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Mechanical-Tunable Capillary-Force Driven SelfAssembled Hierarchical Structures on Soft Substrate Zhaoxin Lao, Deng Pan, Hongwei Yuan, Jincheng Ni, Shengyun Ji, Wulin Zhu, Yanlei Hu, Jiawen Li, Dong Wu, and Jiaru Chu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05024 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Mechanical-Tunable Capillary-Force Driven Self-Assembled Hierarchical Structures on Soft Substrate Zhaoxin Lao, Deng Pan, Hongwei Yuan, Jincheng Ni, Shengyun Ji, Wulin Zhu, Yanlei Hu,* Jiawen Li, Dong Wu,* Jiaru Chu CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei, Anhui 230027, China Email: [email protected], [email protected]

Abstract: Capillary-force driven self-assembly (CFSA) has been combined with many top-down fabrication methods to be alternatives to conventional single micro/nano manufacture techniques for constructing complicated micro/nano-structures. However, most CFSA structures are fabricated on rigid substrate and few attention is paid on the tuning of CFSA, which means that the pattern of structures cannot be regulated once they are manufactured. Here, by combing femtosecond laser direct writing with CFSA, a flexible method is proposed to fabricate self-assembled hierarchical structures on soft substrate. Then, the tuning of self-assembly processing is realized with a mechanical-stretching strategy. With this method, different patterns of tunable self-assembled structures are obtained before tuning and after release, which is difficult to be achieved with other techniques. In addition, as a proof-of-concept application, this mechanical tunable self-assembly of microstructure on soft substrate is used for smart displays and versatile microobject trapping.

Keywords: self-assembly; capillary force; hierarchical structures; soft substrate; laser printing

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Guided bottom-up method has been widely used to construct three-dimensional (3D) hierarchical helical structures that possess diverse and intriguing functions.1 By combining bottom-up approaches with top-down techniques, researchers can address lots of instinctive shortcomings of conventional top-down processes when manufacturing complex 3D micro/nano functional structures, e.g. complex multi-steps and uneconomical cost.2,

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Among these ingenious bottom-up approaches, capillary-force driven self-

assembly (CFSA) has emerged as a promising candidate due to its advantages (simple, economical et al.) compared with other driven forces.4-7 A lot of interesting works have been reported based on the combination of CFSA with top-down techniques. For example, B. Pokroy et al. developed a theoretical model to characterize how geometries, mechanical and surface properties of the pillars favor the adhesive self-organization of bundles with pillars in an evaporating liquid.8 By analyzing adhesive and capillary forces carefully, D. Chandra and S. Yang studied the stability of high-aspect-ratio micropillar arrays with different materials employing UV-lithography and transfer method.9 H. Duan realized ordered complex pattern of nanostructures at 10 nm length scale based on CFSA and electron beam lithography (EBL).10, 11

We also developed a laser printing induced capillary-force driven self-assembly (LPCS) technique,

which greatly increases the flexibility of CFSA method.12-14 However, in these works, main attention was paid to the guidance of capillary-force by designing shape parameters of microstructures and space distribution of basic structures, in which the self-assembled 3D structures are only dependent on the physical properties (e.g. Young modulus) of the materials.15 Besides, most CFSA structures are realized on rigid substrates, which means that the hierarchical structures has been fixed and can no longer be regulated once the fabrication process is completed. Thus, there are still a lack of simple tuning methods to control the pattern of capillary-force self-assembled structures. 2


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In nature, we can get a great deal of inspirations from diverse adaptive phenomenon. By stretching their skins to enlarge the cells embedded in the skins, Chameleon and many other animals can change their body color for communication or escape from predators. Inspired by natural stretching strategy, many artful researches of tunable soft functional structures are reported.16 Based on soft material, the mechanical force tuning method features simplicity, real time and universality for materials compared to other stimuliresponse methods.17-26 For instance, T.P. Vinod et al. employed stretchable substrates and transfer method to assemble polymeric microstructures.27 E. Lee et al. proposed a “stretch-release” strategy to realize a tunable optical window by tilted pillars on wrinkled elastomers.28 However, structures manufactured with mechanical-stretching strategy are relative simple because current fabrication techniques of soft materials heavily rely on imprint and transfer process.29, 30 Moreover, the mechanical stretching method have never been combined with the CFSA technique for controllable fabrication of complex microstructures and functional devices. Here, we present a flexible strategy to construct hierarchical structures on soft substrate. By employing femtosecond laser direct writing, micropillars were fabricated firstly on treated PDMS, which was regarded as stretchable substrates. After development, micropillars on soft substrate assembled into diverse well-designed patterns of complex hierarchical structures owing to CFSA. Then, the mechanical tuning strategy was combined with CFSA so that the controllable transformation of CFSA structures were realized. In the end, it was also demonstrated that this tunable self-assembled structures can be used for display of different patterns and micro-trapping of object with changing size. This tunable CFSA provides a facile and general approach for the on-demand pattern control of hierarchical structure on soft substrates and may be applied in many potential applications such as single cell-trapping and smart displays. 3


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Result and discussion: Fabrication of capillary-force driven self-assembled structures on soft substrate. As shown in Figure 1a, a plasma-soaking processed polydimethylsiloxane (PDMS) slice was used as the soft substrate (Figure S1). PDMS slice was placed on glass slide to keep flat during processing and functionalized by oxygen plasma treatment. Then a mixed solution of poly(vinyl alcohol) (PVA), oxalic acid, and water in a 1:10:500 mass ratio was used to improve the adhesion between photoresist and PDMS according to reference.31 Without this plasma-soaking treatment, microstructures are difficult to be adhered to soft substrate and will be pulled down by evaporating liquid. (Figure S2) Photoresist was dropped on PDMS following by a baking process to remove the solvent. Then, micropillars on soft substrate can be manufactured by focusing a femtosecond laser beam through an objective lens into the photoresist. The sample was placed on a 3D nanotranslation stage to regulate the location and height of structures flexibly. Unpolymerized photoresist that was not exposed to laser can be washed away in the subsequent developing process. In order to simplify the discussion and do not loss generality, two adjacent pillars were treated as a basic unit (Figure 1). The principle of micro-pillars assembling is illustrated in Figure.1b. A meniscus arises between two micropillars when the liquid surface reaches the top of micropillars leading to asymmetric capillary force caused by different distance of adjacent micropillars. There are two capillary force (Fc) between inter-unit and adjacent units, respectively. It is the difference between the two Fc, which can be defined as net capillary force, that dominates the self-assembly. Selfassembly of micropillars occurs when the net Fc exceeds a critical threshold standing force (Fs), defined by the mechanical properties of the micropillars which is related to their shape and Young modulus. After assembly, what resists Fs to hold the structures is the Van der Waals’ force (Fv) that appears when the tips 4


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of micropillars are in contact. According to previous research, the net Fc, which is equal to Fc1 - Fc2, can be calculated with: Fc ~ r 2 cos 2  

d … d  d  d 

(1)

Here, γ, r, θ, d and Δd represent the interfacial tension of the solvent, the radius of the pillars, contact angle, the spacing d between adjacent pillars, and the addition spacing between two adjacent units, respectively, as shown in Figure 1c. Resistance to the capillary force is an elastic force which is proportional to the pillar’s bending distance. Therefore, the critical force occurs when two adjacent pillars contact at their tips. The force, which makes micropillar trend to restore standing, can be presented as: Fs ~

Er 4 d … h3

(2)

where E is Young’s modulus and h is the height of the pillars. Given Eq.1 and Eq.2, if the parameters γ, r, θ and E are fixed, the height and distance of micropillars can be used as control parameters for the regulation of CFSA. The CFSA process can be regulated by introducing an asymmetric arrangement design on distance. Assuming d is the standard distance of two pillars in a basic unit and Δd is the additional distance between two pillars in a unit and two adjacent units which is schematically shown in the inset of Figure 1d. To study the laser printing induced CFSA process on soft substrate, we investigated the dependence of assembly on Δd and d by fixing the height of pillars at 6.5 μm (Figure 1d). For the case of d = 3 μm, more than 90% units assembled successfully under Δd = 1 μm while the percent increased to nearly 100% as Δd increasing to 1.5 μm, indicating that the collapse of the pillars were more directed and controllable. In contrast, if we want to get a success rate of assembly more than 90% under d = 4 μm, the Δd should be 5


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regulated to more than 2.5 μm. To further study the controllability of this strategy, we quantified how ∆d/d affect the self-assembly. As shown in Figure S3, when ∆d/d is bigger than 60%, the net Fc caused by ∆d is so large that the self-assembly is more regular. The more regular assembly can be indicated with a smaller standard errors of success rate which is smaller than 5% regardless of d, proving the method is feasible to produce robust, uniform, and ordered assemblies. Figures 1e-1i are the experimental results corresponding to the typical positions in Figure d. From Figures 1e, 1f and 1g, we can find that when d is fixed, the larger the Δd is, the more regular the assembly is. That is because the increased Δd lead to a more obvious unbalance of force, which benefits the self-assembly. In contrast, a smaller differences of capillary force will lead to a more random assembly (Figures 1f, 1h and 1i). It should be noted that all pillars compose equally spaced square lattices when Δd = 0 μm. Though the capillary forces of an isolate pillar in the center from each direction are identical and finely balanced in such situation, tiny imperfections in pillar structures or instabilities in the evaporating process may break the weak balance and result in uncontrolled bending. Therefore, the assembled pattern of the symmetric square array is random and individual standing pillars as well as 2-, 3-, or 4-pillar units are likely to be formed without distinct regularity (Figure 1e). Because of the high flexibility of fs-laser printing, the spatial distribution of pillars can be easily controlled for generation of diverse ordered patterns. Figure 2 shows a series of assemblies with multielement basic units, from which we can see robustly ordered assemblies were achieved by CFSA. Benefited from the tunability of laser printing method, the exposure dose can also be controlled to obtain structures on soft substrates with tunable diameters and elasticity contributing to different assembly configurations. For example, we can get a smaller diameters (~750 nm) and elasticity with a lightly 6


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weakened laser energy (Figure 2a) while other images show the pillars with a bigger diameter (~850 nm). The SEM images also exhibit fine replication results of the laser printing and high uniformity of the 3D self-assembled structures (More results can be seen in Figure S4-S6). Furthermore, by manufacturing upright pillars according to specific patterns, more complex daisy-like structures self-assembled by 25pillar (Figure 2e) and 9-pillar with different rotation angles (Figure 2f) in each basic unit are realized, demonstrating the favorable designability of the LPCS method to fabricate hierarchical structures on soft substrates, which is difficult to be realized with other fabrication techniques of soft structures based on lithograph-transfer process. It should be noted that the diverse ordered assemblies on soft substrate as Figure 2 are a comprehensive result of both height and distance of pillars. It is also another interesting phenomenon that regular wrinkle patterns will be generated in self-assembled area, which can be used for special applications such as fabrication of maze-like structures32 and artificial fingerprint33. Here, we varied the height only in a small range because unsuitable height will result in failure of assembly like CFSA on rigid substrate and the height is difficult to be regulated after fabrication. In contrast, the distance between adjacent pillars can be easily adjusted with a mechanical tuning method so that tunable CFSA can be realized. Mechanical tuning of CFSA. The mechanical tuning procedure is sketched in Figure 3a. Because the assemblies are retained by the van der Waals force in air, the subtle balance between the short-range intermolecular force and the elastic standing force may be broken when a liquid is dropped on the substrate. That is because liquid molecules enter the contact area of self-assembled structures which results in the decrease of Fv. If the 7


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decreased Fv is smaller than Fs, micropillars restore upright in the liquid. (Detailed discussion can be seen in SI) With a hand-made device (Figure S7) used to stretch the soft substrates, d between pillars can be easily adjusted so that a modulated Fc can be achieved. For two neighboring pillars with a given height h, there is a critical distance (dc), which results in a delicate balance between the capillary force and the standing force. When d < dc, neighboring pillars can assemble together, or remain upright. Giving that mechanical sketching in one direction results in distance variation in both two directions of the substrate, we regard 4-pillar as a basic unit and employ one direction stretching. Therefore, the distance between neighboring pillars in a basic unit can be regulated both in the sketching direction and the vertical tensile direction. Here, we employ the force in X-direction (Figure 3b) to simplify the discussion and stretching device (Figure S7). In the case of X-direction stretching, the distance after tuning can be expressed by the formulas,

d x  a0 (1   ) … (3) d y  b0 (1  k ) … (4) where dx, dy represent the distances between two interunit pillars in X-direction and Y-direction in case of stretching; a0 and b0 are initial spacing of two interunit pillars in X-direction and Y-direction; ε and k refer to the stretch strain applied in X-direction and Possion ratio of material, respectively. Here, a0 and b0 are fixed and k is about 0.45. Therefore, the applied strain (ε) can be used for regulating dx and dy (Figure 3c). Figure 3b shows the self-assembled pattern evolution of pillars (a0 = 4 μm, b0 = 6 μm, h ~ 6 μm) under different stretching ε. Insets in the lower left present typical patterns of corresponding experimental results. When ε = 0, 4-pillar units assemble along X-direction (2-axial pattern) because the designed asymmetry between a0 and b0. As the ε increases, dx increases and dy decreases, leading to the weakening of the 8


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asymmetric capillary forces. So, when ε = 15%, half of units assemble into “X” pattern (Figure 3b) and the ratio raises to more than 90% when ε ~ 30%. When ε further increases (here, 40%), most units assemble in the direction perpendicular to strain force (named as 2-radial in Figure 3e). The evolutions of these 3 main patterns and 6 patterns varying on ε are indicated in Figures 3d and 3e respectively, from which we can see that the self-assembled patterns can be tuned with mechanical stretching method. In order to further explore this mechanical tuning method, we also employed a “tuning-release” strategy to regulate CFSA on soft-substrate. (Figure 4a) Three different 4-pillar (h ~ 5.5 μm) units, whose b0 are 3 μm, 4 μm, 5 μm respectively with the same a0 (3 μm), were applied to verify this concept (Figure 4b). By employing a larger strain (ε ~ 45%) in the third step as shown in Figure 4a, we can change the “X” pattern to “2-radial” patterns after release (the fifth step in Figure 4a). In the experiment, we found that self-assembled structures did not cancel again although the release causes an increase of dy, indicating self-assembled structure is stable in air. Besides, in the case of 2-pillar unit, we can also decrease Fc according to formula 1 by enlarging d so that upright pillars can be realized with this strategy (Figure 4b, more results can be seen in Figure S8). Such assembled patterns of pillars with those parameters (h, d, E and r) cannot be constructed with other CFSA on rigid substrate. In this mechanical tuning method, possible adjustment routes are schematically depicted in Figure 4c. Without mechanical stretch, micropillars self-assemble into patterns as the competitive result of Fc and Fs, as shown with purple dashed line. Obviously, there should be two critical stretch strain (ε1 and εc). When ε < ε1, the variation of difference between dx and dy caused by stretch is too small to change the asymmetry of self-assembly pattern. When ε1 < ε < εc, a larger strain decreases the difference of distance in X-direction and Y-direction resulting in a smaller asymmetry of Fx and Fy, therefore tuning self9


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assembly process will follow the black line. In contrast, as the strain increases and be larger than a critical value (εc), dx is larger than dy so that the tuned self-assembly process will follow the red line. Insets are corresponding self-assembled structures. As mentioned above, when parameters γ, r, θ and E are fixed, self-assembly is a comprehensive result of height and distance. Considering equation 1-4 at the same time, for patterns with fixed a0 and b0, the critical strains only depend on the height of pillars. Here, we perform a series of experiment with 4-pillar patterns with designed distances (a0 = 4 μm, b0 = 6 μm), which are indicated in Figure 4d. By fitting the experimental results, Figure 4d can be divide into three regions which correspond to assembly patterns as shown in insets, respectively. Our investigation reveals that ε1 decreases and εc increases as height raises. Applications of mechanical tunable CFSA Furthermore, the tunable self-assembly pattern can enable the capacity for information display and conversion. Demonstrations of dynamic information recording through deformable patterns based on mechanical tuning CFSA are shown in Figures 5. 6-pillar units in Figure 5a distributes as hexagon with a side length of 3.5 μm. The unit with 5.5 μm height presents “flower” pattern without any stretch strain, which changes into a “butterfly” pattern as sketched when ε = 20%. Then, while ε raises to 40%, many self-assembled patterns can be seen as a “III” pattern (the bottom image of Figure 5a). In the case of 3pillar unit whose h is about 5 μm, self-assembled patterns also comprise a series of evolution, as shown in Figure 5b. Two pillars collapse and touch each other at the tip while an isolated pillar keeps upright when ε = 20%. Because of the force between the isolated pillar and two adjacent pillars, the self-assembly forms an angle (α). The α decrease from 120° to 105° as the ε increase from 20% to 40% which comes from a smaller capillary force between isolated pillar and two adjacent pillars. This may find potential application

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in switchable information recording and ink-free display, such as the fabrication of hidden information by combing with Morse-code. Another possible application of capillary-force driven self-assembled structures is the trapping of variable size micro objects, which are intensively demanded in the fields of biomedical and chemical research, for example, single-cell analysis.34,

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Owing to its absence of an energy beam and free of

possible harm to living samples or influences to chemical reactions, CFSA structure have been applied as grippers for microobjects or cells which results in many interesting works.8, 12 However, while the size of cells is changeable, these CFSA structures on rigid substrate cannot be varied to match the trapping size. The concept of tunable trapping is shown in Figure 5c. Only those particles of suitable size can be effectively caught owing to the space limitation of the microgripper. Large particles are unable to enter the interunit space and can be cleaned out by external force (e.g., blown away by compressed air). It’s worthy to note that the suitable size of a certain 4-pillar unit on the substrate can be adjusted with applied strain (ε). When a0 = 4 μm, b0 = 6 μm, the dependence of trapping size on ε is shown in Figure 5d, which can be divided into three parts. For area I, particle can be dropped into the interunit and be restricted by pillars. In contrast, particles with the size of area II can be limited in one direction by the 4-pillar unit while pass freely in another direction (A detailed quantitative discussion is given in Supporting Information). Small particles can move freely in and out the structures without being trapped in both Xdirection and Y-direction (area III). Figures 5e-5h demonstrate a representative result of adjustable trapping. If ε raises to 20%, the distances in both direction vary to 4.8 μm and 5.5 μm so that particles with 5 μm diameter can be trapped (Figure 5e, corresponding area I). In contrast, while ε = 30%, dx and dy change into 5.2 μm so that 6 μm particles are captured (Figure 5g). Particles with D = 5 μm and D = 6 11


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μm trapped by self-assembled structures after releasing the strain are shown in Figure 5f and Figure 5h, respectively. Furthermore, as-prepared microgrippers can be further tuned during the trapping. For instance, a trapped particle can be released by adjusting applied strain (from area I to area II), or be trapped in a converse process (from area II to area I). It is worth mentioning that such grippers can be reused and employed to capture microobjects with arbitrary shapes and surface properties.

Conclusion: In summary, we proposed a fabrication method of hierarchical structures on soft substrate by combing femtosecond laser printing with CFSA. This top-down/bottom-up hybrid method for construction of 3D complex structures on soft substrate avoids the heavily dependence on multi-steps processes and expensive equipment, which features simplicity, scalability, and high flexibility in comparison with other state-of-the-art approaches based on lithograph-transfer strategy. Inheriting the high flexibility of 3D laser printing, it can be employed to construct various microarchitectures conveniently on soft substrates. In order to realize the adjustability of CFSA on soft substrate, the dependence of CFSA configuration on interunit distance and additional distance of micropillars were studied. Then, we demonstrated this CFSA of microstructures can be easily regulated with a mechanical stretch tuning approach, which is difficult to be achieved with other techniques. As a proof-of-concept demonstration, different patterns of tunable self-assembled structures are realized before tuning and after release. Finally, the applications for deformable information recording and microobjects trapping with changeable sizes was demonstrated using mechanical tunable CFSA method. We believe that this strategy will provide an alternative for many researches, for example, single cell trapping and on-demand tuning of surface properties. 12


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Experimental Section Preparation of Sample: The soft substrate was made by polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning). Firstly, PDMS was mixed with the crosslinker at a ratio of 10: 1 (w/w). Then, the mixture was spin-coated on a cleaned glass at 300 rpm for 150 s, following by curing with 80 °C for 5 h which results in ~120 μm thickness of PDMS. After peeling off from glass and cutting into slice, PDMS slice was placed on glass slide to be functionalized by oxygen plasma treatment (Mingheng PDC-MG, 50 s, 75 W, 100 Pa). Then a mixed solution of poly(vinyl alcohol) (PVA), oxalic acid, and water in a 1:10:500 mass ratio was used to improve the adhesion between photoresist and PDMS according to reference. The liquid on PDMS was blew away after taking from solution. A commercially available zirconium-silicon hybrid sol-gel material (SZ2080, IESL-FORTH, Greece), which was dropped on treated PDMS, was used for 2-photons polymerization. Pre-baking process for evaporating the solvent in SZ2080 was set to 1 h on a 65 °C thermal platform. Unsuitable temperature may cause wrinkles of PDMS which will lead to failure fabrication. The Possion ratio (k) of PDMS plays an essential role during the mechanical tunable selfassembly of structures. Here, by measuring the shape variables in vertical (∆dy) and parallel (∆dx) to stretching directions in experimental figures, the k is measured to be about 0.45 by ∆dy/∆dx. Fabrication: A Ti:sapphire femtosecond laser system (Chameleon Vision-S, Coherent Inc, USA) is used for direct laser writing. The central wavelength, pulse width and repetition rate of laser source are 800 nm, 75 fs, and 80 MHz respectively. Laser with the energy varying from 55 to 70 mW pass through a 50× objective lens (NA = 0.8; Olympus) to polymerize material. The energy was measured before entering the microscope. The laser power or curing time was tuned within the presented ranges to satisfy the appropriate parameters of micropillars (diameter and Young modulus). The sample was mounted on a 13


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nanopositioning stage (E545, from Physik Instrumente (PI) GmbH & Co. KG, Germany) with nanometer resolution and a 200 μm × 200 μm × 200 μm moving range to precisely locate microstructures. After polymerized by femtosecond laser, the sample was developed in 1-propanol for 30 minutes until all of the unpolymerized part was washed away. During the evaporation process after withdrawal from 1-propanol, the micropillar array was induced to assemble into various complex 3D microstructures driven by capillary force. Sample Characterization and Image Acquisition: The SEM images were taken with a secondary electron scanning electron microscope (ZEISS EVO18) operated at an accelerating voltage of 10 keV after depositing ~10 nm gold. An optical microscopy (DMI 3000B, Leica) with a white light source performs imaging experiments of trapped particles. A handmade device (Figure S5) was used to realize the mechanical tunability of self-assembly. Manipulation of Microspheres: Commercialized SiO2 particles (SEQ04, Tianjin Saierqun Technology Co. Ltd, China) with different diameters (4, 5, 6 μm) are mixed in DI water solution with a concentration about 10-4 g/mL. For trapping particles, the solvent containing microspheres was dropped on the sample and the rest solvent was blown after 5 min when most particles sunk to the bottom. Then the sample is flushed by DI water to remove the non-captured particles. The environment condition was kept 23 ± 1 °C in 45% ± 5% relative humidity.

Acknowledge This work was supported by the National Science Foundation of China (nos. 61475149, 61675190 51805509 and 51675503), Youth Innovation Promotion Association CAS (2017495), the Fundamental Research Funds for the Central Universities (nos. WK 2090090012, WK2480000002, 14


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WK2090090021) and National Key R&D Program of China (2018YFB1105400). We also acknowledge the Experimental Center of Engineering and Material Sciences, USTC.

Supporting Information The reversible behavior of capillary-force driven self-assembly can be seen in Video S1 and Video S2. More detail about sample preparation, experimental results and discussions can be seen in the Supporting Information, which is available free of charge on the ACS Publications website. References

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Figures and captions:

Figure 1. Fabrication of capillary-force self-assembled structures on soft substrates with laser printing method. (a) PDMS is used as the soft substrate on a glass slide. The femtosecond laser is focused into the photoresist (SZ2080) with an objective lens. The sample is placed on a 3D nanotranslation stage. (b) Scheme of capillary force induced self-assembly of laser printed microstructures. (c). Illustrations of the factors which impact the result of self-assembly, the height (h) and radius (r) of a micropillar, the distance (d) between two pillars in a basic unit, the additional distance (Δd) between two adjacent units, and the contact angle (θ) of liquid and pillars. There are two different capillary forces (Fc) at the opposite directions because of the different spacing. It’s the net Fc (Fc= Fc1 - Fc2) that conquers the standing force (Fs) and finally realizes the self-assembly. (d). Plot of the capillary-force driven self-assembly (CFSA) 18


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evolution over Δd with different distance d. Measurements are taken at d=3, d=4 and d=5 μm respectively. (Shaded area indicates standard error; counted number of experimental results (N) = 6 at each distance). The distance variations exist in both horizontal and vertical directions, as shown in the inset illustration. All samples are fixed at h = 6.5 μm, unless otherwise indicated. (e-i) The experimental results corresponding to typical position in figure d. From e, f and g, we can see that a larger Δd with the same d results in more regular self-assembly. d and Δd are marked correspondingly at the top of figures. From f, h and i, a larger d with the same Δd results in a more disordered self-assembly.

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Figure 2. Diverse ordered structures prepared on soft substrates with the LPCS approach. (a, b) Selfassembled patterns of 2-pillar, 3-pillar unit. Top view. (c, d) Top view and tilted view of patterns selfassembled by 4-pillar unit. (e) A more complex daisy-like structures self-assembled with 25-pillar in each basic unit. (f) 9-pillar cells with different rotations. (g, h) 6-pillar patterns distributed as hexagon. Tilted view: 45°. All scale bars, 10 μm.

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Figure 3. Evolution of self-assembled patterns on stretching ratio by mechanical tuning method of CFSA. (a) Illustration of mechanical tuning method. When self-assembled structures were immersed in liquid, the Fv decreased because liquid molecules entered contact area. Micropillars stood in liquid again if Fs was larger than decreased Fv. Then, we stretched the substrate in X-direction which would lead to an opposite change in Y-direction. The adjusted distance between adjacent pillars results in a different Fc during the evaporation of liquid which will cause a different self-assembled pattern due to the competition of adjusted capillary-force at X-direction (Fx) and Y-direction (Fy). (b) The illustration and experimental results of tuning self-assembly under different stretching status in X-direction. The illustrations in the lower left present typical patterns of corresponding experimental results. The height of pillars is about 6 μm. a0 = 4 μm, b0 = 6 μm, Scale bars, 20 μm. (c) Evolution of a and b along with stretch strain. (d, e) Three 21


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(d) and six (e) main typical self-assembled patterns changes along with the increase of stretch (ε). The stretch along X-direction will change the difference between a and b. In the case of ε is smaller than a critical value (εc), the difference between a and b decreases when ε increases so that the asymmetry between Fx and Fy decreases. When ε is larger than εc, the asymmetry of distance changes in the opposite direction.

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Figure 4. Regulating self-assembled pattern with mechanical tuning method. (a). Illustration of tuning process. Different self-assembled structures on soft substrate can be obtained after release. (b). Different patterns before tuning and after release with the tuning process as shown in Figure 4a. All pillars, h ~ 5 μm; From left to right, a0 = b0 = 3 μm; a0 = 3 μm, b0 = 4 μm; a0 = 3 μm, b0 = 5 μm; a0 = 3 μm. Applied ε ~ 45%. (c). Without mechanical stretch, micropillars will self-assemble into patterns due to the competitive result of Fc and Fs, as shown with purple line (from 2 to 1 directly). During the tuning method, there are two critical stretch strain (ε1 and εc). When ε < ε1, the stretch does not change the pattern of selfassembly; when ε1 < ε < εc, the tuning self-assembly process follow the black line (4-pillar assemblies are finally obtained); In contrast, the tuning self-assembly process will follow the red line as ε > εc (vertical 2-pillar assemblies are obtained after releasing). Insets are corresponding self-assembled structures. (d) The dependence of critical stretch strains on the height of pillars. Here the distance of adjacent pillars along X-direction and Y-direction (dx, dy) are fixed as 4 and 6 μm respectively. Insets are experimental 23


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results of self-assembled patterns in corresponding area. All scale bars, 10 μm.

Figure 5. Application of mechanical tunable CFSA in smart display and micro-particle trapping. (a, b). Demonstration of dynamic information recording through deformable patterns based on mechanical tuning of CFSA. For the 6-pillar unit, side length is 3.5 μm, h ~ 5.5 μm. For 3-pillar unit, side length is 3.5 μm, h ~ 5 μm. α1 and α2 in (b) are 120° and 105° respectively, because of the different stretch strain. (c). Schematic illustration of micro-object trapping with a stretchable 4-pillar unit. Here, the initial distance at X-direction (a) and Y-direction (b) between adjacent pillars in a 4-pillar unit are fixed as 4 and 6 μm. The height (h) of pillars is about 6.5 μm. (d) The diameter of microobject that can be trapped is a function of stretch strain (ε). The insets are corresponding schematics of trapping model. (e, f) Optical microscopy image and SEM image of trapped 5 μm when ε = 20% corresponding to area I. (g, h) Optical microscopy image and SEM image of trapped 6 μm when ε = 30% corresponding to area I. SEM images

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are taken at release status. All scale bars, 5 μm.

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TOC image: A mechanical tunable capillary-force driven self-assembly (CFSA) method is proposed to fabricate complex hierarchical structures on soft substrate. The CFSA process can be regulated with a mechanical stretching strategy so that self-assembled patterns can be tuned. Such adjustable structures exhibit potential application for variable-size microobjective trapping and switchable information recording.

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Mechanical-Tunable Capillary-Force-Driven Self ... - ACS Publications

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