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Shape induced deformation, capillary bridging and self-assembly of cuboids at fluid-fluid interface Thriveni G Anjali, and Madivala G Basavaraj Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03866 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 1, 2017
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Shape induced deformation, capillary bridging and self-assembly of cuboids at fluid-fluid interface 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 Keywords: cuboidal particles, gel trapping technique, fluid-fluid interfaces, interface deformation, self-assembly, capillary multipoles ABATRACT The controlled assembly of anisotropic particles through shape induced interface deformations is shown to be a potential route for the fabrication of novel functional materials. In this article, the shape induced interface deformation, capillary bridging and directed selfassembly of cuboidal shaped hematite particles at fluid-fluid interfaces are reported. The multipolar nature of the interface distortions is directly visualized using high resolution scanning electron microscopy (HR-SEM) and 3D optical surface profiler. The nature of the interface deformations around cuboidal particles vary from monopolar to octupolar types depending on their orientation and position with respect to the interface. The deformations are of either hexapolar or octupolar type in the face-up, quadrupolar or monopolar nature in the edge-up and monopolar type in the vertex-up orientations. The particles adsorbed at the interface interact through the interface deformations forming capillary bridges that lead to isolated assemblies of two or more particles. The arrangement of particles in any assembly is such that the condition for capillary attraction is satisfied i.e., in accordance with predictions based on the nature of interface deformations. At sufficient particle concentrations, these 1 ACS Paragon Plus Environment
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isolated structures interact to form a percolating network of cuboids. Furthermore, the difference in the nature of the assembly structures formed at the air-water interface and in the bulk water phase indicates that the interfacial assembly of these particles is controlled by the capillary interactions.
INTRODUCTION Colloidal self-assembly refers to the process of self-organization of particles in 1 nm to 1 μm size range into structures with desired particle arrangements achieved through direct specific interactions or by the application of an external field.1,
2
The particle assembly is
dictated by the balance between different types of inter particle interactions. Among different directing fields, interface deformation mediated forces serve as an ideal tool to control the self assembly of particles at planar interfaces. 1, 3, 4 The driving force for the particle assembly at the interface is the reduction in the interfacial energy and the particle assemblies are controlled by the colloidal interactions at the interface.5, 6 The structure and the properties of the particle monolayers that are formed at the interface have potential applications in the fields of foam/emulsion stabilization, drug delivery, food technology, energy conversion and for the synthesis of functional materials with specific optical, electrical, magnetic and mechanical properties.2, 7, 8, 9, 10 The first reports on the adsorption of particles at the interface came more than a century ago11,
12
and for the past few decades it has become a topic of
immense research because of both scientific and technological importance. When positioned at the interface, particles minimize the interfacial energy of the system by reducing the unfavorable contact between the two fluid phases.13 Unlike surfactant molecules, magnitude of the attachment energy of the micron sized colloidal particles is higher than the thermal energy (kBT) by several orders, which makes their adsorption at the interface irreversible.13, 14, 15
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Recently anisotropic particle laden interfaces have attracted interest owing to their promising applications.16,
17, 18, 19, 20
Particle anisotropy either in terms of shape or surface
chemistry makes them better candidate for Pickering emulsion stabilization, fabrication of photonic crystals and in pharmaceutical applications.14, 21, 22, 23, 24 Unlike isotropic particles, anisotropic particles when trapped at the interface distort the interface in order to satisfy the Young’s equation at every point along the three phase contact line.25,
26, 27
The nature of
interface deformation is a function of the wettability, shape, size and aspect ratio of the particles.14,
28
Among different types of anisotropic particles, there is emerging research
interests in the investigation of particles with asymmetric shape owing to the unique shape induced interface deformations and the resulting particle structures that are formed at the interface.14,
21, 24, 25, 29, 30, 31, 32, 33
Upon adsorption, smooth spherical particles which are
partially wetted by both the fluids adopt an equilibrium position at the interface such that the contact line where the three interfaces meet is a circle.4 However, in the case of shape anisotropic particles, the three phase contact line is non-planar.26 This is due to the fact that the Young’s equation has to be satisfied at every point along the three phase contact line of varying curvature. The non-planar contact line is a result of the shape induced interface deformation and thus, the nature of capillary force-distance curves depends on particle shape.34
While a large number of published literature on particle shape induced interface deformations are mainly through numerical simulations,28,
29, 30, 33, 35, 36, 37, 38, 39
been very few experimental observations in recent years.21,
25, 35, 38, 40, 41
there have
The interface
deformation around ellipsoidal and cylindrical particles has been investigated both experimentally and theoretically, and is found to be of quadrupolar type.14, 21, 25, 26, 35, 36, 37, 38, 40
The contact angle and aspect ratio of orthorhombic particles have known to influence their
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stable orientation at the interface. The adsorption, orientation and evolution of the interface upon the attachment of orthorhombic particles as a function of their contact angle and the aspect ratio have been investigated numerically in the context on the stability of thin films.39, 42, 43
When adsorbed at the interface, the cuboidal particles are reported to exhibit three stable
orientations - face-up, edge-up and vertex-up.42,
44
Recently, the interface deformations
around particles of different shapes and aspect ratios have been predicted and the results are correlated to the capillary bond energies and the particle assemblies at the interface, using simulation studies.30 Numerical simulation studies have predicted hexapolar33 or octupolar30 nature of the shape induced interface deformations of micron sized cuboidal particles with uniform surface properties for different contact angle values. The particles exhibiting a hexapolar type of interface deformations self-assemble through capillary interactions to form either hexagonal or honeycomb lattices.33 There is no report on the experimental evidence of the multipolar nature of the interface deformation around cuboidal particles till date; therefore, we investigate shape induced deformations and elucidate the microstructure of such particles at the interfaces.
In the present work, the interface deformation and directed self-assembly of cuboidal shaped hematite particles at the air-water and decane-water interfaces are studied using the gel trapping technique (GTT).45 Micron sized hematite particles are known to possess a magnetic moment and in the dispersion, they aggregate into ordered structures under the influence of magnetic fields.46, 47 But, when adsorbed at the fluid-fluid interface, as we will show, their assembly is controlled by the capillary interactions originating from the shape induced interface deformations. The near field interface deformations around isolated particles in three different orientations, the capillary bridge formation between two interacting particles and the particle assemblies formed by interface mediated capillary
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interactions are investigated. Since the cuboidal particles get adsorbed at different orientations44, 48, 49, their shape induced interface deformations vary as well. The nature of the interface deformation around cuboidal particle is observed to depend on their orientation as well as position with respect to the interface.
We have directly visualized interface
deformations around individual particles using high resolution scanning electron microscopy and a three dimensional (3D) optical surface profiler and show that the deformation profile varies from monopolar to octupolar. In the face-up orientation, depending on the position of the particle with respect to the interface, the deformations are either hexa or octupolar type. Cuboids in the edge-up orientation induce either a quadrupolar or monopolar type deformation depending on the interfacial position whereas in the vertex-up orientation the interface deformation is monopolar in nature. All these types of interface deformations are supported by the typical particle assembly structures formed at the interface. The interface deformations around the particles are directly visualized. When the interface deformations of neighboring particles overlap, they interact to minimize the deformations and a capillary bridge, which holds the particles together, is formed and the possible particle clusters formed by the collective assemblies of many particles in different orientations are reported. All the particle assemblies originate from the shape induced capillary attractions and are according to the notion of capillary multipoles. The cuboidal particles in the face-up orientation assemble in vertex-to-vertex (V-V), side-by-side (S-S) and side-to-vertex (S-V) modes and the assemblies of particles involving the other two orientations are also in line with the sign conventions of capillary interactions. The collective assemblies of many particles forming different configurations are discussed and are found to be in accordance with the predicted energy landscapes.50 The assembly of the particles at the fluid-fluid interface through the shape induced capillary interactions is confirmed from the distinct structures formed at the air-water interface and in the bulk water phase.
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EXPERIMENTAL Hematite particle synthesis: Cuboidal shaped hematite particles in the micron size range are synthesized from condensed ferric hydroxide gel following Sugimoto’s gel-sol procedure.49, 51 In a standard synthesis, 100 ml of 6M NaOH solution was added in 5 minutes to 100 ml of 2M FeCl3.6H2O solution under vigorous stirring. The dark brown mixture formed was then stirred for another 5 minutes. The gel-sol thus formed was aged for 8 days at 100°C. After this period, the particles were recovered by multiple centrifugation and redispersion using Milli-Q water. The sizes of the cuboidal particles were varied by changing the concentration of the NaOH solution. The purity of the chemicals, the stirring speed, the concentration and speed of addition of NaOH control the size and the monodispersity of the particles. The synthesized particles were characterized using a high resolution scanning electron microscopy (HR-SEM, Hitachi-S 4800, Japan) and a transmission electron microscopy (Tecnai-12). Deionised water (resistivity of 18.2MΩ cm) from a Millipore MilliQ plus water purification system was used in all the experiments. Particle characterization for shape induced interface deformation studies: Figure 1 shows the representative SEM and TEM images of the synthesized particles, the size analysis of which are done with ImageJ.55 The physical dimensions of the all particles used in the present work are listed in the supplementary information (Table S1) along with the SEM and TEM images (Figure S1) of the particles that are not shown in Figure 1. The uniform surface properties, partial wettability49, monodispersity and the size range makes them the right system to study the effect of particle shape on the interfacial behavior of particles. The suitability of the synthesized hematite particles for the investigation of the shape induced interface deformations is verified from the Bond number52 (Bo), Capillary number30 (Ca) and the capillary length
38
( λ c ) calculations. The effect of gravitational force on the
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equilibrium position of the particles at the interface can be predicted from the values of the Bo, while the effect of gravitational and hydrodynamic forces on the interface shape can be inferred from the magnitudes of λ c and the Ca calculations respectively. For the system of micron sized hematite particles adsorbed at air-water or decane-water interface, the calculated value of the Bo is 10-7
