Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces

Feb 3, 2009 - Department of Chemical Engineering. , ‡. Department of Metallurgy and Materials Engineering. Cite this:Langmuir 25, 5, 2718-2728 ...
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Langmuir 2009, 25, 2718-2728

Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces Basavaraj Madivala,† Jan Fransaer,‡ and Jan Vermant*,† Department of Chemical Engineering, K. U. LeuVen, W. de Croylaan 46, B-3001, LeuVen, Belgium, and Department of Metallurgy and Materials Engineering, K. U. LeuVen, Kasteelpark Arenbergpark 44, B-3001, LeuVen, Belgium ReceiVed October 25, 2008. ReVised Manuscript ReceiVed December 16, 2008 Colloidal particles confined at liquid interfaces have important applications, for example in the stabilization of emulsions and foams. Also the self-assembly of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. In the present work the particle shape, surface charge, and wetting properties are varied, and the resulting self-assembly of particles at a fluid interface is studied. Model monodisperse micrometer-sized ellipsoidal particles were prepared by a mechanical stretching method. These particles were chosen to be well-suited for investigation by optical microscopy. When deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changing the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembly process. The initial microstructure of charged ellipsoids at a decane-water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. The aggregation behavior in these monolayers was investigated by optical microscopy combined with quantitative image analysis and a dominant tip-tip aggregation was observed. Microstructural information was quantified by calculating the pair-distribution and orientation-distribution functions, as a function of time. Compared to particles at an oil-water interface, particles of the same surface chemistry and charge at an air-water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The latter leads to differences in the capillary forces. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks having as a typical building block ellipsoids connected at their tips in triangular or flower-like configuration. These networks were very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the mechanical properties of these monolayers, as measured by surface shear rheology, showed that the monolayer of ellipsoids exhibit a substantial surface modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties.

Introduction The possibility of tailoring the interaction forces between colloidal particles at fluid-fluid interfaces, by choosing an appropriate combination of fluids, suitable additives, and other formulation parameters, has been exploited to study a range of physical phenomena, such as dislocation dynamics in crystals,1 crystallization,2 and aggregation3-5 in two-dimensional (2D) model systems. The major advantage of using a 2D suspension is that all the microstructural information is contained in one plane, and high resolution, time-resolved studies are possible using simple bright field microscopy over a wide range of surface coverages. When micrometer-sized particles are pinned at an interface between air-water or oil-water, they stick to the interface irreversibly because of capillary energy on the order of 106 times the thermal energy.6 Charged spherical particles at a fluid-liquid interface have been known to form ordered hexagonal crystals that are stable over several days, because of enhancement of the electrostatic interactions.6 Aggregation can be induced by tuning the interparticle interaction by the addition * Corresponding author: [email protected]. † Department of Chemical Engineering. ‡ Department of Metallurgy and Materials Engineering. (1) Lipowsky, P.; Bowick, M. J.; Meinke, J. H.; Nelson, D. R.; Bausch, A. R. Nat. Mater. 2005, 4, 407. (2) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 26, 361. (3) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 46, 2045. (4) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 46, 2055. (5) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 46, 2065. (6) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569.

of salt and/or surfactant to the subphase.7 For spherical particles, the screening of the surface charges in the aqueous phase combined with the changes in wetting properties that control the amount of surface area exposed to the water phase control the electrostatic interactions, as has been demonstrated by direct measurements using optical tweezers.8 In the present work, we will focus on the use of nonspherical particles. The use of shape as a parameter in colloidal systems leads to a number of effects that can be exploited, in 3D as well as in 2D systems. First, one can lower the percolation threshold by using anisotropic particles, which is a critical issue for several applications. Second, the maximum random jammed packing (MRJ) density of ellipsoids varies in a nonmonotonic way as a function of aspect ratio, reaching a maximum of 0.78 for an aspect ratio of 1.5, as shown by recent simulations and experiments.9 A similar nonmonotonic evolution of maximum packing fraction as a function of aspect ratio was also observed experimentally in 2D.10 Also, as many as 10 particle-particle contacts are required to obtain a stable ellipsoidal packing compared to just 6 for spheres; therefore, ellipsoids could be building blocks for stronger materials.9 Also, anisotropic particles can play a very important role in controlling rheology and structure.11-13 (7) Reynaert, S.; Moldenaers, P.; Vermant, J. Langmuir 2006, 22, 4936. (8) Park, B. J.; Pantina, J. P.; Furst, E. M.; Oettel, M.; Reynaert, S.; Vermant, J. Langmuir 2008, 24, 1686. (9) Donev, A.; Cisse, I.; Sachs, D.; Variano, E. A.; Stillinger, F. H.; Connelly, R.; Torquato, S.; Chaikin, P. M. Science 2004, 303, 990. (10) Basavaraj, M. G.; Fuller, G. G.; Fransaer, J.; Vermant, J. Langmuir 2006, 22, 6605.

10.1021/la803554u CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

Ellipsoidal Particles at Interfaces

A systematic study of all these effects in 2D has not yet been carried out, possibly because of the lack of model systems. Currently, a number of synthesis procedures for the preparation of anisotropic particles from various routes such as mechanical stretching,14,15 lithographic methods,16 microfluidics,17 and chemical methods18-20 have become available. With the development of these methods for the preparation of monodispersed anisotropic particles, there has been a surge to explore particle shape effect experimentally in both two and three dimensions. Some recent studies include the effect of capillary interaction at fluid interfaces,21,22 electrophoretic codeposition,23 particle-stabilized emulsions,24 the compression behavior of ellipsoidal monolayers,10 self-assembled structures formed by sedimentation of rods,25 behavior of nanorods at curved interfaces,26 and shape-induced alignment of rods at the water-air interface.27 Particles trapped at a fluid-fluid interface can deform the interface and gives rise to deformation-mediated capillary interactions, either due to gravitational effects, electrodipping,28,29 or chemical and physical heterogeneities on the particle surface.30 For particles with a nonsmooth surface, instead of a circular contact line, the meniscus takes on an irregular shape in order to have the correct contact angle at all points along the perimeter. Even a nonspherical particle deposited at an interface creates nontrivial and more complex interface deformation. As the threephase contact angle needs to be constant and the Young-Laplace equation needs to be obeyed, an undulated contact line is generated to compensate the variations in curvature along the particle surface.22,31 Wetting-induced self-assembly arises from these interactions.32,33 Numerical calculations demonstrate that even a nanoscale deviations from sphericity lead to significant attractions.34 Recent experiments by Loudet et al.21 showed that sufficiently long micron-sized prolate ellipsoids at the water-oil (11) Mohraz, A.; Moler, D. B.; Ziff, R. M.; Solomon, M. J. Phys. ReV. Lett. 2004, 92, 155503. (12) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Soft Matter 2007, 3, 1145. (13) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Soft Matter 2008, 4, 337. (14) Keville, K. M.; Franses, E. I.; Caruthers, J. M. J. Colloid Interface Sci. 1991, 144, 103. (15) Ho, C. C.; Keller, A.; Odell, J. A.; Ottewill, R. H. Colloid Polym. Sci. 1993, 271, 469. (16) Velikov, K. P.; van Dillen, T.; Polman, A.; van Blaaderen, A. Appl. Phys. Lett. 2002, 81, 838. (17) Xu, S.; Nie, Z.; Seo, M.; Lewis, P.; Kumacheva, E.; Stone, H. A.; Garstecki, P.; Weibel, D. B.; Gitlin, I.; Whitesides, G. M. Angew. Chem. 2005, 44, 724. (18) Ozaki, M.; Kratohvil, S.; Matijevic, E. J. Colloid Interface Sci. 1984, 102, 146. (19) Thies-Weesie, D. M. E.; Philipse, A. P.; Kluijtmans, S. G. J. M. J. Colloid Interface Sci. 1995, 174, 211. (20) Perez-Juste, J.; Correa-Duarte, M. A.; Liz-Marzan, L. M. Appl. Surf. Sci. 2004, 226, 137. (21) Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G. Phys. ReV. Lett. 2005, 94, 018301. (22) Loudet, J. C.; Yodh, A. G.; Pouligny, B. Phys. ReV. Lett. 2006, 97, 018304. (23) Stappers, L.; Basavaraj, M. G.; Vermant, J.; Fransaer, J. J. Electrochem. Soc. 2006, 153, C660. (24) Noble, P. F.; Cayre, 0.J.; Alargova, R. G.; Velevand, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092. (25) Mohraz, A.; Solomon, M. J. Langmuir 2005, 21, 5298. (26) He, J.; Zhang, Q.; Gupta, S.; Emrick, T.; Russell, T. P.; Thiyagarajan, P. Small 2007, 3, 1214. (27) Lewandowski, E. P.; Bernate, J. A.; Searson, P. C.; Stebe, K. J. Langmuir 2008, 24, 9302–9307. (28) Oettel, M.; Dietrich, S. Langmuir 2008, 24, 1425. (29) Kralchevsky, P. A., and Nagayama, K. Particles at fluids interfaces and membranes: attachment of colloid particles and proteins to interfaces and formation of two-dimensional arrays; Elsevier: Amsterdam, 2001. (30) Stamou, D.; Duschl, C.; Johannsmann, D. Phys. ReV. E 2000, 62, 5277– 5272. (31) Lehle, H.; Noruzifar, E.; Oettel, M. Eur. Phys. J. E 2008, 26, 151. (32) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233–235. (33) Brown, A. B. D.; Smith, C. G.; Rennie, A. R. Phys. ReV. E 2000, 62, 951.

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interface experience much strong, shaped-induced, long-range capillary attraction, and the local deformations of the interface could be measured by interferometry.22 The ellipsoidal particles are found to interact over distances several times the particle major axis, with an attractive strength of about 104 kT near contact. It was assumed that such strong and long-range attractions are due to shape-induced interface deformation having quadrupolar symmetry. Recent calculations by Lehle et al. show that the capillary forces between two ellipsoids deviate significantly from the quadrupolar approximation for distances smaller than 4 times the particle major axis, reiterating the long-range nature of the shape-induced capillary forces. Moreover, for ellipsoids, the interface deformation and associated capillary interactions have been shown to be strong functions of both particle aspect ratio and contact angle.31 The present work is an experimental study of the self-assembly of anisotropic particles at interfaces. Specifically, the effect of particle charge and wetting properties on self-assembly of particle monolayers are investigated using video microscopy. The experimental design is such that we independently vary the effect of charge and wetting properties. First, the effect of changes in the electrostatic forces is evaluated, using polystyrene particles with similar wetting properties but different surface charge. Second, the capillary forces are varied using polystyrene particles of the same surface chemistry, but with a different equilibrium position at the interface, by comparing results for water-oil and water-air interfaces. The time evolution of self-assembly is monitored to provide mechanistic insight into this phenomena and quantify the structure evolution by image analysis. Finally, we will investigate if the complex interplay between capillary and electrostatic interactions can be exploited to alter the surface rheological properties, as investigated with a magnetic rod interfacial shear stress rheometer and a bicone rheometer to cover a wide range of surface coverages. The potential applications of the resulting surface gels will be briefly discussed.

Materials and Methods Preparation of Ellipsoids. Prolate ellipsoidal particles were prepared from monodisperse spherical polystyrene particles following the approach of Keville et al. 14 and Ho et al.15 In this method the particles are embedded in a PVA film and heated above the glass transition temperature of both particles and matrix, followed by a stretching of the film. The spherical particles in the film undergo a plastic deformation to ellipsoids. The film is subsequently cooled to room temperature and the particles are extracted by dissolving the PVA matrix. However, certain modifications to the original procedure were required. These modifications were aimed at synthesizing sufficient quantities per batch and ensuring adequate PVA-removal from the particle surface for the studies of particles at interfaces. Spherical particles (diameter, 3.1 µm) carrying sulfate groups on their surface were obtained from Interfacial Dynamics Corporation (IDC). To compare the effect of surface chemistry on structure evolution, ellipsoids were prepared from uncharged polystyrene spherical particles. The uncharged spherical particles of 2.8 ( 0.1 µm diameter were synthesized by polymerization of the monomer styrene in a series of alcohols in the presence of azobisisobutironitrile.35 An aqueous polyvinyl alcohol (PVA) solution was obtained by dissolving 8.75 g of PVA (polyvinyl alcohol 40-88 from Fluka, degree of hydrolysis 86.7-88.7, degree of polymerization 4200, molecular weight 205 000) in 250 g of deionized, double distilled water. PVA was added in two equal parts. The system was stirred magnetically at a speed less than 500 rpm to avoid foaming. After (34) Van Nierop, E. A.; Stijnman, N. A.; Hilgenfeldt, S. Europhys. Lett. 2005, 72, 671. (35) Almog, Y.; Reich, S.; Levy, M. Br. Polym. J. 1982, 14, 131.

2720 Langmuir, Vol. 25, No. 5, 2009 dissolution of most of the PVA, the solution was allowed to stand for several hours to allow undissolved PVA grains to sediment. The clear PVA solution was then decanted. About 4 g of polystyrene latex suspension containing 15% particles by weight was added and stirred to get a homogeneous PVA-particle suspension. This dispersion was cast onto a flat glass plate (15 cm × 45 cm × 1 cm). Air bubbles were removed with the help of a spatula, and the film was air-dried. After 2-3 days, a particle-embedded PVA film was formed upon water evaporation. These films were cut into small pieces (4 cm × 5 to 7 cm depending on the desired stretch ratio) and stretched using a film-stretching apparatus similar to the one described in references.14,15 The stretching assembly consisted of a metal frame with a fixed lower plate and a movable upper plate. The plate is connected through a pulley to a rope that can be pulled manually. The two plates contain clamps to load the PVA-particle film. Because of manual stretching, the aspect ratio cannot be reproducibly controlled, but for every batch a sufficient quantity of monodisperse ellipsoids with aspect ratio from 1.25 to 10 can be made. Up to six films can be stretched at once. For a PVA film of about 1 mm thick immersed in an oil-bath, heat transfer calculations show that it takes less than 1 s to reach the oil-bath temperature (T ) 140 °C). It was decided to use an immersion time on the order of only few seconds (less than 10 s) to avoid diffusion of oil into the film. The temperature was lower than that used by Ho et.al.,15 to avoid possible changes to the particle surface or surface charge. After stretching, the polymer-particle film was cooled and cleaned several times with dry soft-tissue to remove oil from the film surface. Subsequently, the particles need to be recovered from the film. Because of nonuniform stretching of the film near the clamps, the entire film could not used. The edges of the stretched film were rejected, and only the center part was selected for further processing. It was cut into small pieces of approximately 1 cm × 1 cm. The pieces were soaked in isopropyl alcohol (IPA) while stirring for 1 to 2 h to remove traces of oil. The IPA solution was decanted and replaced with fresh IPA. This procedure was repeated five to six times. Film strips were then soaked in 3:7 solution by volume of IPA-water for 10 to 12 h under magnetic stirring. At the end of this period, the sample was heated to about 85 °C for 45 to 60 min to dissolve the PVA matrix completely. The dispersion was centrifuged to sediment the particles, and the clear high viscous PVA-rich supernatant at the top was poured off. The same washing procedure was repeated three more times. In the final washing step, particles were dispersed in water at about 90 °C under magnetic stirring, for 1 h to remove traces of PVA. After centrifugation at 4000 rpm, ellipsoidal particles thus obtained were redispersed in appropriate amount of deionized double distilled water depending on the particle concentration needed. Optical microscopy and zeta-potential measurements were used to evaluate the effectiveness of the washing procedure. A PVA particle film containing spherical particles was first dipped in hot oil maintained at 140 °C and taken out without stretching. The spherical particles in this film were recovered by subjecting it to the identical PVA removal procedure as that used for extracting ellipsoidal particles from the stretched films. The effectiveness of the washing procedure was tested in two ways. The zeta-potential of the particles was measured before and after washing, the values corresponding to -53 ( 2 mV before and -57 ( 3 mV after the treatment. Second, as an indirect check, the particles were spread at an oil-water interface and the resulting microstructures, as well as their stability in time, were compared. Both the particles as received and those which underwent the procedure indentical to ellipsoidal particle synthesis gave ordered hexagonal lattices with identical structural features. Also the stability in time of the colloidal crystal was identical in both cases. About 50 particles in a SEM image of each batch were counted to get the particle size. A SEM image of particles having an aspect ratio of 5.3 (major axis of 8.75 ( 0.50 µm, minor axis of 1.65 ( 0.15 µm) obtained from one such stretch is represented in Figure 1 and is representative for the particles prepared.

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Figure 1. SEM micrograph of charged polystyrene particles with an aspect ratio of 5.3, obtained starting from 3.1 µm spheres.

Preparation of Monolayer Containing Ellipsoidal Particles. A monolayer containing ellipsoidal particles was prepared by spreading a dispersion onto a fluid-fluid interface. About 10 to 100 µL of dispersion with a particle content of 1.6 to 2.0% by weight was used in each experiment. The spreading solvent was an equal volume mixture of water and isopropyl alcohol (IPA). Analytic grade isopropyl alcohol (BDH) was used as received, and the water was deionized and double distilled. The conductivity of water used was measured to be 1.5 ( 0.5 µS/cm, leading to an estimated Debye length of 150 to 200 nm, and the surface tension was measured to be 72 ( 1 mN/m using a Wilhelmy balance. Decane was used as the oil phase and passed three times through an aluminum oxide column to remove polar contaminants that might have been present. Glass cells used to carry out the aggregation experiments were partially silanated with dimethyldichlorosilane to render the upper part of its walls hydrophobic, to obtain a flat the water-oil interface. A suspension containing ellipsoidal particles, dispersed in an equivolume mixture of water-IPA, was carefully injected at an interface between the two fluids using a microsyringe. For the charged particles at the water-decane interface, the monolayer of ellipsoids was homogeneous and could be characterized by a single value of surface coverage. Also, the surface coverage obtained by dilution of the stock suspension was consistent with that obtained from microscopy images using quantitative image analysis. Optical Video Microscopy and Surface Pressure Measurements. Bright field video microscopy was used to observe the structure and dynamics of the particles at the fluid interface both during aggregation as well as during compression/expansion experiment. An Olympus (BX51WI) fixed stage microscope equipped with a CCD camera (Hamamatsu, C8800-21C) was used. With this camera, 12-bit gray scale images of a spatial resolution of 1000 pixels × 1000 pixels were obtained. A 20× long distance objective having a working distance of 21 mm, a numerical aperture of 0.4 and a field depth of 5.8 µm was used in all experiments. The combination of simple bright field microscopy and a fast CCD camera enables fast acquisition with frames rates up to 50 fps. The compression and expansion of the monolayer was performed in a micro Langmuir trough (KSV instruments, Finland) equipped with a glass window that makes it possible to visualize monolayer structure by optical microscopy. The trough was filled with water, and a Wilhelmy plate was suspended at the air-water interface, so that it is partially immersed in the subphase (water). The particles dispersed in an equivolume mixture of water-IPA were slowly spread onto the air-water interface using a microsyringe. Upon spreading the dispersion containing ellipsoids onto the air-water interface, the attractive capillary interaction forces lead to the formation of aggregates. While the particles were added on the interface, the surface tension of water decreased from its initial value of 72 to 66 mN/m, because of both the presence of particles and IPA. The system was allowed to stand for about 15 min before compression was initiated to ensure that IPA evaporated from the

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surface. Subsequently, the compression of the monolayer was performed using Delrin barriers which confine the particles and are driven by a stepper motor. Simultaneous surface pressure measurements and bright field video microscopy images were recorded during these experiments. The whole setup during aggregation and compression experiments was placed on a vibration-isolated table. Position and Orientation Correlation. A quantitative image analysis method has been developed for complete characterization of ellipsoids in a microscopy image. The algorithm consists of three steps, first locating pixels inside an ellipsoid using the method of Crocker and Grier36 followed by systematically identifying pixels at the edge of an ellipsoid by searching first in horizontal and vertical directions. For more dense suspensions, this needs to be combined with the use of erosion filters. The detection of pixels at the edges is followed by a least-squares fitting of the contour to an ellipsoid which then gives the position of the center and the orientation of the major axis. A detailed description of this algorithm and its performance is given in Supporting Information. From the position and orientation of each ellipsoid, the pair and orientation correlation functions, respectively, were calculated. Pair Correlation Function. The pair correlation function gives the normalized probability of finding a particle at a certain distance from the other. The pair correlation function g(r) was calculated from the following equation:

g(r) )

n(r) n

(1)

where n(r) is the number of particles that are within a spherical shell r and r + δr normalized by average particle number density, n. The routines were implemented in MATLAB. Orientation Correlation Function. The orientation correlation function was calculated from the following relation38

Q(r) ) 〈cos(2∆Θij)〉

(2)

where ∆Θij is the difference in the orientation between two particles i and j. This difference was calculated for each particle with respect to the other, whose center to center distance was within r and r + δr and averaged over all particles. Any kind of orienational order present in the system appears in the calculated orientation correlation function. If the particles were perfectly aligned, for which ∆Θij takes the value of 0, Q(r) would take a value of 1 and would be equal to 0 if particles are randomly oriented.38 Algorithms for 2D suspension by Hoekstra et al.37 were also used to calculate the number of ellipsoids in an aggregate, N, and its radius of gyration, Rg for each aggregate present in the microscopy image. Gel Trapping Technique. The knowledge of the equilibrium position of a particle at the fluid-fluid interface, as determined by the wetting characteristics, is essential to understand the interaction forces. The concept of the three-phase contact angle has been widely used in literature to quantify the wetting properties of colloidal particles trapped at an interface. Experimentally, various methods have been used to determine the three-phase contact angle of microand nanometer-sized colloidal particles.39,40 In the present manuscript, in order to realize the contact angle and nature of interface deformation by ellipsoids, one of the phases was gelled in order to trap the particles at the interface to enable in situ visualization by SEM. In the gel trapping technique,39 a low surface activity hydrocolloid that does not affect the interface properties was added to the subphase before particles are spread onto an interface. The whole system was maintained at sufficiently high temperature to avoid the gel formation, i.e. 50 °C, until the particles reach their equilibrium position. The system is then cooled quickly to room temperature allowing the subphase to gel so that it traps or arrests the particles in their (37) Hoekstra, H.; Vermant, J.; Mewis, J.; Fuller, G. G. Langmuir 2003, 19, 9134. (38) Stokely, K.; Diacou, A.; Franklin, S. V. Phys. ReV. E 2003, 67, 051302. (39) Paunov, V. N. Langmuir 2003, 19, 7970. (40) Stancik, E. J.; Fuller, G. G. Langmuir 2004, 20, 4805.

equilibrium position at the interface. The top phase (either oil or air) was then replaced with mixture of PDMS and a curing agent, which acts as a cross-linker. The system was allowed to stand at room temperature for two days, after which the PDMS layer could be peeled off. The PDMS layer now carries the trapped particles. The PDMS-particle samples were cut into 1 cm × 1 cm parts and prepared for imaging with SEM after coating with a gold nanolayer. SEM experiments can now reveal the part of the particles that was originally exposed to water. In this manner, the contact angle for spherical polystyrene particles at the water-decane interface can be obtained and was found to be 134 ( 5°, which is consistent with that obtained by other methods.40 For the air-water interface a contact angle of 98 ( 5° was found. When the samples are tilted on the SEM stage, the interface deformations around an ellipsoid can be viewed. Interfacial Rheology. A bicone geometry attached to Physica MCR 501 (Anton Paar Physica, Austria) controlled stress rheometer was used to measure the interfacial rheology of dense monolayer of ellipsoids at oil-water and air-water interfaces. For each measurement a fresh dispersion was prepared. The spreading solution consisted of an aqueous suspension of ellipsoids (2% by volume) mixed with equal volume of ethyl alcohol. Typically 10 to 200 µL of suspension was used in each experiments. Before each measurement, a waiting time of 15 min was used after creating the monolayer. Contributions of the bulk phases were taken into consideration by a complete analysis of the flow field for the bicone geometry.41,42 For lower surface coverages, the sensitive magnetic rod interfacial stress rheometer was used.43,44 The surface rheological properties of particulate monolayers at the water-air interface were studied by performing oscillatory shear experiments. The details of measurement on the monolayers of spheres have been presented elsewhere.45 The monolayers of ellipsoids were prepared in a 100 mm diameter Petri dish. At the center of this dish, a rectangular channel of width 2R ) 20 mm and length L ) 70 mm was placed. A very dense percolating network was observed soon after spreading ellipsoids at the interface. The percolating networks of ellipsoids were allowed to age for 48 h before interfacial rheological measurements to compare the data with the surface rheological properties of monolayer of spheres. An optimum sensitivity was achieved by using a glass capillary of 400 µm diameter filled with highly magnetic material at the center as the rheological probe.45,44 The whole setup was placed at the center of two coils of the magnetic needle rheometer. The magnetic needle held firmly at the interface by the surface tension was oscillated by applying a sinusoidal current through one of the coils. The net force F, exerted on the needle, and the amplitude of displacement were measured as a function of time. The storage and loss moduli can then be calculated from the dimension of the channel, the needle, the force, and displacement data.44,45 In all surface rheological measurements, the ratio between the surface contribution and the bulk needs to be considered. This is typically characterized by a dimensionless number, the Boussinesq number, which characterizes the ratio between the contributions of surface drag versus the bulk contribution to the drag on the measurement geometry. When the Boussinesq number was too small, the raw experimental data were corrected for the subphase drag using adequate numerical procedures.44,45

Results and Discussion Effect of Particle Shape, Wetting, and Surface Chemistry. Uncharged Ellipsoids at the Water-Decane Interface. Figure 2a shows the initial configuration of a monolayer obtained soon after depositing uncharged ellipsoids at a water-decane interface. The micrograph in Figure 2a shows that ellipsoids form an open (41) Oh, S. G.; Slattery, J. C. J. Colloid Interface Sci. 1978, 67, 516. (42) Erni, P.; Fischer, P.; Windbah, E. J.; Kusnezov, V.; Stettin, H.; Lauger, J. ReV. Sci. Instrum. 1998, 69, 4916. (43) Brooks, C. F.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1999, 15, 2450. (44) Reynaert, S.; Brooks, C. F.; Moldenaers, P.; Vermant, J.; Fuller, G. G. J. Rheol. 2008, 52, 261. (45) Reynaert, S.; Moldenaers, P.; Vermant, J. Phys. Chem. Chem. Phys. 2007, 9, 6463.

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Figure 3. Self-assembly of ellipsoids with low surface charge at higher surface coverage on a water-decane interface. (a) Initial structure. (b) After 12 h. The aspect ratio of the ellipsoids is 5.5. The scale bar corresponds to 50 µm.

Figure 2. Self-assembly of ellipsoids with a low surface charge at the water-decane interface: Structure evolution at low surface coverage as a function of time. (a) Initial structure soon after spreading. (b) Locally densified structure after 11 h. (c) Further densified structure after 30 h. The aspect ratio of the ellipsoids is 5.5. The scale bar corresponds to 50 µm.

network having predominantly linear chain backbones. Such open networks have been observed earlier for similar uncharged ellipsoids at the water-oil interface.21 Apart from the dominant tip-tip arrangement of particles, some particles form 2D flowerlike structures, which could be viewed as 2D micelles, with multiple particles connected by their tips. Some particles were found to be arranged side by side. For particles of similar dimension, the lateral capillary interaction energy has been measured both for tip-tip and side-side configuration.21 This energy is very large, of the order of 104 kT for both for tip-tip and side-side contact, with a more pronounced interface deformation and attractive force at close range near the tip.31 As time proceeds, the open structure in Figure 2a densified and became more heterogeneous. For the particles in the field of view, the local surface coverage increased with time. The system tends toward phase separation, and dense regions coexist with open voids. Figure 2 b,c shows the evolution of the microstructure after 11 and 30 h, respectively. Figure 2c shows a locally densified network consisting predominantly of flower-like arrangments and arrays of ellipsoids in side-side configuration and a few remaining linear chains.

The spatial variations in surface concentration and its evolution could not be quantified, as the length scale of heterogeneity was large compared to the field of view at these concentrations. Overall, the structural evolution was slow. At higher concentrations, the initial microstructure showed arrays of particles with side-side arrangements as shown in Figure 3a. When the particles are side to side, they will interact over more of their perimeter, leading to a more pronounced reduction of the total energy in the interface, and this should hence correspond to the minimal energy situation. It is clear that for these uncharged ellipsoids, capillary forces dominate. For more complex particle assemblies such as the flower-like structures, a significant reduction of the interfacial distortion may be achieved as well by the more complex assemblies, where particles are arrested in local energy minima. The structure evolved very slowly to a network, seen in Figure 3b, after 12 h, which was similar to that in Figure 2c and seems to be a final, most probably metastable, state. Charged Ellipsoids at the Water-Decane Interface. To manipulate the structure further, charged spheres were used to prepare the ellipsoids, to elucidate the effect of particle charge on the monolayer microstructure. They are first studied at the water-decane interface; hence, the only major difference to the samples studied in the previous paragraph are the effects of surface charge. It is well-known, and shown for reference in Figure 4a, that charged spherical particles at the water-decane interface give rise to an ordered crystalline monolayer, stable over several days as already observed by Pieranski.6 The structure of a monolayer of charged ellipsoids of AR ) 5.3 at a surface coverage of 11.7 ( 0.5%, 18.7 ( 0.6%, and 30.4 ( 0.7% soon after spreading particles onto the water-decane interface is given for comparison respectively in Figure 4b-d. The initial microstructure was homogeneous and could be characterized by a box-size-independent surface coverage. At all surface coverages

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Figure 4. Charged particles at the water-decane interface: Microstructure obtained right after spreading particles at the interface (a) for spheres, (b) for ellipsoids of aspect ratio 5.3 at a surface coverage of 11.7 ( 0.5% with the insert showing details of the local order, triangular and quadrilateral arrangements, (c) for ellipsoids of aspect ratio 5.3 at a surface coverage of 18.7 ( 0.6%, and (d) for ellipsoids of aspect ratio 5.3 at a surface coverage of 30.4 ( 0.7%, respectively. The scale bar corresponds to 50 µm.

Figure 5. Charged ellipsoids at the water-air interface: (a) Microstructure obtained soon after spreading particles (scale bar 50 µm); (b) the same microstructure at a higher magnification (scale bar ) 20 µm). The aspect ratio of the ellipsoids is 5.3.

studied, linear strings of ellipsoids were observed. It must be noted that at low and moderate surface coverage (up to about 18%), many individual particles were present, suggesting that the long-range electrostatic repulsion is of the same order of magnitude as the capillary forces. With charged ellipsoids, no long-range order was observed in the initial structure. But, as shown in the inserts in Figure 4b ellipsoidal particles occupying the vertices of a triangle, quadrilateral, and higher order polygons were repeatedly observed. Yet unlike monolayers of charged spheres, charged ellipsoids in the monolayer aggregate even in the absence of salt/surfactant. At a higher surface coverage of 31% percolating network with linear chains as the backbone was observed. The initial microstructure shown in Figure 4b, 4c, and 4d densified with time, similar to that for the uncharged ellipsoids. This shows that although the electrostatic forces create a barrier

for aggregation, the final structure seems to be determined by the capillary forces. Charged Ellipsoids at the Water-Air Interface. When the same charged ellipsoidal particles are dispersed at the water-air interface, it can be expected that changes in both the electrostatic and capillary forces will occur. As a consequence, the resulting structures can be expected to differ. Figure 5 shows the structure obtained soon after spreading charged ellipsoids at a water-air interface. The particles are the same as the ones used in Figure 4; only the absence of the oil phase changes the contact angle and increases the magnitude of the surface tension somewhat. Compared to the ellipsoids at the water-decane interface, the microstructure is very dense from the start. Surprisingly, neither linear chains nor individual ellipsoids were present in the initial structure. The dense networks predominantly have triangular

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backbones and only some ellipsoids stacked side-to-side. The triangular backbones consisted of three ellipsoids with their tips connected and arranged in the form of a triangle. These structures did not evolve over a time period of days. A more detailed microscopy image of the initial structure is shown in Figure 5b. Effect of Shape, Charge, And Wetting Properties on Interaction Forces. When comparing the remarkably different structures in Figures 2, 4, and 5, the effects of particle shape, surface chemistry, and changes in the wetting properties on the microstructure of monolayers containing ellipsoids at fluidfluid interfaces are clearly significant. It is well known that charged spherical particles at the water-decane interface give rise to a crystalline monolayer having long-range order (Figure 4a), because of an enhanced electrostatic repulsion.6,29,46-48 Similar structures are observed at the water-air interface, although the repulsion is typically weaker. Yet for charged spheres, the addition of salt, surfactant, or both are required to induce aggregation in these monolayers.3-5,7 However, figures 4 and 5, which are for charged ellipsoids, demonstrate that the shape-induced capillary forces for anisotropic particles21,31,34 can cause aggregation even in the absence of salt and surfactant, and this even for charged ellipsoids. It is clear that as as aggregated structures are observed under all conditions when using ellipsoidal particles, the shapeinduced capillary interactions dominate the enhanced electrostatic properties. For the particles at a water-decane interface, the electrostatic interactions are only slightly smaller compared to the capillary interactions; moreover, they can be expected to be anisotropic. For example, using similar reasonings as for spheres,28 suppose the ellipsoid cross-section area with the interface is modeled as being covered by electric dipoles; the ellipsoids would be expected to repel each other strongly when parallel to each other, which would rationalize the predominant tip-tip aggregation. Comparing the microstructure of charged and uncharged ellipsoids at a water-decane interface in Figure 2a and 4b to 4d gives a direct assessment of the effect of charge for particles with similar wetting properties. The presence of space-spanning large aggregates immediately after spreading in Figure 2a are consistent with the absence of any significant repulsion, combined with pronounced shape-induced capillary interactions. Whereas for charged spheres the electrostatic interaction at the oil-water interface is very strong, for ellipsoids the electrostatic repulsion does not suffice to overcome this capillary attraction. This is because of the strength of the capillary force, combined with a possible weakening of the dipole interaction relative to the case of the spheres. The latter may be due to two reasons. First, when a sphere is stretched to form an ellipsoid, there is an increase in surface area. In obtaining an ellipsoid of aspect ratio 5.3 (major axis ) 8.75 µm, minor axis 1.65 µm) starting from a sphere of 3.1 µm diameter, there is approximately 20% increase in surface area. Hence, the number of charges per unit surface area decreases. Second, the electrostatic interaction is a predominant dipoledipole interaction and hence depends on this surface charge density but also the height of the charge with respect to the interface.48-50 Yet, the local order shown in Figure 4b shows that for the particles at the water-decane interface the resulting electrostatic interactions are of the same order of magnitude as the capillary attraction, and because of long-range nature of the forces, multibody (46) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (47) Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Weitz, D. A. Nature 2002, 299, 420. (48) Hurd, A. J. J. Phys. A 1985, 18, 1055. (49) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16, 1969. (50) Oettel, M.; Dominguez, A.; Dietrich, S. Phys. ReV. E 2005, 71, 051401.

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interactions and orientation-dependent interactions would probably need to be considered. Another way of assessing how the capillary forces and electrostatic forces are altered can be obtained from the differences in structural evolution between the ellipsoids at the air-water in Figure 5 and the water-decane interface in Figure 4. Typically, for the PS particles studied here, the electrostatic interactions at the water-air interface are weaker compared to the oil-water interface.28 Yet also the capillary forces are changed as the equilibrium position of the particles relative to the air-water interface differs from the oil-water interface. The equilibrium position above the interface can be estimated following the method proposed by Piernaski.6 The determination of the height of a particle above or below the interface corresponds to an energy minimization problem. The energy by which a particle is attached to an interface is given by the product of the surface area and its interfacial tension. When particles are sitting at an interface between two fluids, two additional interfaces are created, that between particle and the aqueous subphase (p/W) and that between the particle and the upper phase (p/U), whereas part of the fluid-fluid interface is being occupied by the solid particle. For the case of ellipsoids at an interface, the surface area of particle/subphase and that of particle/upper phase is the surface area of an ellipsoidal segment cut by a plane. The area of the missing interface is the cross-sectional area of a two-dimensional ellipse, obtained by cutting a three-dimensional ellipsoid at the interface. The surface energy of the polystyrene/air interface (EpA) is given by (σpA)(SpA) and that of the polystyrene/water interface (EpW) is (σpW)(SpW), and the decrease in interfacial energy due to the reduction of the overall water/air interface, EWA, is (σWA)(SWA), where σpA, σpW, and σWA are the surface tension of particle-air, particle-water, and water-air, respectively. The overall energy difference, δE ) (EpA + EpW - EWA) is a measure for the energy of attachment to the interface. The surface areas of ellipsoidal segments were calculated by numerical integration of the elliptical integral, as no analytical expression is available. To calculate SpW, SpA, and SpD, the following expression to calculate the surface area of a prolate ellipsoid was used.

{

S ) 4πb a

}

sin-1(√δ) +b √δ

(3)

with δ being the eccentricity, defined as (1 - a2/b2)1/2 with a and b the dimensions of the major and minor axis. Figure 6a shows the results of the numerical calculations energy difference δE as a function of immersion depth, z0, normalized by b, the length of the particle minor half-axis (the coordinate system employed is shown in the inset of Figure 6a). The minimum in the curve yields a good estimate of the position of the particle with respect to the interface. The dotted vertical line corresponds to a line through the major axis of the ellipsoid. For both water-air and water-oil interfaces, the hydrophobic particles are expected to stick out of the water into the air or decane phase. Comparing the results for the ellipsoids in the water-air to the water-decane air systems, we can conclude that the three-phase contact line moves up as schematically represented in Figure 6b. For the water-air interface, the ellipsoid intersects the interface along its major axis close to the tip. For the water-decane interface, the intersection with the interface takes place a small distace away from the tip. The equilibrium position at the interface is important, as it has an effect on the lateral capillary interaction. By solving the linearized Laplace equation, Lehle et al. showed that the meniscus shape strongly depends on the aspect ratio and the contact angle.31 The rise of the interface on the side of the particles is expected

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Figure 6. Position of the particles at the interface and resulting interface deformation: (a) Surface attachment energy for ellipsoids at the water-decane and water-air interface as a function of the height with respect to the interface. The minimum in energy corresponds to the equilibrium position. (b) Schematic representation of the positions of the particle at the water-air and water-decane interfaces corresponding to the energy minima. (c) SEM picture of a gel-trapped ellipsoidal particle originally at the water-decane interface, on a tilted stage and viewed from the front, revealing opposite changes in height near the side and near the tip. (d) Side-view of ellipsoidal particles trapped at the water-decane interface visualized by the gel trapping technique, showing the part of the particles originally exposed to water. Table 1. Equilibrium Position of Ellipsoids at the Water-Decane Interface major axis (a) µm

minor axis (b) µm

aspect ratio

A ) a1/a

B ) b - 2b1/b

5.60 8.75

2.20 1.65

2.55 5.30

0.93 0.83

0.81 0.86

to be at its maximum for contact angles which differ from neutral wetting by about 40°. So based on this argument, the side-side capillary forces should be maximal for the oil-water interface. However, the interfacial tension for the air-water interface is significantly higher. Also, one would also expect that for particles touching near the tips, for the oil-water case the particles will touch each other in the oil phase, whereas the deformation of the contact line occurs a distance away from the tip, below the center line of the particles. The capillary force will hence only be able to act from the tips onward, possibly also weakening the magnitude of the tip-tip interaction relative to the water/air case. Overall the reduction of the strength of the electrostatic interaction and the significant presence of the capillary force suffice to create very dense structures right after spreading for the ellipsoids at the air-water interface. The gel trapping technique was used to compare the results of the calculation to the experimentally observed equilibrium position of ellipsoids at the interface and also to visualize the nature of interface deformation. The part of the particle that sticks out of PDMS in the SEM images shown in Figure 6c and 6d corresponds to the part that was in water. The data obtained averaging over 25 such SEM images for particles at a water-decane case have been listed in Table 1 for ellipsoids of aspect ratios 2.5 and 5.3. For the two aspect ratio particles studied here, distances a1 and b - 2b1, normalized by the major axis and

the minor axis, were of the same order. For the water-air case, because of the overwhelming presence of very dense aggregates, reliable data could not be obtained. A curved ellipsoid tip was visible in some SEM images; hence, we assume that the particles were pushed up, away from the aqueous phase, consistent with the sketches in Figure 6b. This indeed is expected based on the energy calculations as shown in Figure 6a. The nature of the interface deformation by ellipsoids showing the opposite changes in height near the side and near the tip is given in Figure 6c, similar to the observations of Loudet et al.21,22 This deformation of the interface at the ellipsoid tip and at the sides of the ellipsoid induces prounounced and long-range capillary attraction. Microstructure Evolution of Charged Ellipsoids at a Water-Decane Interface. After particles were spread onto the water-decane interface, the structure of the monolayer was followed in time for various initial surface coverages. The time evolution of the structure is shown for an initial surface coverage of 11.7 ( 0.5% as calculated from 30 images. Figure 7a depicts the initial configuration of the microstructure. The particles assemble predominantly by connecting at their tips. This differs from earlier observations on uncharged ellipsoids, where both side-side and tip-tip orientations were observed.21 In the latter case, the particle trajectories upon approach were straight. In the current case, ellipsoids approaching one other were observed to rotate because of torque generated by the combined effect of the capillary and electrostatic forces, leading to tip-tip contact. A characteristic angle of 150 ( 10° was observed. This held true even when aggregate chains approached each other. Hence, the self-assembly proceeds by the aggregation of two particles, singlet-doublet, doublet and linear aggregate, and their combinations, to yield linear strings of ellipsoids as shown in Figure 7b

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Figure 7. The structure evolution in a monolayer containing charged ellipsoids at the water-decane interface: (a) soon after spreading; (b) 9 h; (c) 26 h; (d) 130 h; (e) 190 h; (f) 360 h.

and 7c. The linear chains grow in time until they encounter other chains to form interconnected aggregates which percolate as in in Figure 7d. This space-spanning, interconnected network was found to locally densify and become more heterogeneous in time, as seen in parts of Figure 7e. Local density differences on the scale of an image amounted to as much as 5%. As the network becomes denser, the structural changes were very slow. Figure 7f shows a dense zone in the network containing ellipsoids arranged in the form of flower- or star-like structures, formed after particle rearrangement. These flower-like arrangements were found to consist of 3 to 10 particles with all their tips held together at one end. The system subsequently evolved to such a state in about 360 h after spreading. Aging of the structure up to 720 h could be observed, and the resulting network was heterogeneous with dense flowerlike arrangements and an increased number of ellipsoids stacked in a side-side configuration. These are shown in more detail in Figure 8. These stacks formed by particles that rotated around the tips. It has been argued by Brown et al. that tip-tip and side-side configurations lead to the same amount of interface

Figure 8. The metastable, final microstructure of charged ellipsoids at the water-decane interface showing ellipsoids arranged in micellar structures and in side-side arrangments (720 h after spreading).

deformation for particles in contact.33 The large curvatures at the tip of ellipsoids combined with the electrostatic interactions

Ellipsoidal Particles at Interfaces

Figure 9. The pair (a) and orientation (b) correlation functions corresponding to the structures in Figure 7a,c,e.

dictate their tip-tip contact at early stages of aggregation. But particle rearrangement due to rotation leads to dense structures with side-side arrangement which is stable, because the particles are exposed to a larger perimeter of each other which increases overall capillary interactions and increases the area over which van der Waals forces can act. The presence of flower-like structures, stacks of ellipsoids attached side-to-side, and more importantly triangular connections characterize the structure of the final aggregated network. The details of the force balance between the electrostatic and the capillary forces in quantitative terms are not understood as yet and should stimulate further research, such that the self-assembly process can be optimized. Quantitative Image Analysis. Images as in Figure 7 were used to obtain the pair and orientational correlation functions. The radial pair and orientation distribution function corresponding to the structures shown in Figure 7a,ce at a surface coverage of 11.7% are calculated using the algorithm presented in Supporting Information. The particle-particle separation distance (d) has been normalized with respect to the ellipsoid major axis (a). The first peak in the pair correlation function corresponds to the distance to the first neighbor of particles in a chain. At time zero, there is also some order at larger distances, corresponding to structures observed in the inserts in Figure 4b, with particles occupying the vertices of a triangle or a quadrupole. With aging of the monolayer, the position of the second peak shifts inward, the height of the first maximum increases, and the orientational correlation increases, because of an increase in the number of linear chains. The results at 18.65% and at lower surface coverage (4.22%) were qualitatively similar. The linear structures start to aggregate, and the structure becomes locally more dense. The peak in the correlation function at d/a ∼ 1 shifts to smaller values because of a side by side packing, which for the present aspect ratio starts at about ∼0.4. Image analysis was also used to characterize the larger scale structures. To this end the number of ellipsoids in an aggregate, N, and its radius of gyration, Rg, were measured for the images before percolation of the structure for the experiments in Figure 7. A power law was obtained of the type N ∝ Rgdf, typical behavior for a fractal-like aggregate, with an average fractal dimension of df ) 1.65. Compression of the Aggregated Network of Charged Ellipsoids at the Water-Air Interface. A monolayer of aggregated ellipsoids obtained by depositing charged ellipsoidal particles at the water-air interface was subjected to compression

Langmuir, Vol. 25, No. 5, 2009 2727

and expansion cycles in a Langmuir trough. The initial microstructure of charged ellipsoids at the water-air interface before initiating compression was similar to that shown in Figure 4. The shape of the surface pressure-area isotherms recorded during compression was found to be nearly identical to those observed earlier for uncharged particles at the oil-water interface.10 An example of the surface pressure versus rescaled area (ARS) is shown in Figure 10a; the trough area was rescaled by the surface coverage at the first inflection point, as in ref 10, and no substantial increase in surface pressure was observed during initial stages of compression until a space spanning network was formed when large aggregates touch each other. Upon further compression, the surface pressure increased very gradually. Compression of a partially aggregated network leads to complicated geometric packing, similar to the densification observed during selfassembly, with an increasing number of triangular arrays, flowerlike structures, and side by side packed ellipsoids. Some ellipsoids are also observed to flip out of the plane, to an upright position with their major axis perpendicular to the interface, which relieves compressional stress as reported earlier.10 These combined effects lead to gradual increase in surface pressure as in Figure 10a. In the final stages of compression, flippers arranged in a line perpendicular to compression direction and spanning the entire width of the Langmuir trough were observed. Finally, buckling of the monolayer as a whole was observed. The flipping and buckling was observed in denser regions. Remarkably, in the compressed monolayer, buckled and dense regions coexisted with open structures, as shown in Figure 10b. Many of the triangular or micellar structures are very strong and able to sustain a large load, which suggests that when monolayers composed of such building blocks could be made, substantial elastic properties could be achieved. Interfacial Rheology of Aggregated Suspensions of Ellipsoids. Analysis of the surface rheological measurements on a percolating network of spheres at the water-air and water-decane interface has been reported previously.45 Oscillatory measurements on dense networks of ellipsoids were done at frequency of 0.05 and 0.2 Hz. As the Boussinesq numbers were relatively high (∼500), based on the analysis of Reynaert et al.,44 it can be concluded that measurements reported here are well within the working range and sensitivity of the instrument. Figure 11a shows the elastic and viscous modulus as a function of strain for three surface concentrations measured using the ISR. The linear viscoelastic moduli increase as the surface coverage is increased, becoming predominantly elastic. A very small linearity limit is observed, which becomes smaller as the surface coverage increases. Oscillatory shear experiments at higher surface coverage were performed with a bicone geometry. The surface elastic and viscous modulus as a function of strain obtained at a fixed frequency of 1 Hz for higher surface concentrations for ellipsoidal networks at the water-air interface are shown in Figure 11b and confirm the trends observed with the results obtained using the ISR. The loss modulus also shows a clear increase as the strain amplitude is increased, which is a typical rheological signature of a flocculated network breaking up.51-53 Figure 12 compares in more detail the low frequency linear viscoelastic surface storage modulus of ellipsoids at a water-air interface compared to earlier reported data of the spheres from which the ellipsoids were prepared.45 Ellipsoidal particles yield (51) Buscall, R.; Mills, P. D. A.; Goodwin, J. W.; Lawson, D. W. J. Chem. Soc., Faraday Trans. 1988, 84, 4249. (52) Larson, R. G. In The Structure and Rheology of Complex Fluids; Oxford University Press: Oxford, 1998. (53) Grant, M. C.; Russel, W. B. Phys. ReV. E 1993, 47, 2606–2614.

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Figure 10. Compression behavior of ellipsoid monolayers. (a) Pressure-area isotherm. (b) Micrograph of the microstructure of the buckled monolayer after compression. Note that not all the monolayer is in focus because of buckling, and flipped ellipsoids appear as circles in the image. The scale bar in the microscopy image corresponds to 25 µm.

Figure 12. Low frequency modulus (G′s,0) as a function of surface coverage for spherical particles (from ref 45) and spheroidal particles of AR ) 5.5 at the water-air interface.

properties can be further tailored by changing the wetting properties and size of the aggregates and how the resulting surface gels can be exploited in stabilization of high interface systems.

Conclusions

Figure 11. (a) G′S (closed symbols) and G′′S (open symbols) as a function of strain at various surface coverages obtained using a magnetic rod rheometer (b) G′S (closed symbols) and G′′S (open symbols) data at higher surface coverages obtained using a bicone geometry.

surfaces with significantly higher moduli than spheres at the same surface coverage, even when accounting for the increase in effective surface coverage (not shown). The latter is probably due to the specific geometric nature of the films with the intrinsically rigid triangular and flower-like building blocks as in Figure 5, combined with the strong shape-induced capillary interactions. Nonspherical particles of suitable wetting conditions selfassemble at the interface to create strong interfacial gels. These properties can be used for example, in Pickering emulsions stabilized by anisotropic particles,24,54 which can be rationaly designed. Further work could focus on how these rheological (54) Madivala, B. Vandebril, S. Fransaer, J. Vermant, J. Soft Matter, in press.

The behavior of prolate ellipsoidal particles at the planar fluid-fluid interface has been studied. Using either adapted wetting conditions, or time, even charged ellipsoidal particles can be assembled into dense structures, with micellar or side-side arrangement. These features can be quantified by quantitative image analysis. Surface compression and surface shear rheology have been used to demonstrate that the resulting monolayers are highly elastic and brittle. These self-assembled networks, formed by capillary effects and tailored by wetting and surface charge, could hence be of relevance to stabilize interfaces against deformation, e.g., in Pickering emulsions. Acknowledgment. We thank Dr. Sven Reynaert and Steven Vandebril for their assistance in surface rheology measurements. Dr. J. C. Loudet and Dr. M. Oettel are acknowledged for stimulating discussion. We thank the Research Council of the K. U. Leuven for financial support through GOA-2003/06 and 2008/01. This work was performed in the framework of a Network of Excellence ‘SOFTCOMP’ (EU-6Fp). Supporting Information Available: Additional details. This material is available free of charge via the Internet at http://pubs.acs.org.

LA803554U