Control of Orientation, Formation of Ordered Structures and Self

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Interface-Rich Materials and Assemblies

Control of Orientation, Formation of Ordered Structures and SelfSorting of Surface-Functionalized Microcubes at the Air-Water Interface Qimeng Song, and Holger Schönherr Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00792 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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Control of Orientation, Formation of Ordered Structures and Self-Sorting of SurfaceFunctionalized Microcubes at the Air-Water Interface Qimeng Song, Holger Schönherr*

Physical Chemistry I and Research Center of Micro and Nanochemistry and Engineering (Cμ), Department of Chemistry and Biology, University of Siegen, AdolfReichwein-Str. 2, 57076, Siegen, Germany.

KEYWORDS: Particle self-assembly, microcube self-assembly, wettability, capillary force

Abstract. The dependence of the orientation of microscale PS cubes, which are surface functionalized on only 5 faces, at the water/air interface and the ordered aggregates

ACS Paragon Plus Environment

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formed by capillary force assembly are reported. Depending on the wettability of the faces, the cubes were shown to adopt a preferred orientation that changes with decreasing wettability from face up to edge up and further to vertex up. Concomitantly stable aggregates with different structures were formed by capillary force self-assembly. The unmodified bottom face of the cubes was localized by fluorescence labeling. Finally, self-sorting of differently surface functionalized microcubes was realized for the first time, due to the stronger capillary interactions of quadrupole-quadrupole and hexapole-hexapole compared to quadrupole-hexapole interaction.

1. INTRODUCTION

Particle self-assembly at liquid/liquid and liquid/air interfaces driven by capillary forces has gained a great deal of attention in chemistry, physics and biology for more than two decades.1,2 In particular, interfacial self-assembly, studied mostly with spherical colloids,3,4,5 and the obtained monolayer lattices were widely applied in colloidal


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lithography for micro- or nano-patterning fabrication.6,7 With various particle shapes and properties, large area aggregates exhibiting different structures, such as e.g. hexagonally close-packed8 or linear structures9,10 were reported.

Particles trapped at liquid/liquid or liquid/air interfaces induce interface distortion. Because of surface roughness, surface chemical heterogeneity, shape or orientation of the particles, distortions with different capillary multipoles are obtained: monopole, dipole, quadrupole, hexapole etc..11 The capillary force and particle interaction arise, when the distortions induced by two particles overlap.12 Depending on the contours of the

overlapping

menisci,

capillary

interactions

can

be

either

attractive

or

repulsive.13,14,15,16

Capillary interactions between colloidal particles at the liquid/liquid or liquid/air interface have been investigated for decades.15,17,18,19 Ordered structures formed by self-assembly of 2D millimeter-scale tile-shaped objects was studied in detail by Whitesides and coworkers.13,14,20,21,22,23 Based on the different menisci generated from hydrophilic or hydrophobic sides of polydimethylsiloxane (PDMS) objects, different


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aggregates could be formed predictably. However, the interaction between anisotropic particles, especially cubic ones, was rarely studied due to the complexity of the 3 dimensional orientation and the concomitant distortions at the liquid/liquid or liquid/air interfaces. The orientation of single anisotropic particles was reported to depend strongly on a range of particle properties, for instance particle size, shape and wettability.24,25,26,27 Three stable orientations of cubic particle, namely face up, edge up and vertex up, have been theoretically predicted and experimentally observed. By controlling one or more factors, such as wettability, preferential orientations may be obtained at liquid/air or liquid/liquid interfaces.

The self-assembly of aggregates of nanoscale cubes driven by capillary force was recently observed by Ling et al. at the liquid-liquid interface. By varying the surface chemical composition (afforded by varied surface modification giving rise to different wettability on the one hand,28 and solvent polarity on the other hand,29 three expected lattice structures were observed: square close-packed, linear, and hexagonal lattice. Meanwhile, the dependence of cube orientation on wettability at the liquid/liquid


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interface was demonstrated by Roij et al.,30,31 and the hexagonally close-packed and honeycomb-like lattice structured aggregates were predicted to be self-assembled with vertex up orientated cubes. We exploited this prediction in a previous study, where the orientation of microscale PS cube at the water/air interface could be controlled analogously to the cited work of Ling et al. via wettability.32

Here we expand on this initial report by systematically investigating cube orientation and capillary interactions for surface functionalized polystyrene (PS) microcubes that were fabricated via nanoimprint lithography (NIL) technique followed by self-assembled monolayer-based surface modification to control the wettability of five sides of the cubes. The location of unmodified bottom side of the microcubes was determined in different self-assembled aggregates. In addition to unveiling the capillary interactions and assembly of microcubes with different wettability, self-sorting of differently surface functionalized microcubes was observed for the first time.

2. EXPERIMENTAL SECTION


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Materials. Microscope slides were obtained from Thermo scientific, Menzel-Gläser (ISO 8037/1; 76 mm × 26 mm). Polystyrene was purchased from Sigma Aldrich (Mw ≈ 350,000 g/mol, Mn ≈ 170,000 g/mol); Toluene (≥ 99.5%) was obtained from ROTH. 1Octadecanethiol (98%, ODT) and 16-Mercaptohexadecanoic acid (99%, MHDA) were purchased from Sigma Aldrich (≥ 95%); Fluorescein (≥ 95%) was purchased from Merck, poly(dimethyl siloxane) (PDMS) was obtained from DOW CORNING (SYLGARD 184). Milli-Q water with a resistivity of 18.0 MΩ cm obtained from a Millipore Direct Q8 system (Millipore Advantage A 10 system, Schwalbach, with Millimark Express 40 filter, Merck, Germany) was used in all experiments. A homemade metal blade with 10 um step was used for detaching the microcubes from the glass substrate.

Fabrication of PS microcubes. The curing agent and prepolymer of PDMS were mixed in the ratio of 1:10 w/w. The mixture was poured onto the pattern master or petri dish after being degassed in the desiccator under vacuum ( ≤ 10 mbar, 30 min). The PDMS stamp and flat PDMS were obtained after curing in the oven at 60 °C for 2 hours (1 bar). Microscope slides were cut to 2 × 2 cm2, and cleaned with chloroform (99%, Carl Roth), ethanol (96%, Carl Roth) and Milli-Q water. Afterwards, the glass substrates


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were placed in UV/Ozone Cleaner (ProCleaner TM system, supplied by BIOFORCE NANOSCIENCES) for 30 min. A thin film of PS was deposited on the glass substrate by spin coating (0.3 mL, 15 wt% PS in toluene (99.8%, Carl Roth) solution) with 1200 rpm for 30 s. Toluene was evaporated in the vacuum oven (100 °C,10 mbar) overnight. Hot embossing process was done using the Nano Imprint Lithography (NIL) system (Eitre 3, Obducat, Sweden) with a temperature of 175 °C and a pressure of 10 bars for 60s, according to a previous report.33 The PDMS stamp was peeled off after the temperature cooled down to 75 °C.

Modification on PS microcubes surface. For microcubes with different wettability: PS microcube substrate was coated with ~30 nm Au using the sputter coater (S 150B, Edwards, London, UK) under 10−2 mbar vacuum. Afterwards, the substrate was immersed into 1 mM alkane thiol (ODT / MHDA) solution in ethanol for 4 hours, whereby the mole fraction of ODT (xODT) in the solution varied from 0% to 100%. Before use, the substrates were rinsed with ethanol and Milli-Q water.

For cubes utilized to investigate the location of the unfunctionalized bottom side: Sudan III (85%, purchased from Sigma Aldrich) was dissolved in the solution of PS in


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toluene (PS concentration 10 mg/mL) to fabricate dye labeled cubes. The cube substrate covered with ~30 nm Au after the hot embossing process, was immersed into a 1 mM ODT / MHDA binary thiol solution with xODT = 0.2 and xODT = 0.8. Before use, the substrates were rinsed with ethanol and Milli-Q water.

X-ray photoelectron spectroscopy (XPS). XPS measurements were performed with a SSX-100 S-probe photo electron spectrometer (S-probe ESCA SSX-100S Surface Science Instruments, USA) with Al Kα X-ray radiation of 200 W to obtain the elemental composition of the surface. All elements present were identified from the survey spectra (0 - 1200 eV) with an energy resolution of 1.0 eV. The data were analyzed with the software Casa XPS.

Contact angle (CA) measurement. Static water contact angle measurements were done at ambient conditions using the sessile drop technique (Dataphysics OCA-15, Filderstadt, Germany). 2 µL Milli-Q water was dropped on the surface for each measurement. Three different positions in each sample were measured and the values are reported as the mean ± standard deviation.


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Cube self-assembly at the water/air interface. PS cubes were released from the glass substrate after modification with the help of a homemade metal plate. The released cubes, which stayed on the blade surface (due to electrostatic forces) were flushed with 2 mL water to the water/air interface in a petri dish (4 mL of water, ⌀ = 2 cm). The water was stirred using a Teflon-coated stirring bar (length: 10 mm, ⌀ = 3 mm) for at least 2 hours with a speed of 400 rpm.

3. RESULTS AND DISCUSSION

PS microcubes with a base size of 30 ± 1 m were fabricated by nano imprint lithography (NIL) and hot embossing process using a PDMS master, according to previous reports,32,33 as is shown in Figure 1a. The cubes remained on the glass substrate after the imprinting (Figure 1b). Selected microcubes were labeled with fluorescence dyes, such as Nile red, by mixing the dye into the PS solution (Figure 1c).


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Figure 1. PS microcubes fabricated by NIL. a) Schematic of NIL / hot embossing (the temperature was ramped up to 175 °C, 10 bar were applied during the process for 60 s, stamp: microstructured PDMS see Figure S-1, Supporting Information). b) SEM images of the microcubes on glass substrate. c) Fluorescence microscope image of released PS microcubes, labeled with Nile red.

It is well known that the static water contact angle of clean PS surfaces is ~ 90°.34 However, a static water contact angle of 102 ± 1° was observed (Figure S2) on a flat PS


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film, which was contacted under the exact same nanoimprint condition by a flat featureless PDMS stamp. The observed value of the water contact angle is close to that of the stamp material, i.e. 105°.35,36 Subsequently XPS measurements were performed to unveil possible changes in surface composition during nanoimprinting. From the XPS spectrum of the original PS surface, only one peak was observed at 285 eV, which is the typical C 1s peak for PS (Figure 2a). However, after imprinting, additional elements, possibly originating from the PDMS stamp, were noted in the spectrum. Indeed, the presence of new peaks at 979 eV, 533 eV, 151 eV, 99 eV attributed to O KLL (Auger), O 1s, Si 2s and Si 2p, respectively were noted. The insets in Figure 2 show the spectral range for the Si 2s and Si 2p peaks before and after imprinting. The ratio of Si to O signal corresponds to 4.2 : 5.5, which is consistent with an equivalent thickness of – (Si(CH3)2-O)n- of ~1.6 nm. The increased water contact angle can hence be attributed to transfer of PDMS oligomers from the stamp to the PS surface during imprinting.36


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Figure 2. XPS spectra of a) PS surface before imprinting and b) PS microcubes after imprinting (the ratio of Si to O signal is 4.2 : 5.5). The insets show enlarged sections of the spectra in the range 50 eV – 200 eV.

The released microcubes were transferred to water/air interface by flushing with water. Driven by capillary forces, the microcubes assembled and formed large aggregates (Figure 3). Unlike spherical particles, which possess only one orientation, the PS microcubes were observed to reside at the water/air interface with three different orientations: face up, edge up and vertex up. These orientations can be deduced from the top view optical microscopy images obtained, which exhibit square, rectangular, and hexagonal shapes (Figure 3). Because the light source is below the petri dish, the face


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up cubes can be identified as square shaped bright features that allow light passage to the microscopy objective. For cubes with edge up and vertex up orientation, part of the light was reflected, because the cube faces were not perpendicular to the direction of the light. However, the symmetry of the image revealed the orientation in these cases.

Figure 3. a) Top view optical microscopy image of unfunctionalized PS microcubes dispersed at the water/air interface. b) Zoomed sections from panel a) exhibiting three different top view of cubes at the water/air interface: square, hexagon, rectangle and schemes of the corresponding cube orientation at the water/air interface: face up, vertex up and edge up.


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Based on the capillary forces at the water/air interface, the microcubes assembled together into a larger cluster. Because the capillary forces between micron- and submicron particles are comparatively strong, kinetically trapped configurations were usually observed that aged somewhat with time (Figure 4).11 The clusters, which were in a metastable state, aged slowly (Figures 4a and 4b), unless extra energy was supplied,

e.g. in the form of stirring. Here the assemblies change to an energetically more stable state (Figures 4c and 4d). This energy does not only increase the overall energy of the cubes in the system, but may also change the energy potential that the cubes experience. Following the analysis of more than 2,000 cubes, it was found that without energy input, the trapped configurations aged slightly with time in terms of cube orientation. Differences of -16%, +2% and +14% with respect to the orientation observed after 24 hours were observed for face up, edge up and vertex up orientation, respectively (Figure 4e). By contrast, when additional energy was supplied to the system by stirring, a dramatic change of the microcube orientation was observed with time (Figure 4f). During stirring, the population of face up cubes decreased from 57% to 18% within two hours and to less than 9% (-48%) after 24 hours. The population for


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edge up cubes decreased from 19% to 17% in 24 hours, while the population ratio of vertex up cubes increased from 24% to 73% (+49%). 47% of cubes changed orientation from face up to vertex up, indicating that for 30 µm sized PS cubes with a static water contact angle of 102°, the vertex up orientation is more stable at the water/air interface compared to the face up orientation. This observation is in very good agreement with the simulation results reported by Cilliers et al.24 Moreover, the structure of the aggregates changed from a disordered structure, dominated by face up orientated cubes (~60%), to a hexagonally close-packed structure with virtually exclusive vertex up orientation after energy input.


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Figure. 4 Optical microscopy images of self-assembled PS microcube aggregates at the water/air interface. a, c) before energy input and after 2h b) without or d) with energy


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input. Histogram analyses of cube orientation as observed with time: e) without energy input, f) with continuous energy input. Dashed lines indicate the limiting value of the probability of each orientation that is reached for infinite time (~24 h in this work). The analyses were done with more than 2,000 cubes in each case.

From the literature it is known that cubes with defined water contact angle possess preferred orientations at the water/air interface.24,30 However, the initial orientation of the cubes in the aggregates does not only depend on the wettability, but also on how the cube enters into the interface and on the surrounding cubes.24 Extra energy input, like stirring used here, offers the cubes enough energy to overcome the barriers between different orientations and neighbor cube interactions. This may result in self-rotation of the cubes, which re-orient in the interface as well as in the vicinity of the cube aggregates to reach a minimum free interface energy state. In order to investigate, how the orientation of the cubes at the water/air interface and the structure of the cluster depends on the wettability,28,30,37 PS microcubes were coated

with

gold

and

modified

with

self-assembled

monolayers

(SAMs)

of


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octadecanethiol / 16-mercaptohexadecanoic acid (ODT / MHDA) to achieve different static water contact angles  (Figure .

Figure 5. a) Schematic of gold-coated polystyrene microcubes, whose wettability was controlled by the deposition of binary thiol self-assembled monolayers (ODT/MHDA). b) Top view optical microscopy images of self-assembled microcube aggregates at the water/air interface formed by cubes with different wettability. The insets show schematics of the cubes’ orientation and structure of the aggregates formed (top view).


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With varying mole fraction of ODT in the binary thiol solution from 0% to 100% (0 ≤ xODT ≤ 100 %), the water contact angle of the surface changed from < 10° to 110°. Before releasing the cubes from the glass substrate, the cubes slide was covered with ~30 nm Au and functionalized with binary thiol SAMs (Figure 5a). It has to be emphasized that each cube contains five sides with the designated contact angle and one unfunctionalized side exposing clean PS with  ≈ 90° (bottom PS side). After modification, the cubes were released from the substrate and dispersed at the water/air interface. It was observed from the optical microscopy images that most of the cubes with  < 10° showed face up orientation at water-air interface (Figure 5b). These cubes self-assembled into a ‘flat plate’ structure. Since the density of PS (ρPS = 1.05 g / cm3) is slightly higher than water (ρH2O = 1.0 g / cm3), the entire cube is immersed into water and the capillary force is not strong due to a comparatively small interface distortion. When the hydrophobicity of the cube surface is increased (Figure 5b, 𝜃 = 52°), the cubes self-assembled into ‘tilted linear’ aggregates, because their preferred orientation at the water/air interface changed from face up to edge up. When the hydrophobicity of the cube surface was further increased to a water contact angle of 77°, ‘close-packed


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hexagonal’ aggregates were observed to coexist with ‘tilted linear’ aggregates (compare optical microscopy image in Figure 5b, = 77° ). Cubes with a water contact angle of 102° showed almost exclusively vertex up orientation at the water/air interface and selfassembled into ‘close-packed hexagonal’ aggregates (Figure 5b, 𝜃 = 102°). Since both cubes with PDMS layers and ODT/MHDA modification preferred a vertex up orientation and self-assembled to ‘close-packed hexagonal’ aggregates at the water/air interface, the chemical composition does not play a significant role compared to wettability in determining the orientation of the cubes at the water/air interface.32 The schematic of capillary interactions of cubes with different orientation is shown in Figure 6. With face up orientation, a monopolar interface deformation (depression) is created around the cube. Capillary interactions between monopolar face up cubes give rise to the formation of ‘flat plate’ aggregates (Figure 6a).15 When cubes float at the water/air interface with edge up orientation, a quadrupolar interface deformation with two strong menisci at the vertical sides and two weak ones at the edges is generated. Strong capillary interactions arise between the vertical sides of the two cubes. Hence a line-like aggregate is formed (Figure 6b). The formation of linear aggregates based on


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quadrupole- quadrupole capillary interaction was also observed by Stamou et al.38 and Brown et al.,39 with spherical colloids and curved disks, respectively. As simulated by Morris et al.24,25,26 and Soligno et al.30,31 a hexapolar interface deformation with a homogeneous distribution of three elevations and three depressions is created around the single cube, when the cube possesses a vertex orientation at the liquid/air or liquid/liquid interface. As a result, hexagonally close-packed aggregates are formed (Figure 6c).

Figure 6. Schematic of cubes with different orientation (top view): a) face up, b) edge up , c) vertex up, and the corresponding interface distortions and capillary interactions at the water/air interface. The signs “+” and “−” denote elevation and depression of the


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meniscus, respectively, while the thick and thin arrows indicate strong and weak capillary interactions.

The cube aggregates were found to be stable at the water/air interface and could be transferred to a solid substrate by removing the water. The configuration of the differently structured aggregates after transfer to the solid substrate was investigated by SEM (Figure 7). During the water evaporation, the morphology of ‘flat plate’ and ‘close-

packed hexagonal’ aggregates was maintained (Figure 7a and 7c). However, the force from the menisci between cube side and substrate pulled down the entire ‘tilted linear’ aggregate. Therefore, the cubes, which were transferred to a solid substrate were not edge up, but instead face up (Figure 7b). The aggregates retained their linear structure due to the strong capillary force between each two cubes, which are attributed to the smaller radii of curvature of the menisci (Figure S3).


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Figure 7. SEM images of self-assembled cube aggregates with a) flat plate, b) tilted linear and c) close-packed hexagonal structure on glass substrate. The cube aggregates with different structures were thermally fixed onto the glass substrates at 150 °C for 3 min. For SEM analysis they were sputtercoated with ~10 nm Au.

Analyses with over 1,000 cubes for each contact angle afforded the probability for each cube orientation for the aggregates at the water/air interface (Figure 8a) and for the aggregates after transfer to glass (Figure 8b). With increasing water contact angle of the cube from less than 10° to 102°, the probability of face up cube orientation was decreasing from 98.5% to 0.9%. By contrast, the probability of vertex up orientation cubes increased from 0.2% to 89.5%. The probability of cubes with edge up orientation was observed to first increase from 1.3% to 83.5%, when water contact angle increased


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from less than 10° to 52° and then to decrease to 10% with further increase of the water contact angle to 102°(Figure 8a). Different analysis results were obtained for cubes aggregates, which were transferred to glass (Figure 8b). Edge up cubes, which selfassembled into ‘tilted linear’ aggregates at the water/air interface, were pulled down during water evaporation. Thus a huge increase of the probability of face up cubes on solid substrate compared to the water/air interface was observed for cube contact angle of 52°. The cubes in ‘close-packed hexagonal’ aggregates are supported by the six neighboring cubes, expect for the cubes at the boundary of aggregates. Thus, no big difference was observed for the probability of vertex up cubes in aggregates at the water/air interface or in aggregate on glass.


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Figure 8. Histogram analysis of microcube orientation as observed for microcubes with different water contact angles in assemblies at the water/air interface (after more than 2 h stirring): face up, edge up and vertex up. a) Analysis for the aggregates at the water/air interface; b) analysis for the aggregates after transfer to glass. More than 1000 cubes were analyzed for each contact angle studied.


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In the modification procedure employed in this study, the cubes were modified with Au-ODT/MHDA before release from the substrate, which means that each cube possesses five faces with a designated contact angle and one face exposing PS with an unaltered contact angle of ~ 90°. The position of the PS face in different microcubes aggregate, ‘tilted linear’ and ‘hexagonally close-packed’, could be determined by fluorescence labeling. PS microcubes were labeled with the fluorescent dye Sudan III. Since five faces of each cubes are covered with ~30 nm of Au, which blocks the transmission of the excitation light. Thus the released microcubes only show a fluorescence signal that originates from the unfunctionalized PS bottom face under the fluorescence microscope. The PS bottom face was observed on different positions in the tilted linear aggregates (Figure 9b), as well as in close-packed hexagonal (Figure 9c) aggregates. Since the fluorescence signal was detected from below the sample by fluorescence microscopy, cubes showing strong fluorescence have the PS bottom face in the water, while this face is out of water for weak fluorescent cubes. The orientation of a single cube was defined with the parameters z and ϕ. z = 0, 1 and 2 indicates that the unfunctionalized PS bottom face is below, above or in the water/air


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interface. φ represents the azimuthal angle of the cubes. φ equals 0° or 180° for cubes in ‘tilted linear’ aggregates and φ = 0°, 120° or 240° for cubes in ‘closed packed

hexagonal’ aggregates. Compared to the other five faces with water contact angle of 52°, the unfunctionalized PS bottom face with a water contact angle of ~90° is more hydrophobic in ‘tilted linear’ aggregates. By contrast, the unfunctionalized PS bottom face of microcubes in ‘close-packed hexagonal’ aggregates is slightly more hydrophilic in comparison to other five sides, which exhibit a water contact angle of 102°. According to the analysis of more than 2,000 cubes in assembled aggregates it was found that 29.5% cubes have the unfunctionalized PS bottom face below the water/air interface. 67.2% cubes have the unfunctionalized PS bottom face above water/air interface and 3.3% cubes have it in the interface. In ‘close-packed hexagonal’ aggregates 59.3% were observed with the unfunctionalized PS bottom face below the water/air interface, while 40.7% cubes have ‘this face above the water/air interface.


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Figure 9. a) Schematic of cubes with labeled PS face and their orientation at the water/air interface (after more than 2 h stirring) with z and φ. z = 0, 1 and 2 indicate that the labeled side is below, above or cross the water/air interface. φ indicates the orientation of the cubes: φ = 0°, 180° for cubes in tilted linear aggregates; φ = 0°, 120°, 240° for cubes in close-packed hexagonal aggregates. Fluorescence microscopy images of cube aggregates with b) ‘tilted linear’ structure c) ‘closed packed hexagonal’ structure at the water/air interface. Histogram analysis of cube orientation as observed


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in d) the ‘tilted linear’ structure and e) the ‘closed packed hexagonal’ structure. The analysis was done with more than 2,000 cubes for each contact angle studied.

In theory, all cubes should possess an orientation with the unfunctionalized PS bottom face out of water in ‘tilted linear’ aggregates and all cubes should have this face inside the water in ‘close-packed hexagonal’ aggregates to obtain the lowest free energy of the system. Because the PS face is more hydrophobic and hydrophilic compared to the other five faces of the microcubes in ‘tilted linear’ aggregates and ‘close-packed

hexagonal’ aggregates, respectively. However, only 67.2% of cubes in ‘tilted linear’ aggregates and 59.3% of cubes in ‘close-packed hexagonal’ aggregates show the expected orientation, which has the lowest free energy. Furthermore, 32.8% and 40.7% of cubes showed a metastable state in the ‘tilted linear’ and ‘close-packed hexagonal’ aggregate, respectively. This metastable state can be explained by the stabilizing effect of neighboring cube. Inside the aggregates, an individual cube is strongly affected by the neighboring cubes based on the capillary force: two neighboring cubes for tilted linear aggregates and six neighboring cubes for close-packed hexagonal aggregates.


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The large surface tension of water surface induces an energy barrier that prevents the reorientation of cubes from a metastable state to a minimum energy configuration.40 Capillary interactions of polar multipoles with identical order, for instance monopolemonopole and quadrupole-quadrupole, are well understood.15,38 As observed in this work, microcubes with a water contact angle of 52° prefer an edge up orientation at the water/air interface and microcubes favor a vertex up orientation, when the water contact angle is 102°. Capillary interactions of polar quadrupoles and polar hexapoles, which are created by edge up and vertex up oriented microcubes give rise to the selfassembly of ‘tilted linear’ aggregates and ‘close-packed hexagonal’ aggregate, respectively. The assembly of cubes in mixtures, has not been addressed experimentally. Theoretical expressions for capillary interaction energy of multipoles with arbitrary order have been derived by Kralchevsky and collaborators. According to their equation, lower capillary interaction energy of quadrupole-quadrupole and hexapole-hexapole, compared to quadrupole-hexapole was revealed. Here we investigate

the

quadrupole-hexapole

capillary

interaction

experimentally

with


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microcubes and attempt to verify the self-sorting of cubes with different surface modification that was predicted theoretically. Two types of cubes with different wettability, surface contact angle of 52° and 102°, were obtained by functionalizing the cubes surface with ODT/MHDA. In order to distinguish different cubes, the cubes with contact of 102° were labeled with Sudan III (red). The cubes with water contact angle of 52° and 102°, which could assemble to ‘Hexagonal’ and ‘Linear’ aggregates, are named as ‘H-cubes’ and ‘L-cubes’, respectively. The mixture of cubes clustered in large disordered aggregates at the water/air interface (Figure S4) before energy input. However, after stirring for at least two hours, aggregates with two distinctly different structures were observed in the system: ‘Fiber-like linear’ aggregates and hexagonally closed packed aggregates (Figure 10b). Furthermore, the linear aggregates appeared dark in color, while the hexagonally closed packed aggregates were red in color. Fluorescence microscopy images were taken to investigate the detailed arrangement of the cube aggregates. It was observed that most of the cubes in the linear aggregates did not show any fluorescence signal (Figure 10a). This indicates that these cubes were L-cubes with


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water contact angle of 52°; by contrast, most of the cubes in the hexagonal structure emitted fluorescence (Figure 10c). This shows that the cube in hexagonal aggregate are mostly cubes with water contact angle of 102°.

Figure 10. Cubes with different wettability self-sorting at the water/air interface, after more than 2 h stirring. b) Overview of self-sorted cube mixture, where the non-labeled (black) cubes possess a water contact angle of 52°, while Sudan III labeled (red) cubes


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possess a water contact angle of 102°. Fluorescence microscopy images of cubes a) self-assembled into ‘tilted linear’ aggregates; c) self-assembled into ‘close-packed hexagonal’ aggregates. Histogram analyses of cubes as observed in d) ‘tilted linear’ aggregates and e) ‘closed packed hexagonal’ aggregates based on fluorescence microscopy images (L-cubes: cubes with water contact angle of 52°; H-cube: cube with water contact angle of 102°) The analyses were done with more than 500 cubes each. In linear aggregates more than 90% of the cubes were non-fluorescent (Figure 10d). In hexagonal aggregates, more than 95% of the cubes were fluorescing (Figure 10e). This revealed that stronger forces were created among quadrupole-quadrupole and hexapole-hexapole capillary interactions, in comparison to quadrupole-hexapole capillary interactions. For symmetrical interface deformations, such as hexapole, dipoledipole, even tripole-tripole interaction could be created within hexapole-hexapole capillary interaction,30,31 but not within quadrupole-hexapole capillary interaction. In addition, with the help of external energy input, which broke up the nonaligned arbitrary capillary interaction, self-sorting of cubes with different surface functionality (wettability) was observed for the first time.


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4. CONCLUSION

In microscale PS cubes fabricated via NIL the wettability of five of the cube face was varied from hydrophilic to hydrophobic by covering the cube surface with a thin Au layer and depositing binary SAMs of alkane thiols. The orientation of the cubes at the waterair interface was found to be preferentially face up, edge up or vertex up, which resulted in the assembly of ‘flat plate’, ‘tilted linear’ and ‘close-packed hexagonal’ aggregates, driven by capillary force, respectively. Furthermore, different locations of the unmodified ‘sixth side’ of the cubes were unveiled by fluorescence labeling in ‘tilted linear’ and ‘close-packed hexagonal’ aggregates. Stronger capillary interactions of quadrupolequadrupole and hexapole-hexapole compared to quadrupole-hexapole for cubes contact angles of 52° and 102° afforded self-sorting of microcubes for the first time.


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ToC graphics


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ASSOCIATED CONTENT

Supporting Information. Temperature and pressure ramp for NIL, Contact angle data, SEM data of transferred tilted linear aggregates; optical microscopy images of clusters assembled with non-labeled and Sudan III labeled cubes at the water/air interface before energy input.

AUTHOR INFORMATION

Corresponding Author Prof. Dr. H. Schönherr, E-mail: [email protected]; Fax: +49(0)271 740 2805. Physical Chemistry I and Research Center of Micro and Nanochemistry and Engineering (Cμ), Department of Chemistry and Biology, University of Siegen, AdolfReichwein-Str. 2, 57076, Siegen, Germany.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The authors gratefully acknowledge financial support from the European Research Council (ERC project ASMIDIAS, Grant no. 279202) and the University of Siegen.

ACKNOWLEDGMENT The authors thank Dipl.-Ing. Gregor Schulte for his technical support. We thank Dr. Marc Steuber for the initial experiments, Dr. Stephanie Müller, M.Sc. Siyu Jiang, and M.Sc. Anna Schulte for XPS measurements. We thank Dr. Sergey Druzhinin and M.Sc. Zhiyuan Jia for discussion. Part of this work was performed at the Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen.


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