Ordering of Solid Microparticles at Liquid Crystal−Water Interfaces

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Ordering of Solid Microparticles at Liquid Crystal-Water Interfaces I-Hsin Lin,† Gary M. Koenig, Jr.,† Juan J. de Pablo, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, UniVersity of Wisconsin-Madison, 1415 Engineering DriVe, Madison, Wisconsin 53706 ReceiVed: August 14, 2008; ReVised Manuscript ReceiVed: October 8, 2008

We report a study of the organization of solid microparticles at oil-water interfaces, where the oil is a thermotropic liquid crystal (LC). The study was motivated by the proposition that microparticle organization and LC ordering would be coupled at these interfaces. Surfactant-functionalized polystyrene microparticles were spread at air-water interfaces at prescribed densities and then raised into contact with supported films of nematic 4-pentyl-4′-cyanobiphenyl (5CB). Whereas this method of sample preparation led to quantitative transfer of microparticles from the air-water interface to an isotropic oil-water interface, forces mediated by the nematic order of 5CB were observed to rapidly displace microparticles laterally across the interface of the water upon contact with nematic 5CB, thus leading to a 65% decrease in the density of microparticles at the LC-water interface. These lateral forces were determined to be caused by microparticle-induced deformation of the LC, the energy of which was estimated to be ∼104 kT. We also observed microparticles transferred to the LC-water interface to assemble into chainlike structures that were not seen when using isotropic oils, indicating the presence of LC-mediated interparticle interactions at this interface. Optical textures of the LC in the vicinity of the microparticles were consistent with formation of topological defects with dipolar symmetry capable of promoting the chaining of the microparticles. The presence of microparticles at the interface also impacted the ordering of the LCs, including a transition from parallel to perpendicular ordering of the LC with increasing microparticle density. These observations, when combined, demonstrate that LC-mediated interactions can direct the assembly of solid microparticles at LC-water interfaces and that the ordering of the LC is also strongly coupled to the presence of microparticles. 1. Introduction When solid particles are confined to interfaces between two isotropic liquids, effective interparticle interactions (including capillary,1 dipolar electrostatic,2 and elastic forces3) can drive the formation of particle assemblies.4-9 These assemblies influence a variety of physical phenomena, including stabilization of emulsions (so-called Pickering emulsions).10-14 Other interfacial assemblies have been shown to exhibit optical,15-18 electronic,19,20 or mechanical21-25 properties that are determined by particle organizations at these interfaces. Whereas the studies noted above have focused on particles at the interfaces of isotropic liquids, several recent studies have demonstrated that glycerol microdroplets condensed at a liquid crystal (LC)-air interface can spontaneously form hexagonal and chain-type patterns.4-6 The ordering of the microdroplets was attributed to forces mediated by the elastic deformation of the LC. These observations obtained by nucleation and growth of microdroplets led us to predict that LC-mediated forces between solid microparticles deposited at LC-water interfaces may also lead to microparticle organizations that are not seen when using isotropic oils. In the study reported in this paper, we test this proposition by developing a method to transfer well-defined densities of solid microparticles onto a LC-water interface, and subsequently we characterize the extent of coupling between the order of the nematic LC phase and the interfacial organization of the microparticles. Whereas interactions between solid microparticles dispersed in bulk LCs are known to be strongly * To whom correspondence should be addressed. E-mail: abbott@ engr.wisc.edu. Tel: 608-265-5278. Fax: 608-262-5434. † These authors contributed equally to this work.

influenced by elastic deformations of the LCs,26,27 to our knowledge there have been no prior studies of solid microparticles at LC-water interfaces. 2. Experimental Section Preparation of Chemically Functionalized Solid Microparticles. We prepared polystyrene (PS) microparticles with diameters of 19 µm that were coated with the surfactant dimethyldioctadecylammonium bromide (DODAB) so as to cause perpendicular ordering of the LC at the microparticle surface. The PS microparticle surfaces were saturated with DODAB by immersing the microparticles into a 260 µM aqueous solution of DODAB (the critical aggregation concentration of DODAB is 3.7 µM).28 Following immersion in the DODAB solution for 4 h (with occasional vortexing and sonication), the PS microparticles were separated from the solution by centrifugation and rinsed six times in water to remove DODAB not strongly bound to the microparticle surfaces. To confirm the perpendicular ordering of LC at the surfaces of the DODAB-coated PS microparticles, a small number of microparticles were dispersed into thin films of nematic 4-pentyl-4′-cyanobiphenyl (5CB) and observed under an optical microscope. These microparticles exhibited optical textures (not shown) associated with formation of dipolar hyperbolic hedgehog defects, a result that is consistent with strong perpendicular ordering of 5CB at the microparticle surfaces.26,29 Microparticles not functionalized with DODAB did not form hyperbolic hedgehog defects, confirming that the DODAB was responsible for the perpendicular ordering of 5CB at the microparticle surfaces.

10.1021/jp807286s CCC: $40.75  2008 American Chemical Society Published on Web 12/02/2008

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Figure 1. Experimental setup used to transfer PS microparticles spread at the surface of water onto the LC-water interface. A Teflon miniLangmuir trough with two barriers was used to prepare predefined densities of PS microparticles at the surface of water. The level of water in the trough was then raised to bring the microparticle-decorated surface of the water into contact with LC that was contained within a metallic specimen grid.

Transfer of Solid Microparticles to the Liquid CrystalWater Interface. We used a modified Langmuir-Schaefer method to transfer DODAB-coated PS microparticles spread at the surface of water onto the LC-water interface. Details of the method can be found in prior publications in which amphiphilic molecules spread at the surface of water were transferred onto LC-water interfaces.30,31 In brief, 20 µm thick films of nematic 5CB were prepared by hosting the LC within a gold grid (20 µm thick, 280 µm × 280 µm grid squares) that was supported on an octyltrichlorosilane (OTS)-treated glass microscope slide. The OTS-treated glass provided perpendicular ordering of the LC at the LC-OTS interface.32 The free surface of the LC hosted in the grid was oriented toward water that filled a Langmuir trough (Figures 1 and 2G). Next, DODABcoated microparticles were dried, dispersed in 2-propanol, and spread onto the surface of water within the Langmuir trough. Following spreading, the monolayer of microparticles was compressed in the Langmuir trough to achieve the desired density of PS microparticles prior to transfer onto the LC interface. We note here that measurement of surface pressure isotherms for the microparticles at the air-water interface was not possible because the PS microparticles spontaneously pinned to the Wilhelmy plate and prevented accurate measurements of surface pressure.7 To achieve transfer of the microparticles onto the LC interface, water was added through an inlet located at the bottom of the trough at a controlled rate using a microlitersyringe pump. Addition of water to the trough raised the height of the surface of the water along with the PS microparticles at a constant rate (∼2 µm/sec) until the microparticles contacted the LC. When using microparticles, we found this method of transfer led to more reproducible results than what was achieved using a previously reported method in which the LC-filled grid was plunged down through the air-water interface.30,31 The home-built trough used in these studies was 5 cm × 3.5 cm × 1 cm in size and was mounted on a polarized light microscope specimen stage. Results and Discussion The first experiment we performed sought to determine if the nematic order of the LC influenced the transfer of microparticles from the surface of water onto the LC-water interface. Figure 2A shows an optical micrograph of PS microparticles spread at the surface of water at a density of 2250 ( 150 microparticles/mm2. The microparticles were then transferred to the nematic 5CB-water interface using the modified Langmuir-Schaefer method, as described in the Experimental Section. After transfer to the nematic 5CB-water interface, the PS microparticle density was measured to decrease to 790 (

Figure 2. (A-F) Brightfield micrographs of DODAB-coated PS microparticles (diameters of 19 µm) at the air-water interface (panels A and D) and after transfer to a nematic 5CB-water interface (panels B and C) or an isotropic silicone oil-water interface (panels E and F). The brightfield micrographs were digitally enhanced to accentuate the contrast between the PS microparticles and background regions at the interfaces. Scale bars correspond to 150 µm. (G-I) Illustrations depicting the process of transfer of PS microparticles from the air-water interface to the LC-water interface via the Langmuir-Schaefer method.

200 microparticles/mm2, indicating that only ∼35% of the microparticles at the surface of the water were transferred to the 5CB-water interface (Figure 2B). In contrast, inspection of Figure 2C reveals that the density of microparticles outside of the area of contact of the water and nematic 5CB (i.e., the interface between water and OTS-treated glass) was much greater than the density of microparticles at the nematic 5CB-water interface (see Figure 2I). This observation indicates that the microparticles were expelled laterally from the area of contact between the nematic 5CB and water. To determine if the lateral expulsion of microparticles was due to nematic forces,

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Lin et al.

Figure 3. Brightfield micrographs of DODAB-coated PS microparticles after transfer from an air-water interface to (A) a LC-water interface or (B) a silicone oil-water interface. Scale bar corresponds to 150 µm.

PS microparticles at the surface of water were brought into contact with an isotropic (silicone) oil using procedures identical to those used with nematic 5CB. In this control experiment, the initial microparticle density at the air-water interface was 1500 ( 170 microparticles/mm2 (Figure 2D). After transfer of the microparticles to the isotropic oil-water interface (Figure 2E), the microparticle density was 1570 ( 100 microparticles/ mm2, indicating complete transfer of PS microparticles to the isotropic oil-water interface. Consistent with this result, when using the isotropic oil, the density of microparticles outside of the grid was not seen to be elevated (Figure 2F), as in the case of the nematic oil (Figure 2C). The results presented above led us to conclude that the ordering of the 5CB molecules in the nematic phase influenced the process of transfer of the microparticles to the LC-water interface. Our observations suggest that the process of transfer of microparticles to the nematic 5CB-water interface occurs as illustrated in Figure 2G-I. As shown in Figure 2H, upon initial contact of the LC and water phases, an air-water interface is directly adjacent to the newly formed LC-water interface (because the grid protrudes beyond the surface of the OTStreated glass). Our results indicate that upon initial contact of the LC with the water, microparticles are expelled laterally from the 5CB-water interface to this air-water interface. Subsequently, as the water level in the Langmuir trough is raised further, the microparticles expelled laterally to the air-water interface are deposited onto the interface that forms between the OTS-treated glass and water, where they were imaged (Figure 2I). Our observations also reveal that the microparticles are expelled laterally when using nematic 5CB but not the isotropic oil, indicating that the nematic ordering of the LC is the origin of the lateral forces that lead to transfer of the PS microparticles from the 5CB-water interface to the air-water interface. Attempts to transfer microparticles to a 5CB-water interface with 5CB heated above the nematic-to-isotropic transition temperature were unsuccessful due to condensation of water at the 5CB-air interface prior to contact with the aqueous phase. We estimate the energetic penalty associated with distortion of the LC around the microparticles at the LC-water interface to be ∼KA, where K (10-11 N for 5CB) is the elastic constant of the LC and A is the microparticle radius (9.5 µm for the microparticles used in our experiments).33 For the system described above, this energetic penalty corresponds to ∼20 000 kT and is thus likely at the origin of the large forces that lead to the rapid lateral displacement of the PS microparticles. This force does not exist for the experiments performed with the isotropic oil. The results described above demonstrate that it is possible to prepare microparticle-decorated LC-water interfaces using the Langmuir-Schaefer method, and that (i) the microparticle density is lower than that initially spread at the surface of the water, and (ii) forces mediated by the nematic order of the LC are acting on the microparticles at the LC interface. In addition

to laterally expelling the microparticles from the LC-water interface, LC-mediated interparticle interactions were also observed to organize the microparticles into extended chainlike structures (Figure 3A). Several observations suggest that the nematic order of the LC plays a role in the observed chaining of microparticles. First, chaining of PS microparticles was not observed at the interface between the OTS-treated glass and water (regions adjacent to the 5CB-filled gold grid). Second, when PS microparticles at a similar interfacial density to that shown in Figure 3A were transferred to an isotropic oil (silicone)-water interface, no evidence of comparable chaining was found (Figure 3B). Although aggregation of microparticles is evident at the isotropic oil-water interface, the aggregates do not comprise chains as seen in the experiments employing nematic 5CB. The chaining of microparticles that we observed was unique to the 5CB-water interface and thus we attribute the formation of these structures to LC-mediated interparticle forces. Later in this paper, we return to consider in more detail the origin of these forces. Next, we sought to determine the processes leading to the organization of microparticles at the LC interface, as well as the influence of the microparticles on the ordering of the LC. The results of a typical experiment employing polarized and brightfield microscopy are shown in Figure 4A-L (the images on the left are micrographs obtained with crossed polars and the images on the right are brightfield micrographs), where the initial density of DODAB-coated PS microparticles at the air-water interface was 1640 ( 220 microparticles/mm2 and the resulting density of microparticles at the LC-water interface was 620 ( 200 microparticles/mm2. Figure 4A shows a polarized light micrograph of two grid squares containing 5CB just prior to contact of the LC with the surface of water on which the PS microparticles were spread. In Figure 4A, the nematic 5CB appears dark because of perpendicular ordering of the LC at both the 5CB-OTS interface and the 5CB-air interface (as shown in Figure 5A). At the instant of contact between the nematic 5CB and water phases, the appearance of the 5CB between crossed polars became bright (Figure 4C). The change in optical appearance of the nematic film was caused by an ordering transition in the LC induced by contact with water: past studies have established that nematic 5CB orients parallel to an interface with water.34 Contact of the LC with water thus introduced splay and bend distortions into the LC film (see Figure 5B for a schematic illustration). Inspection of Figure 4E,G,I reveal that this state of the LC was a transient one, and that over a period of 180 s that followed contact of the LC with the water, a second ordering transition occurred within the LC (evident as the appearance of dark regions spreading across the LC in Figure 4E,G,I). Conoscopic inspection of the dark regions revealed that they corresponded to uniform perpendicular ordering of the LC film, thus indicating that the introduction of the DODAB-coated PS microparticles triggered a slow transition (∼180 s) of the LC to ordering that

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Figure 4. Polarized light (panels A,C,E,G,I and K, all with crossed polars) and bright-field (panels B,D,F,H,J and L) micrographs of the timedependent organization of DODAB-coated PS microparticles following transfer of the microparticles from an air-water interface to a LC-water interface. Times are indicated in units of seconds and t ) 0 corresponds to the instant of contact of the microparticle-decorated surface of water with the LC. Scale bar corresponds to 300 µm.

was perpendicular to the interface with water (Figure 4E, G, I, and K). In contrast to the above results, when DODAB-coated PS microparticles were transferred to the 5CB-water interface at densities that were less than 50 microparticles/mm2, we did not observe a transition to perpendicular ordering of the film of 5CB, but instead the LC film retained an optical appearance similar to that seen in Figure 4C (see Supporting Information, Figure S2C).

The results above, when combined, clearly indicate that the ordering of the LC was influenced by the presence of the DODAB-coated PS microparticles. By using brightfield microscopy (Figure 4B,D,F,H,J,L), we also observed the process of microparticle chaining. In Figure 4B, the blurred outlines of the PS microparticles at the air-water interface are evident just prior to transfer onto the LC interface. Figure 4C shows the microparticle organization immediately after transfer. Inspection

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Figure 5. (A and B) Schematic illustrations of 5CB confined to specimen grids (side views). The orientation of 5CB is perpendicular at the OTS-treated interface and (A) perpendicular at the interface to air or (B) parallel at the interface to water. Side view (C) and top view (E) of the ordering of nematic 5CB (dashed lines) after the transfer of DODAB-coated PS microparticles to a LC-water interface that causes parallel ordering of the LC. Side view (D) and top view (F) of the ordering of 5CB around a DODAB-coated PS microparticle with perpendicular ordering of LC at the LC-water interface. Small dots indicate in the plane of the page. (G) Polarized light micrograph of DODAB-coated PS microparticle with a dipolar defect at the LC-water interface. Optical micrographs of chains of DODAB-coated PS microparticles at a LC-water interface that causes a perpendicular ordering of the LC; (H) crossed polars and (I) brightfield. The particles shown in G-I have diameters of 19 µm.

of the grid on the left in Figure 4C reveals that some microparticles are rapidly organized into chainlike structures upon contact with the nematic LC. In contrast, in the grid square on the right side of the image in Figure 4C, the process of assembly of the PS microparticles into chainlike structures is seen to be slower. Red arrows have been added to Figure 4C-H to highlight the slow process of organization of microparticles that occurs over a period of 78 s following contact of the LC with the water. Over this interval of 78 s, the 3 trimers evident in Figure 4C assemble into a 9-mer in Figure 4E, which

Lin et al. eventually associates with a 4-mer on the edge of the grid, as seen in Figure 4H. Inspection of Figure 4K reveals that the organization of the microparticles does not change over periods of time longer than 78 s. Here, we emphasize that both the fast process of organization (rapid formation of chains seen in the left grid) as well as the slow processes of assembly (right grid) were typical and representative of multiple experimental observations. The results described above and shown in Figures 3 and 4 clearly demonstrate that LC-mediated interparticle interactions influence the organization of microparticles at LC-water interfaces. Past studies have shown that interparticle interactions in bulk LCs are mediated by topological defects that form around the microparticles.5,26,27,29,33,35,36 When the DODABfunctionalized microparticles initially contacted the LC-water interface in our experiments, the ordering of the LC was locally perpendicular at the microparticle surface and parallel to the LC-water interface (see Figure 5C for a side view and Figure 5E for a top view; these cartoons are discussed in more detail below). Our optical observations also indicate that the midplane of the microparticles was close to the LC-water interface. The symmetry of the director profile at the midplane of the microparticles, when viewed from above, is thus similar to microparticles with perpendicular anchoring dispersed in bulk LC, leading to prediction of either a quadrupolar (saturn ring) defect or a dipolar (hyperbolic hedgehog) defect.26,35 Experimentally, it was difficult to observe the defects during the microparticle assembly process when using high microparticle densities because the microparticles contacted one another and quickly formed chains. We also note that the defect structures of ensembles of microparticles in close proximity are more complex than isolated microparticles and are not necessarily additive.37 However, we were able to clearly image isolated microparticles deposited at the LC-water interface and the resulting defect structures were determined to process dipolar symmetry (Figure 5G and see discussion in Supporting Information) and thus are consistent with the director profiles shown in Figure 5C and 5E. The observation of dipolar defects parallel to the LC-water interface give rise to the chaining observed for high densities of microparticles. We also note that the origins of the dipolar interactions observed in our experiments differ from those reported to lead to the organization of glycerol microdroplets at interfaces between nematic LCs and air.4-6 The interdroplet forces in past studies with glycerol droplets were attributed to dipolar interactions between boojum defects caused by parallel ordering of the LC at the interface of the glycerol microdroplets.5 Chaining of the glycerol microdroplets was proposed to be due to tilting of the surface dipoles away from the surface normal with decreasing thicknesses of LC. As mentioned above, we observed transfer of DODAB-coated microparticles onto the LC-water interface to cause a transition in the orientation of the LC from parallel to perpendicular to the LC-water interface. At long times, the LC film was observed to adopt uniform perpendicular ordering in all regions of the interface except for very near the microparticle surface (Figure 5F, top view). The symmetry of this physical situation should result in either a surface ring or a dipolar defect (Figure 5D) orthogonal to the LC-water interface. A dipolar defect oriented parallel to the surface normal would result in interparticle repulsion. Close inspection of Figure 5H reveals that the microparticles have a bright corona with a cross near the microparticle surface, consistent with strong perpendicular ordering of the LC at the microparticle surface. Additionally, inspection of Figure 5I reveals that many of the microparticles

Ordering of Solid Microparticles (noted with red arrows) have point defects below them, an observation that is consistent with Figure 5D, though some of the defects are displaced from the center of the microparticle. This result suggests that the dipoles associated with the point defect seen in Figure 5G rotates to an orientation close to the normal of the LC-water interface when the orientation of the LC at the LC-water interface transitions to the perpendicular. Although this orientation of dipoles should lead to repulsive interactions between microparticles, we observed the chain-like assemblies of microparticles formed at the LC-water interface to persist when the 5CB was heated to 50 °C, a temperature that is greater than the nematic-to-isotropic transition temperature (33.5 °C38) (see Supporting Information, Figure S1D-F). This result suggests that short-range interparticle interactions, which are not dependent on the nematic order of the LC, act between the microparticles and preserve the organization that is initially directed by the nematicity of the LC. We also note that some of the microparticles evident in the experiments shown in Figures 3A and 4 L were already in contact with one another at the air-water interface, prior to transfer onto the LC-water interface. A surprising result of our study, as seen in Figure 4K, is the perpendicular ordering of the LC evident in regions of the interface that are distant from the microparticles. To determine if DODAB liberated from the surfaces of the PS microparticles plays a role in the perpendicular ordering of the LCs observed after transfer of the microparticles (see Supporting Information, Figure S2), we performed several additional experiments. First, we sought to determine if DODAB dissociated from the microparticles at the air-water interface prior to contact with the LC. To test this possibility, we spread a high density (>1000 microparticles/mm2) of DODAB-coated PS microparticles at the air-water interface and then contacted 5CB supported within a gold grid with the surface of water in a region where there were few microparticles. The resulting optical textures of the LC (Supporting Information, Figure S2D) were indistinguishable from those seen using water on which microparticles were not spread (Figure 5B, Supporting Information, Figure S2A). This result indicated that DODAB liberated from the microparticles at the air-water interface was not responsible for the LC ordering transition seen in Figure 4K. Next, we performed an experiment to determine if DODAB dissociated from the PS microparticles upon contact with 5CB. To this end, DODABfunctionalized microparticles were immersed in 5CB, and then the 5CB (free of the microparticles) was used to fill a grid supported on an OTS-treated glass surface. Contact of the 5CB with water resulted in optical textures of 5CB (Supporting Information, Figure S2E) that were again similar to those obtained with 5CB free of DODAB. While these control experiments did not yield evidence of a role for free DODAB in the LC ordering transition seen in Figure 4K, we speculate that DODAB is in fact playing a role in this phenomenon. It seems unlikely to us that the microparticles would promote perpendicular ordering of the LC over a distance that extends ∼100 µm from microparticles when the thickness of the film of LC is ∼20 µm. In future studies, we will use fluorescently labeled amphiphiles to determine if they are liberated from the microparticles when located at the LC-water interface. 4. Conclusions In summary, the main findings reported in this article are fourfold. First, we have demonstrated a method that permits transfer of solid microparticles from the surface of water onto a planar LC-water interface, thus enabling investigations of

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16557 the coupling that occurs between the order of the LC and the organization of the microparticles. Second, we have found that during the process of transfer of microparticles to the LC-water interface, the nematicity of the LC generates lateral forces that act on the microparticles to expel them from the LC-water interface. This result is a particularly significant one as it suggests new ways to manipulate microparticles in monolayers at fluid interfaces. Third, the nematicity of the LC leads to interparticle forces that promote the formation of chain-like structures of microparticles at the LC-water interface. These organized structures are not seen when using isotropic oils, thus demonstrating that nematic LCs can be exploited to generate new types of microparticle organizations that are not seen with isotropic oils. Optical textures of the LC in the vicinity of the microparticles are consistent with formation of defects with dipolar symmetry capable of promoting the chaining of the microparticles. Fourth, we have observed that DODAB-coated microparticles transferred to the LC-water interface trigger an ordering transition within the LC; the ordering transition is surprisingly slow and may be influenced by the liberation of DODAB from the surfaces of the microparticles. Overall, these results demonstrate the strong coupling that occurs between microparticles and LC ordering at interfaces. Acknowledgment. This research was partially supported by the University of Wisconsin, Nanoscale Science and Engineering Center (DMR-0425880). We would like to thank Mr. John Cannon for help with fabrication of our Langmuir trough. Supporting Information Available: Description of LC brush textures, brightfield micrographs of microparticles transferred to a silicone-water interface and transferred to a 5CB-water interface and heated above the nematic-to-isotropic transition temperature of 5CB, and polarized light micrographs of liberated DODAB control experiments. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kralchevsky, P. A.; Nagayama, K. AdV. Colloid Interface Sci. 2000, 85, 145. (2) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569. (3) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (4) Nazarenko, V. G.; Nych, A. B.; Lev, B. I. Phys. ReV. Lett. 2001, 87, 075504. (5) Smalyukh, I. I.; Chernyshuk, S.; Lev, B. I.; Nych, A. B.; Ognysta, U. M.; Nazarenko, V. G.; Lavrentovich, O. D. Phys. ReV. Lett. 2004, 93, 117801. (6) Nych, A. B.; Ognysta, U. M.; Pergamenshchik, V. M.; Lev, B. I.; Nazarenko, V. G.; Musevic, I.; Skarabot, M.; Lavrentovich, O. D. Phys. ReV. Lett. 2007, 98, 057801. (7) Oettel, M.; Dietrich, S. Langmuir 2008, 24, 1425. (8) Park, B. J.; Pantina, J. P.; Furst, E. M.; Oettel, M.; Reynaert, S.; Vermant, J. Langmuir 2008, 24, 1686. (9) Onoda, G. Y. Phys. ReV. Lett. 1985, 55, 226. (10) Danov, K. D.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2006, 22, 106. (11) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226. (12) Giermanska-Kahn, J.; Schmitt, V.; Binks, B. P.; Leal-Calderon, F. Langmuir 2002, 18, 2515. (13) Tarimala, S.; Lenore, L. D. Langmuir 2004, 20, 3492. (14) Vignati, E.; Piazza, R. Langmuir 2003, 19, 6650. (15) Kim, B.; Tripp, S. L.; Wei, A. Mater. Res. Soc. Symp. Proc. 2001, 676, Y61.1. (16) Stein, A.; Schroden, R. C. Curr. Opin. Solid State Mater. Sci. 2002, 5, 553. (17) Sidhaye, D. S.; Kashyap, S.; Sastry, M.; Hotha, S.; Prasad, B. L. V. Langmuir 2005, 21, 7979. (18) Hosoki, K.; Tayagaki, T.; Yamamoto, S.; Matsuda, K.; Kanemitsu, Y. Phys. ReV. Lett. 2008, 100, 207404. (19) Kim, J.; Lee, D. J. Am. Chem. Soc. 2006, 128, 4518.

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