Article Cite This: Langmuir 2019, 35, 7791−7797
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Reconfigurable Microcube Assemblies at the Liquid/Air Interface: The Impact of Surface Tension on Orientation and Capillary-ForceInteraction-Driven Assembly Qimeng Song,† Mengdi Zuo,† and Holger Schönherr* Physical Chemistry I and Research Center of Micro and Nanochemistry and Engineering (Cμ), Department of Chemistry and Biology, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany Downloaded via UNIV OF SOUTHERN INDIANA on July 27, 2019 at 16:11:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: The systematic investigation of the dependence of the orientation and capillary interaction of hydrophobized polystyrene microcubes at the liquid/air interface on the surface tension of the aqueous subphase is reported. By decreasing the subphase surface tension, the preferential orientation of the cubes was observed to change independent of the surfactant type from the vertex up to the edge up and finally to the face up. Concomitantly, the structure of the aggregates obtained by cube assembly was observed to change from a close-packed hexagonal to tilted linear and finally to flat plate. In particular, the preferential orientation of the cubes was virtually independent of the surfactant charge at a constant surface tension. In addition, reconfigurable microcube assemblies at the liquid/air interface, which respond to the surface tension of the subphase, were observed for the first time. The dynamic reconfigurability of preformed microcube aggregates induced by adding surfactant to the subphase may open new pathways to dynamic assemblies at liquid/air interfaces, which may be interesting, e.g., for sensing applications.
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INTRODUCTION Reconfigurable matter has been recently identified as an important new subclass of materials in the area of smart materials.1,2 For instance, smart hydrogels, which respond to environmental changes like temperature or pH by shape transformations were exploited by the groups of Kumacheva3 and Agarwal.4 Likewise, temperature-induced self-folding materials were explored by Zhang et al.5 A second field of interest is comprises hybrid materials formed by particle assembly.6,7 For instance, Ling et al. have exploited threedimensional ordered particle arrays, which were assembled with nanoparticles, as self-supported free-standing bridges on topographically patterned substrates.8 By using sequential capillarity-assisted particle assembly, versatile composite particle arrays and particle gradients with confined structures were obtained by Wolf and co-workers.9−11 Within the area of particle assembly mentioned above, the self-assembly of colloids adsorbed at the liquid/liquid or the liquid/air interface has been studied for a long time and has been widely utilized in emulsion stabilization, drug delivery, and fabrication of functional materials.6,12,13 Unlike the selfassembly of homogeneous spherical particles, which possess only one orientation at the interface and are hence well understood, the self-assembly of anisotropic particles, for instance, cubic particles, was only rarely investigated, presumably due to the complex orientation at the interface. Particles adsorbed at liquid/liquid or the liquid/air interfaces cause interface deformations as a consequence of (partial) © 2019 American Chemical Society
wetting. This deformation gives rise to capillary forces, which become increasingly important once the deformed interfaces start to overlap. As a result, the particles may self-assemble into ordered aggregates if the interactions are attractive and the particle shape favors ordered assemblies. By controlling the shape of the particles on the one hand and the wetting via controlled chemical properties of different particle faces on the other hand, capillary multipoles, such as monopoles, dipoles, quadrupoles, or hexapoles, were obtained at those interfaces, which led to the assembly of particles into aggregates with different structures.14,15 Landmark work in this area was reported in the 1990s by the group of Whitesides, who used flat tilelike particles.16,17 The main factors that govern the type of assembly obtained at the water/air or liquid/liquid interfaces are particle wetting, interfacial tension, and density. With increasing density of the aqueous phase, the dominating capillary force for tilelike millimeter-scale objects at the water/perfluorodecalin interface was reported to change from hydrophobic side−hydrophobic side interaction to hydrophilic side−hydrophilic side interaction by Mao et al.18 Also, Ag nanocubes, functionalized with a mixed monolayer of thiol-terminated poly(ethylene glycol) and hexadecanethiol, were observed to change the configuration and the structure of the self-assembled metacrystals at Received: April 15, 2019 Revised: May 15, 2019 Published: May 23, 2019 7791
DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
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Figure 1. (a) Surface tension of aqueous SDS solutions as a function of SDS concentration. (b) Static contact angles measured on a hydrophobized PS thin film (θ measured with pure water is 102°) as measured with different concentrations of aqueous SDS solutions as a function of surface tension. The surface tension data were measured with the Wilhelmy plate technique (n = 3). The red line corresponds to a linear least-squares fit.
Here, we investigate the dependence of the orientation and self-assembly of hydrophobized 30 μm polystyrene (PS) microcubes at the aqueous subphase/air interface on the surface tension and on the type of surfactant applied to the aqueous subphase. The marked dependence of the orientation of the cubes on surface tension and the dynamics of the selfassembly process afforded a first example of dynamically reconfigurable microcube assemblies at the water/air interface.
the oil/water interface by varying the solvent polarity, which gives rise to a conformation change on the nanocube surfaces.19 Finally, it was reported that different capillary multipoles could be obtained from identically shaped cubic particles when their orientation changed from face up to edge up or vertex up.20−22 This leads to the formation of different structures at the corresponding interfaces, such as squareshaped, linear, or honeycomb (graphene-like) close-packed hexagonal structures.22−25 Also, for micrometer-sized cubes, similar experimental observations were reported.26,27 Likewise, the capillary interaction of vertex-up-oriented cubes was investigated theoretically. Soligno et al. reported a prediction of close-packed hexagonal and honeycomb-structured aggregates.28,29 According to Pieranski’s free-energy change theory,30 the free-energy change of a particle adsorbed at the water/air interface (ΔE) can be derived as ΔE = γwa × (A pa × cos θ − Ac) + A × γpw
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EXPERIMENTAL SECTION
The materials and the experimental procedures for the fabrication of PS microcubes used in this work were adopted from a previous work and are described in refs 26 and 27. Sodium dodecyl sulfate (SDS) was purchased from Sigma-Aldrich. Tetradecyltrimethylammonium bromide (TTAB) was purchased from Fluka. Triton X-100 (Mw = 647 g/mol) was purchased from VWR. Surface Tension Measurements. Aqueous stock solutions of the three different surfactants, SDS, TTAB, and Triton X-100, were prepared by dissolving the surfactants into Milli-Q water and placing the solutions into an ultrasonic bath for 20 min. A set of surfactant solution with different concentrations was prepared by diluting these stock solutions with Milli-Q water. Surface tension measurements were carried out at 21.2 ± 0.7 °C using a DCAT 11EC tensiometer (Dataphysics, Filderstadt, Germany) with a Wilhelmy plate (platinum−iridium, a = 19.90 mm, b = 0.20 mm). Three individual measurements were done for each solution, and the values are reported as the mean ± standard deviation, σ. Contact Angle Measurements. The static contact angles were determined by the sessile drop technique (Dataphysics OCA-15, Filderstadt, Germany) at ambient conditions. Two microliters of Milli-Q water or aqueous SDS solution with different concentrations was dropped onto the hydrophobized PS film for each measurement. For each sample, three different positions were measured, and the values are reported as the mean ± σ. Microcube Self-Assembly at Liquid/Air Interfaces with Different Surface Tensions. The microcubes fabricated according to ref 27 were released from the substrate and dispersed into aqueous surfactant solutions with different concentrations in a Petri dish (tissue culture polystyrene (TCPS), Sarstedt, Germany). Energy was supplied by stirring with a stirring bar (length: 10 mm, ⌀ = 3 mm) for at least 2 h at a speed of 400 rpm. SDS solutions with concentrations of 1.0, 2.0, 3.0, 4.0, 6.0, and 8.0 mM; TTAB solutions with a concentrations of 0.2, 0.3, 2.0, and 4.0 mM; and Triton X-100 solutions with a concentrations of 6.00, 33.0, 237, and 590 μm were used. Reconfiguration of Cube Orientation and Assemblies. PS microcubes were first self-assembled at the water/air interface in a TCPS Petri dish (⌀ = 2 cm) with 6 mL of Milli-Q water for at least 2 h. Afterward, the SDS stock solution was added to the liquid phase successively to increase the concentration of SDS to 1.0, 2.0, 3.0, 4.0, and 6.0 mM. After reaching each concentration of the SDS solution,
(1)
where γwa and γpw denote the interfacial free energies per unit area of the water/air and particle/water interfaces, respectively; Apa is the surface area of the particle in contact with air; Ac is the area of the water/air interface, which is replaced by particle; A is the total area of the particle surface; and θ is the particle’s contact angle according to Young’s equation31 (eq 2). γ − γsl cos θ = sv γlv (2) where θ is the contact angle and γsv, γsl, and γlv are the interfacial tensions between solid and vapor, solid and liquid, and liquid and vapor, respectively. It was found in a previous study that the orientation of 30 μm microcubes and their capillary interaction at the water/air interface are strongly influenced by the wettability of the cube surfaces.26,27 When the wettability of the cube surface was changed from hydrophilic to hydrophobic, the preferred orientation of microcubes was observed to change from face up to edge up and further to vertex up, which resulted in the assembly of aggregates with flat-plate, tilted linear, and closepacked hexagonal structures. Equation 1 implies that the surface tension of the liquid phase should also play an important role. Surface tension is one of the main parameters in Young’s equation (eq 2), which defines the contact angle of a probe liquid on a solid surface, and it is relevant for the calculation of the corresponding free-energy change. Hence, surface tension should also influence the orientation of cubic microparticles and their assembly into ordered aggregates at the liquid/air interface. 7792
DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
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Figure 2. Optical micrographs showing hydrophobized PS microcubes (θwater = 102°) self-assembled at the interface of aqueous SDS solutions with air for different SDS concentrations: (a) 0 mM, (c) 3.0 mM, and (e) 6.0 mM. Optical microscopy images of self-assembled cube aggregates with (b) close-packed hexagonal, (d) tilted linear, and (f) flat-plate structure at the interface of aqueous SDS solutions with air, with SDS concentrations of 0, 3.0, and 6.0 mM, respectively. the solution was stirred for at least 2 h at a speed of 400 rpm. Then, the cubes were observed by optical light microscopy (Primovert, Carl Zeiss, Oberkochen, Germany).
were observed to decrease with decreasing surface tension of the aqueous SDS solution. A linear relationship between cos θ and surface tension was obtained, which is in a good agreement with the Zisman plot,36 even though the Zisman equation was originally developed for pure liquids. PS microcubes (30 μm, θwater = 102°) were fabricated by nanoimprint lithography (NIL) (for details, see ref 27). After detachment from the substrate, the microcubes were dispersed into the interface of aqueous SDS solutions and air for different concentrations of SDS. It was observed that close-packed hexagonal aggregates assembled at the interface of pure water after 2 h of continuous stirring (Figure 2a,b). However, at the interface of an aqueous SDS solution and air with an SDS concentration of cSDS = 3.0 mM, the cubes were observed with an edge-up orientation, which resulted in the assembly of tilted linear aggregates (Figure 2c,d). A number of fiberlike linear cube aggregates at the liquid/air interface can be clearly observed in the overview image (Figure 2c). Furthermore, it can be observed in Figure 2f that most of the cubes possess a face-up orientation at the interface with a SDS concentration of cSDS = 6.0 mM. The assembled aggregates showed a flatplate structure. Cubes at the liquid/air interface changed the orientation from vertex up to edge up and further to face up with the decreasing surface tension from 72.6 ± 0.1 mN/m (pure water) to 49.2 ± 1.0 mN/m (cSDS = 3.0 mM) and further to 37.1 ± 1.1 mN/m (cSDS = 6.0 mM). These changes did not occur stepwise, but instead a coexistence of two types of preferred structures was noted (Figure S2). Statistical analyses of the orientation as observed for microcubes in assembled aggregates at the interface of an aqueous SDS solution and air with different SDS concentrations are shown in Figure S3. This trend is similar to the change in cube orientation with decreasing cube contact angle, which was reported in previous studies.26,27 It is known that single cubes with specific surface
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RESULTS AND DISCUSSION The surface tension of the aqueous subphase should affect the orientation of the microcubes at the aqueous subphase/air interface. Hence, to investigate the dependence of the cube orientation and resulting capillary interactions on the surface tension of the liquid subphase, the surface tensions of the surfactant solutions at different concentrations approaching the critical micelle concentration (CMC) were determined for all three surfactants used. Figure 1a shows the data for aqueous solutions of the anionic surfactant SDS, while the data for the cationic surfactant TTAB and the nonionic Triton X-100 are shown in Figures S1. The well-known trend of decreasing surface tension with increasing concentration of the surfactant was confirmed in all cases. For SDS, TTAB, and Triton X-100, the approximate CMC values of ∼8.0, ∼2.7, and ∼0.47 mM, respectively, were observed. These values are in full agreement with the results reported by others (8.0, 3.3, and 0.45 mM for SDS, TTAB, and Triton X-100, respectively),32−34 although it is known that the CMC varies in practice due to small concentrations of surface-active impurities.35 To simulate the contamination of the PS surface by PDMS oligomers,26,27 which renders the PS surface more hydrophobic compared to neat PS, hot embossing was carried out on the PS thin film with a flat featureless PDMS. The influence of the surface tension of liquid on the static contact angle on imprinted PS thin films was investigated by contact angle measurements with different concentrations of SDS solution. The result is shown in Figure 1b. Due to the inverse relationship between the contact angle and the surface tension based on Young’s equation, the contact angle values on PS 7793
DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
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Figure 3. Optical micrographs of self-assembled hydrophobized PS microcubes at the interface of different aqueous surfactant solutions with identical surface tension of 58.3 ± 0.5 mN/m (for) and air: (a) SDS, (b) TTAB, and (c) Triton X-100. (d) Statistical analyses of orientation as observed for microcubes in assembled aggregates: face up, edge up, and vertex up (more than 800 of cubes were analyzed for each surfactant aqueous solution).
wettability possess a preferred orientation.20,21,28 Thus, presumably single cubes possess a preferred orientation at the aqueous subphase/air interface for a specific surface tension. However, the energy barriers between the states of preferred orientation and other orientations, which are still unknown so far, are different for subphases with different surface tensions. Several orientations of cubes likely possess similar interface free energy. In that case, the total interface free energy of all aggregates should be considered. In addition, the different orientations observed for different SDS concentrations, namely, vertex up, edge up, and face up, correspond to different patterns of surface deformation. The associated hexapolar, quadrupolar, and monopolar surface deformations result in the self-assembly of microcubes into aggregates with close-packed hexagonal, tilted linear, and flatplate structure, respectively. Although the orientation of the microcubes at the liquid/air interface depends mainly on surface wettability26,27 and surface tension (see above), the role of the negative charge in SDS remains uncertain. Madivala et al. reported for instance that the charge on shape-anisotropic colloids could affect the interaction between colloids and the obtained assembled aggregates.37 To unveil the potential effect of charge on the orientation of microcubes and the assembled structures formed at the liquid/air interface for similar surface tension values, surfactant solutions with differently charged surfactant were applied. The data obtained for the anionic surfactant SDS were compared to those of the cationic surfactant TTAB (Figures S4 and S5) and the nonionic surfactant Triton X-100 (Figures S6 and S7). By controlling the concentration of the surfactants in the aqueous solution, different solutions with identical surface tensions, namely, 45.4 ± 0.3, 58.3 ± 0.5, and 68.0 ± 0.9 mN/ m, were prepared. Subsequently, the orientation of the cubes and the structure of the self-assembled aggregates formed after stirring were studied by optical microscopy (Figure 3).
No obvious differences of cube orientation and structure of self-assembled cube aggregates were observed among the various aqueous solutions with identical surface tensions of the differently charged surfactants. For instance, cubes at the liquid/air interface of SDS, TTAB, and Triton X-100 at a constant surface tension of 58.3 ± 0.5 mN/m (Figure 3a−c) assembled dominantly in the vertex-up orientation. Those cubes assembled into close-packed hexagonal aggregates, which coexisted with a small number of edge-up-oriented cubes in linear aggregates. Microcubes with θwater = 102° prefer a vertex-up orientation at the water/air interface when the subphase has a surface tension γ = 72.6 ± 0.1 mN/m. Driven by capillary interactions between hexapolar interface deformations, the cubes selfassembled into close-packed hexagonal aggregates. However, with decreasing surface tension of the liquid phase, the cubes start to prefer the edge-up orientation. Based on the interaction between quadrupolar interface deformations of edge-up cubes, linear aggregates were assembled. The change in cube orientation caused by different surface tensions of the liquid phase shows a reasonable agreement with the changes induced by cube surface wettability. The probability of each of the three cube orientations, i.e., face up, edge up, and vertex up, at the liquid/air interface with different surfactants but identical surface tension (γ = 58.3 ± 0.5 mN/m), was analyzed for more than 800 cubes. More than 70% of the cubes were observed with the vertex-up orientation in close-packed hexagonal aggregates for different surfactant solutions. Ten to twenty percent of the cubes were observed with the edge-up orientation and assembled into linear aggregates. Finally, less than 10% of the cubes showed a face-up orientation for different surfactant solutions. No obvious difference between different surfactant solutions was observed at 58.3 ± 0.5 mN/m surface tension. Similarly, no systematic differences of cube orientation and aggregate structure were found for the different surfactant 7794
DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
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Langmuir solutions with γ = 68.0 ± 0.9 mN/m (Figure S8). Of course, due to the change of wetting and surface tension, more cubes were observed with a vertex-up orientation. The data for all of experiments are summarized in the histograms in Figure 4.
Figure 4. Statistical analysis of the orientation (face up, edge up, and vertex up) as observed for hydrophobized PS microcubes in assembled aggregates at the interface of various aqueous surfactant solutions and air with different surface tensions: 45.4 ± 0.3, 58.3 ± 0.5, and 68.0 ± 0.9 mN/m (more than 800 cubes were analyzed for each surfactant and surface tension).
Figure 5. (a) Schematic of the continuous transformation of cube orientation by decreasing surface tension at the liquid/air interface. (b) Optical microscopy images of self-assembled hydrophobized PS microcube aggregates, which changed their structure dynamically by increasing the concentration of SDS in the aqueous subphase. The insets show schematics of the cube orientation and aggregate structure (top view). (c) Statistical analysis of the orientation as observed for microcubes in self-assembled aggregate. (∼1000 cubes were analyzed for each surface tension. The dotted lines serve as guide for the eyes).
Only for a surface tension of 45.4 ± 0.3 mN/m, the nonionic surfactant Triton X-100 showed a significant difference. Here, more face-up cubes were observed compared to the other surfactant solutions (Figure S9). This can be attributed to the difference in the wetting behaviors of nonionic and ionic surfactant aqueous solutions. It was reported that in comparison to ionic surfactants, nonionic surfactants showed enhanced spreading on highly hydrophobic surfaces, which resulted in a lower contact angle.38,39 Thus, presumably Triton X-100 was adsorbed on the PS microcube surface, which should contribute to a slightly altered surface wettability.40 We can conclude at this stage that the charge of the surfactant plays at most a marginal role in cube orientation at the liquid/air interface. Due to the negligible electrostatic forces between cubes compared to the capillary force, the charge of surfactants does not influence the capillary interaction. The marked change in cube orientation for different surface tensions discussed above can afford assemblies of different structures, which are stable enough to be transferred to solid supports. However, it is a priori not clear how cubes, which are already in the most stable orientation or cubes within the aggregates at the liquid/air interface, respond to a change in surface tension. To answer this question, hydrophobized PS microcubes were first dispersed at the interface of pure water and air. After stirring for ≥2 h to obtain stable assemblies, a certain amount of SDS was added to the aqueous subphase to change the surface tension. The orientation of microcubes and the structure of the aggregates formed after stirring again for at least 2 h were determined with optical microscopy. As can be observed in Figure 5, the microcubes self-assembled into closepacked hexagonal aggregates at the water/air interface, which is a result of the vertex-up cube orientation (Figure 5b, 0 mM SDS). However, when SDS was added to the aqueous solution, a fraction of the cubes changed their orientation from vertex up to edge up after stirring. This in turn resulted in an altered aggregate structure. The assemblies observed changed from close-packed hexagonal to tilted linear. Furthermore, as more
SDS was added to the subphase, the fraction of the cubes with changed orientation increased. When the concentration of the SDS aqueous solution reached 3.0 mM, cubes with the vertexup orientation were only rarely observed; most of the cubes were observed with the edge-up orientation in tilted linear aggregates, which coexisted with a small fraction of the face-up cubes (Figure 5b, 3.0 mM). With a further increase in SDS concentration, the orientation of the cubes changed from edge up to face up. Eventually, at 6.0 mM SDS (γ = 37.1 ± 1.1 mN/ m), most of the cubes showed a face-up orientation. These conclusions are corroborated by the analysis of more than 1000 cubes for each surface tension. It was observed that more than 90% of the cubes show a vertex-up orientation at the liquid/air interface for high values of surface tension, like 72.6 mN/m. With decreasing surface tension, the orientation changed progressively from vertex up to edge up and further to face up. At 49.2 mN/m, only 2% of the cubes were in the vertex-up orientation, 70% in edge-up, and 28% in face-up orientation. When the surface tension was further decreased to 37.1 mN/m, all of the cubes changed their orientation to face up from vertex up or edge up. By comparing the corresponding data (Figure S10), the probability of each orientation of cubes at the interface of SDS solution and air with different surface tensions and for individual SDS solutions with different concentrations that were successively changed, no obvious difference was observed. This implies that an equilibrium was achieved after agitation. Hence, with decreasing surface tension of the aqueous subphase, induced by added SDS, the microcubes changed their orientation successively from vertex up to edge up and then to face up, which means that the cubes and cube aggregates at the liquid/air interface responded dynamically to the change of surface tension. Thus, the 7795
DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
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structure of the assemblies could be dynamically reconfigured, which was demonstrated here for the first time. At this point, we can only speculate about the mechanism of reconfiguration. For a given surface tension, stable aggregates were formed, as shown above. At the high surface tension of pure Milli-Q water, the vertex-up orientation and concomitantly the hexagonally close-packed structures possess the minimum free energy. After the addition of a surfactant, the cubes at the edges of the aggregates are affected much more than cubes in the interior of the close-packed arrays, since those in the interior are stabilized by six neighboring cubes. We speculate that these cubes at the edges dissociate from the aggregate and change their orientation. This leads to a continuous shrinking of the aggregate size and exposure of new cubes at the edges and at the same time to the formation of extended linear structures of cubes in tilted linear orientation. When the surface tension is reduced even further, the linear aggregates disassemble, starting preferentially form the ends to form close-packed and flat-plate aggregates of faceup-oriented cubes.
The authors gratefully acknowledge financial support from the European Research Council (ERC project ASMIDIAS, Grant no. 279202) and the University of Siegen. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the contributions of Dipl.Ing. Gregor Schulte for excellent technical support. We thank Dr Sergey I. Druzhinin and M.Sc. Zhiyuan Jia for valuable discussions.
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(1) Balazs, A. C.; Aizenberg, J. Reconfigurable soft matter. Soft Matter 2014, 10, 1244−1245. (2) Ko, H.; Javey, A. Smart Actuators and Adhesives for Reconfigurable Matter. Acc. Chem. Res. 2017, 50, 691−702. (3) Thérien-Aubin, H.; Wu, Z. L.; Nie, Z.; Kumacheva, E. Multiple shape transformations of composite hydrogel sheets. J. Am. Chem. Soc. 2013, 135, 4834−4839. (4) Liu, L.; Jiang, S.; Sun, Y.; Agarwal, S. Giving Direction to Motion and Surface with Ultra-Fast Speed Using Oriented Hydrogel Fibers. Adv. Funct. Mater. 2016, 26, 1021−1027. (5) Zhang, X.; Pint, C. L.; Lee, M. H.; Schubert, B. E.; Jamshidi, A.; Takei, K.; Ko, H.; Gillies, A.; Bardhan, R.; Urban, J. J.; Wu, M.; Fearing, R.; Javey, A. Optically- and thermally-responsive programmable materials based on carbon nanotube-hydrogel polymer composites. Nano Lett. 2011, 11, 3239−3244. (6) Vogel, N.; Retsch, M.; Fustin, C.-A.; Del Campo, A.; Jonas, U. Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions. Chem. Rev. 2015, 115, 6265−6311. (7) Wintzheimer, S.; Granath, T.; Oppmann, M.; Kister, T.; Thai, T.; Kraus, T.; Vogel, N.; Mandel, K. Supraparticles: Functionality from Uniform Structural Motifs. ACS Nano 2018, 12, 5093−5121. (8) Ling, X. Y.; Phang, I. Y.; Maijenburg, W.; Schönherr, H.; Reinhoudt, D. N.; Vancso, G. J.; Huskens, J. Free-standing 3D supramolecular hybrid particle structures. Angew. Chem., Int. Ed. 2009, 48, 983−987. (9) Malaquin, L.; Kraus, T.; Schmid, H.; Delamarche, E.; Wolf, H. Controlled particle placement through convective and capillary assembly. Langmuir 2007, 23, 11513−11521. (10) Holzner, F.; Kuemin, C.; Paul, P.; Hedrick, J. L.; Wolf, H.; Spencer, N. D.; Duerig, U.; Knoll, A. W. Directed placement of gold nanorods using a removable template for guided assembly. Nano Lett. 2011, 11, 3957−3962. (11) Timpu, F.; Hendricks, N. R.; Petrov, M.; Ni, S.; Renaut, C.; Wolf, H.; Isa, L.; Kivshar, Y.; Grange, R. Enhanced Second-Harmonic Generation from Sequential Capillarity-Assisted Particle Assembly of Hybrid Nanodimers. Nano Lett. 2017, 17, 5381−5388. (12) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions stabilised solely by colloidal particles. Adv. Colloid Interface Sci. 2003, 100-102, 503− 546. (13) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Colloidosomes: selectively permeable capsules composed of colloidal particles. Science 2002, 298, 1006− 1009. (14) Bowden, N.; Arias, F.; Deng, T.; Whitesides, G. M. SelfAssembly of Microscale Objects at a Liquid/Liquid Interface through Lateral Capillary Forces. Langmuir 2001, 17, 1757−1765. (15) Bowden, N.; Choi, I. S.; Grzybowski, B. A.; Whitesides, G. M. Mesoscale Self-Assembly of Hexagonal Plates Using Lateral Capillary Forces: Synthesis Using the “Capillary Bond”. J. Am. Chem. Soc. 1999, 121, 5373−5391. (16) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. SelfAssembly of Mesoscale Objects into Ordered Two-Dimensional Arrays. Science 1997, 276, 233−235.
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CONCLUSIONS The surface tension of the subphase was shown to play a pivotal role in the orientation and self-assembly of hydrophobized PS microcubes into aggregates at the liquid/air interface. The preferential orientation of the cubes was virtually independent of the charge of the surfactant and differed with increasing surfactant concentration from vertex up at 72.6 ± 0.1 mN/m, to edge up at 49.2 ± 1.0 mN/m, and finally to face up at 37.1 ± 1.1 mN/m. Concomitantly, the cubes assembled into aggregates that changed their structures from close-packed hexagonal, to tilted linear, and finally to flat plate. When energy is supplied by stirring, preformed aggregates were shown to be dynamically reconfigurable by adding a surfactant to the subphase, which may open the pathway to dynamic assemblies at the interfaces, e.g., for sensing applications by signal amplification.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01104. Surface tension data of TTAB and Triton X-100 aqueous solutions; optical microscopy images of clusters self-assembled at various liquid/air interfaces with different surface tensions; and statistical analysis data of cube orientation (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +49(0)271 740 2805. ORCID
Holger Schönherr: 0000-0002-5836-5569 Author Contributions †
Q.S. and M.Z. contributed equally to this work.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 7796
DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
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DOI: 10.1021/acs.langmuir.9b01104 Langmuir 2019, 35, 7791−7797
