. 109. Yang, Y.; Fang, Z.; Chen, X.; Zhang, W.; Xie, Y.; Chen, Y.; Liu, Z.; Yuan, W., An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications. Front. Pharmacol. 2017, 8, 287.


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110. Tang, J.; Quinlan, P. J.; Tam, K. C., Stimuli-Responsive Pickering emulsions: Recent Advances and Potential Applications. Soft Matter 2015, 11, 3512-3529. 111. Bollhorst, T.; Rezwan, K.; Maas, M., Colloidal Capsules: Nano-and Microcapsules with Colloidal Particle Shells. Chem. Soc. Rev. 2017, 46, 2091-2126. 112. Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D., Foam Superstabilization by Polymer Microrods. Langmuir 2004, 20, 10371-10374. 113. Liu, H.; Wang, C.; Zou, S.; Wei, Z.; Tong, Z., Simple, Rreversible Emulsion System Switched by pH on the Basis of Chitosan without any Hydrophobic Modification. Langmuir 2012, 28, 11017-11024. 114. Zhang, Y.; Chen, Z.; Bian, W.; Feng, L.; Wu, Z.; Wang, P.; Zeng, X.; Wu, T., Stabilizing Oil-in-Water Emulsions with Regenerated Chitin Nanofibers. Food Chem. 2015, 183, 115-121. 115. Wongkongkatep, P.; Manopwisedjaroen, K.; Tiposoth, P.; Archakunakorn, S.; Pongtharangkul, T.; Suphantharika, M.; Honda, K.; Hamachi, I.; Wongkongkatep, J., Bacteria Interface Pickering Emulsions Stabilized by Self-Assembled Bacteria–Chitosan Network. Langmuir 2012, 28, 5729-5736. 116. Kim, J.; Cote, L. J.; Kim, F.; Yuan, W.; Shull, K. R.; Huang, J., Graphene Oxide Sheets at Interfaces. J. Am. Chem. Soc. 2010, 132, 8180-8186. 117. Kalashnikova, I.; Bizot, H.; Bertoncini, P.; Cathala, B.; Capron, I., Cellulosic Nanorods of Various Aspect Ratios for Oil in Water Pickering Emulsions. Soft Matter 2013, 9, 952-959. 118. Daware, S. V.; Basavaraj, M. G., Emulsions Stabilized by Silica Rods via Arrested Demixing. Langmuir 2015, 31, 6649-6654. 119. Datskos, P.; Polizos, G.; Bhandari, M.; Cullen, D.; Sharma, J., Colloidosome Like Structures: Self-assembly of Silica Microrods. RSC Adv. 2016, 6, 26734-26737. 120. Lou, F.; Ye, L.; Kong, M.; Yang, Q.; Li, G.; Huang, Y., Pickering Emulsions Stabilized by Shape-Controlled Silica Microrods. RSC Adv. 2016, 6, 24195-24202.


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121. Pang, M.; Cairns, A. J.; Liu, Y.; Belmabkhout, Y.; Zeng, H. C.; Eddaoudi, M., Synthesis and Integration of Fe-soc-MOF Cubes into Colloidosomes via a Single-Step Emulsion-Based Approach. J. Am. Chem. Soc. 2013, 135, 10234-10237. 122. Cai, X.; Deng, X.; Xie, Z.; Bao, S.; Shi, Y.; Lin, J.; Pang, M.; Eddaoudi, M., Synthesis of Highly Monodispersed Ga-soc-MOF Hollow Cubes, Colloidosomes and Nanocomposites. Chem. Comm. 2016, 52, 9901-9904. 123. Nicolai, T.; Murray, B., Particle Stabilized Water in Water Emulsions. Food Hydrocoll. 2017, 68, 157-163. 124. Hijnen, N.; Cai, D.; Clegg, P. S., Bijels Stabilized Using Rod-Like Particles. Soft Matter 2015, 11, 4351-4355. 125. Binks, B. P.; Clint, J.; Mackenzie, G.; Simcock, C.; Whitby, C. P., Naturally Occurring Spore Particles at Planar Fluid Interfaces and in Emulsions. Langmuir 2005, 21, 8161-8167. 126. Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I., New Pickering Emulsions Stabilized by Bacterial Cellulose Nanocrystals. Langmuir 2011, 27, 7471-7479. 127. Zhang, L.; Li, Z.; Wang, L.; Sun, D., High Temperature Stable W/O Emulsions Prepared with In-Situ Hydrophobically Modified Rodlike Sepiolite. J. Colloid Interface Sci. 2017, 493, 378-384. 128. Briggs, N.; Raman, A. K. Y.; Barrett, L.; Brown, C.; Li, B.; Leavitt, D.; Aichele, C. P.; Crossley, S., Stable Pickering Emulsions Using Multi-Walled Carbon Nanotubes of Varying Wettability. Colloids Surf. A 2018, 537, 227-235. 129. Tsuji, S.; Kawaguchi, H., Thermosensitive Pickering Emulsion Stabilized by Poly (NIsopropylacrylamide)-Carrying Particles. Langmuir 2008, 24, 3300-3305. 130. Peng, J.; Liu, Q.; Xu, Z.; Masliyah, J., Synthesis of Interfacially Active and Magnetically Responsive Nanoparticles for Multiphase Separation Applications. Adv. Funct. Mater. 2012, 22, 1732-1740.


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131. Chen, Z.; Zhou, L.; Bing, W.; Zhang, Z.; Li, Z.; Ren, J.; Qu, X., Light Controlled Reversible

Inversion

of

Nanophosphor-Stabilized

Pickering

Emulsions

for

Biphasic

Enantioselective Biocatalysis. J. Am. Chem. Soc. 2014, 136, 7498-7504. 132. Jiang, J.; Ma, Y.; Cui, Z.; Binks, B. P., Pickering Emulsions Responsive to CO2/N2 and Light Dual Stimuli at Ambient Temperature. Langmuir 2016, 32, 8668-8675. 133. Sabapathy, M.; Kollabattula, V.; Basavaraj, M. G.; Mani, E., Visualization of the Equilibrium Position of Colloidal Particles at Fluid–Water Interfaces by Deposition of Nanoparticles. Nanoscale 2015, 7, 13868-13876. 134. Paunov, V. N.; Cayre, O. J., Supraparticles and “Janus” Particles Fabricated by Replication of Particle Monolayers at Liquid Surfaces Using a Gel Trapping Technique. Adv. Mat. 2004, 16, 788-791. 135. Jiang, S.; Granick, S., Controlling the Geometry (Janus Balance) of Amphiphilic Colloidal Particles. Langmuir 2008, 24, 2438-2445. 136. Ge, L.; Friberg, S. E.; Guo, R., Recent Studies of Janus Emulsions Prepared by One-Step Vibrational Mixing. Curr. Opin. Colloid Interface Sci. 2016, 25, 58-66.


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Figure 1. Illustrations show various aspects concerning colloidal particles at interfaces typically investigated: (A) An example of a pendant drop of aqueous colloidal dispersion in contact with a nonpolar oil medium, one of the many methods used to study adsorption of particles to interfaces (B) Equilibrium positions occupied by particle of different wettability characterized by three-phase contact angle. (C) Colloidal interactions between particles adsorbed at interface: (1) Dipolar repulsion (2) Image charge repulsion (3) Capillary interactions. (D) Self-assembly of particles at interfaces - charged


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polystyrene (PS) particles assemble into hexagonal crystal lattices at decane-water interface, reproduced with permission from ref (70). Copyright (2009) American Chemical Society. (E) Behaviour of particle monolayers under compression and shear studied using (1) Langmuir Blodgett trough (2) Pendant drop compression (3) Interfacial rheology


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Figure 1. Adsorption of non-spherical particles to interfaces: (A) Pendant drop tensiometry of drops containing aqueous hematite ellipsoids at pH 2, 4 and 6.5 in contact with decane. Reproduced with permission from ref (44). Copyright (2016) Royal Society of Chemistry. (B) The image of the vials after emulsification of aqueous dispersions of hematite peanuts and decane of 2:1 volume ratio. The pH of the dispersions is varied, other parameters being the same, particle stabilized emulsions are formed only at pH 6.5.45 Reproduced with permission from ref (45). Copyright (2017) Royal Society of Chemistry.


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Figure 3. Equilibrium position and orientation of shape anisotropic hematite particles at fluidfluid interfaces:57 The scanning electron microscopy images (A-E) show the part of the particle surface originally immersed in the aqueous sub-phase obtained following gel trapping procedure for particles deposited at decane-water Interface. (D1-D3) and (E1-E3) respectively are the top and side view of cuboids at decane-water interfaces in three different orientations. (F) Frequency of occurrence of different particle orientations. Reproduced with permission from ref (57). Copyright (2016) Elsevier.


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Figure 4. Experimental visualization of interface deformation around interfacially trapped cuboidal particles in face-up, tilted edge-up and vertex-up orientations:58 (A1, B1, and C1) are the gel trapping visualization of particles at interfaces. (A2, B2, and C2) are the 3D optical profiler images of interface deformations around the particles in the face-up, tilted edge-up and vertex-up orientations respectively. (A3, B3, and C3) are the height profiles of the interface deformations around cuboidal particles along the lines connecting the capillary poles in the respective orientations. Particles in the face-up orientation deform the interface in a hexapolar manner, those in the tilted edge-up orientation induce quadrupolar deformation and the deformation is of monopolar type in the vertex-up orientation. Reproduced with permission from ref (58). Copyright (2017) American Chemical Society.


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Figure 5. Directed 2D self-assembly of particles via shape-induced interface deformations: (A) and (B) are the optical microscopy images of monolayers of charged spheres and charged ellipsoids at decane-water interface that demonstrate the effect of particle shape.70

At

sufficiently large surface coverage, polystyrene spheres form ordered crystalline structure (A) and polystyrene ellipsoids assemble to form percolating network with predominantly tip-to-tip connections (B).70 Self-assembly of polystyrene particles of low surface charge density at decane-water interface (C).70 Reproduced with permission from ref (70). Copyright (2009) Royal Society of Chemistry. Optical microscopy image (D1) and SEM image (D2) of the assembly of hematite cuboids at air-water interface.58 Cuboids are observed to form locally ordered square and hexagonal lattices. Reproduced with permission from ref (58). Copyright (2017) American Chemical Society.


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Figure 6. Compression isotherms and associated microstructural changes in monolayer of spheres and ellipsoids at air-water interface:21 (A) Surface pressure plotted against through area (rescaled) for monolayers of spherical particles and different aspect ratio (rp) ellipsoids. (B-D) Bright field optical microscopy images showing the structural changes in compressed monolayer of ellipsoids of 5.5 aspect ratio: (B) The microstructure of the monolayer soon after deposition of ellipsoids (C) The microstructure at the inflection point IP2 (when AT = 100 cm2) – the ellipsoidal particles that appear as small circles in top view are “flippers” and are oriented with their long axis perpendicular to the interface. (D) Typical microstructure of the monolayer compressed beyond the inflection point IP3 (at AT = 15 cm2) shows buckling and the monolayer does not remain planar any more. Reproduced with permission from ref (21). Copyright (2009) Royal Society of Chemistry.


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Figure 7. Pendant drop compression – drop shape changes and the microstructure of the wrinkled particle monolayer.44 (A) Compression of pendant drop of aqueous dispersion of ellipsoids at pH 2 does not show buckling even at high compression ratio. (B) Compression of pendant drop of aqueous dispersion of ellipsoids at pH 6.5 shows reversible buckling and wrinkling transitions. (C) Low magnification SEM image show the wrinkles in the particle laden film floating at water surface soon after pendant oil drop in compressed state is detached from the syringe tip (D) High magnification SEM image confirms that the particle laden film is single particle thick and the local nematic ordering. Reproduced with permission from ref (44). Copyright (2016) Royal Society of Chemistry.


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Figure 8. Interfacial rheology of monolayer of ellipsoids:70 (A) Surface storage modulus, G′S and surface loss modulus, G′′S represented respectively by filled and open symbols measured for monolayers of charged ellipsoids at higher surface coverage (B) Comparison of surface storage modulus of monolayer of ellipsoids and spheres spread at air-water interface in the linear viscoelastic regime. Reproduced with permission from ref (70). Copyright (2009) Royal Society of Chemistry.


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Figure 9. Pickering emulsions stabilized by shape anisotropic particles: (A) The digital image demonstrates the effect of hematite particle aspect ratio (AR) on the formation of emulsions.74 (B) Cryo-SEM images of the water droplet covered with polystyrene ellipsoids74 and (C) SEM images of hollow shells prepared by drying silica rod stabilized lutidine droplets.118 (D) W/O emulsion stabilized by polystyrene ellipsoids,74 (E) O/W emulsion stabilized by peanut shaped hematite particles,45 (F) O/W emulsion stabilized by hematite ellipsoids,45 (G) O/W emulsion droplets stabilized silica rods.45 74 The optical microscopy images of the arrangement of particles on emulsion droplets reveal the formation of close packed dense particle layer of (H) polystyrene ellipsoids74 (I) peanut shaped particles45 (J) spherocylderical particles45 and (K) cuboidal hematite particles45 on the drops surface. Reproduced with permission from ref (45), Copyright (2017), ref 74, Copyright (2009) and ref (118), Copyright (2015), American Chemical Society.


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Figure 10. pH responsive Pickering emulsions:

45

(A) O/W emulsions stabilized by peanut-

shaped hematite particles. The emulsions formed at pH - 6.5 demulsify when the pH of the continuous phase is adjusted to either 2 or 12. Upon complete destabilization, the stable emulsions can again be formed after adjusting the pH of the continuous phase to 6.5. (B) O/W emulsions stabilized by silica rods: The emulsions form at pH - 2 and destabilize when the pH is adjusted to 12. Stable emulsions are formed gain after adjusting the pH back to 2. Reproduced with permission from ref (45). Copyright (2017) Royal Society of Chemistry.


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GRAPHICAL ABSTRACT (TABLE OF CONTENTS)


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Madivala G Basavaraj is an Associate Professor in the Department of Chemical Engineering at the Indian Institute of Technology Madras. He obtained his Ph.D. from the KU Leuven, Belgium and carried out post doctoral research at the University of Delaware, USA. His research interests are in particles at interfaces, self-assembly of colloids, desiccation cracks, colloidal gels, emulsion fuels, Pickering emulsions and cellulose-ionic liquid solutions.

Thriveni G. Anjali is a PhD candidate in the Department of Chemical Engineering at the Indian Institute of Technology Madras. Her research interests include synthesis of non-spherical particles, self-assembly of particles at interfaces and Pickering emulsions.


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Shape anisotropic colloids at interfaces

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