Letter pubs.acs.org/Langmuir
Programming Tilting Angles in Shape Memory Polymer Janus Pillar Arrays with Unidirectional Wetting against the Tilting Direction Chi-Mon Chen, Chang-Lung Chiang, and Shu Yang* Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States ABSTRACT: By coating a thin layer of metal, including gold and gold−palladium alloy, of different thickness on the deformed shape memory polymer (SMP) pillars, we manipulate the degree of recovery of the SMP pillars. Pillars of different tilting angles were obtained as a result of balancing the strain recovery energy of the SMP pillars that favor the original straight state and the elastic energy of the metal layers that prefer the bent state. With this selective coating of a metal layer on the tilted pillars, we report a unique anisotropic liquid spreading behavior, where the water droplet is fully pinned in the direction of pillar tilting but advances in the reverse direction. This phenomenon is explained by the interplay of the surface chemistry and topography.
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INTRODUCTION Anisotropic wetting that directs liquid flow to a certain direction is of interests for water shedding, water spreading, and water collection, as well as the applications in microfluidics and optical components. To induce directional wetting, anisotropic surface structures, including wrinkles,1,2 microgrooves,3−5 microprisms,6 ratcheted surface patterns,7−10 and tilted micropillar arrays,11−14 have been fabricated to guide the liquid propagation. It has also been shown that the surface with inclined patterns can either hold or release the liquid droplet by changing the direction15 or direct deformation.14 Besides surface topography, surface chemistry also plays an important role to manipulate liquid wetting behaviors.16−21 For example, using angled vapor deposition of self-assembled monolayers (SAMs), a gradient SAMs is created on a flat substrate, leading to the uphill traveling of the water droplet.17 On a patterned surface that is partially coated with metal, a chemical contrast is introduced to direct the liquid propagation in one or multiple directions.22 By functionalizing the microstructured surface with two type of SAMs on Si nanopillars,20 or grafting thermoresponsive polymers on one side of a prism and coating metal on the other side,6 Janus structures that have two types of surface chemistry have been demonstrated to direct the anisotropic flow of fluid. Among different strategies to induce anisotropic liquid wetting, tilted high-aspect-ratio (HAR) pillar arrays have shown unidirectional wetting in the direction of tilting angles. They are created by shearing,14,23 oblique irradiation,24 angled etching,25 and selective deposition of metal.13,26 When exposed to e-beam irradiation in an inclined arrangement, the HAR polymer pillars are selectively degraded in the exposed side, thus, bending the pillars toward the irradiation direction.24 Alternatively, metal can be selectively deposited on one side of the polymer or silicon pillars, causing the pillars to bend upon cooling13,26 or after thermal annealing12 due to residual thermal © 2015 American Chemical Society
stresses from the difference in coefficient of thermal expansion (CTE) between metal and polymer or silicon. Despite the advances in methods of tilting the pillars, most of the prior work on unidirectional wetting on titled pillars is from pillars with uniform surface chemistry. Therefore, liquid wets and spreads along the pillar tilting direction with one edge being pinned to the substrate. In addition, the reported methods to fabricate tilted pillars often involve partial degradation or change of materials properties (e.g., hardening or wrinkle formation) during e-beam irradiation and plasma etching, which are not necessarily desirable. It will be interesting to create Janus HAR pillar arrays without altering pillar’s intrinsic physical properties, and more importantly, to investigate the interplay between the surface topography and surface chemistry to the wetting directionality.6 By taking advantage of the significant change of elastic modulus of shape memory polymer (SMP) pillars upon heating and shape recovery upon reheating, as well as the large difference in elasticity between SMP and metal, here, we finetuned the degree of tilting of SMP pillar arrays to study directional wetting. By depositing a thin layer of gold or gold− palladium alloy of variable thicknesses onto the fully deformed SMP pillars and reheating them, we restricted the degree of recovery of the SMP pillars by the metal layer thickness, resulting in Janus pillars tiled at different angles. Since the pillar surface had different chemistry on each side (SMP vs metal), we observed an unusual anisotropic liquid spreading behavior: the water droplet propagates in the opposite direction of the pillar tilting. The wetting behavior was completely different from the literature studies, where water always spread along the direction of pillar tilting angle.11,13 Received: July 16, 2015 Revised: August 20, 2015 Published: August 25, 2015 9523
DOI: 10.1021/acs.langmuir.5b02622 Langmuir 2015, 31, 9523−9526
Letter
Langmuir
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EXPERIMENTAL METHODS
Materials and Fabrication. The epoxy-based SMP pillars (10 μm in diameter, 30 μm side-to-side spacing, and 30 μm in height in a square lattice) were fabricated by replica molding from photocurable liquid epoxy resin (D.E.R 354, Dow Chemical) following the procedure reported earlier.27 After they were completely deformed to the ground by mechanical shearing,14 gold or gold−palladium alloy (Ted Pella Inc.) was sputtered onto the deformed SMP pillars. Gold of different thickness was coated using Quorum Q150T ES turbopumped sputter coater for 20−100 s at 20 mA. The gold−palladium alloy is coated by Cressington 108 sputter coater for 30 s at 30 mA. Characterization. Angled SMP structures were examined using FEI Quanta FEG ESEM and FEI Strata DE235 scanning electron microscopes (SEM). The liquid wetting behaviors on the original and metal coated SMP pillars were characterized by a ramé-hart automated goniometer model 290 with DropImage Advanced v1.5. The thickness of sputter-coated metal films was measured by Veeco Dimension 3100 atomic force microscope.
Figure 2. Tilting angles of Au-coated SMP pillars as a function of gold layer thickness.
prevent the complete recovery of SMP pillars. The collective bending moment balance between the SMP pillars and the coated metal layer led to a partially recovered state, where the degree of recovery depended on the thickness of the metal layer. For a flat thin plate, linear elasticity predicts that the bending moment M scales with plate thickness t as M ∼ t3.30 Apparently, the bending strain here is too large to be applicable in the linear regime. Also, the metal coating on the cylindrical pillars should be treated as a curved shell rather than a flat thin plate. Nevertheless, it is clear from the simple scaling law that the bending moment of the metal layers should increase drastically with the layer thickness, allowing for control of the pillar tilting angles. The method of tilting pillars demonstrated here is superior to the oblique irradiation approach24 and selective metal deposition on the HAR polymer pillars13,26,12 reported in literature, which would inevitably damage the original polymer structures. Our method is also very flexible since there are rich types of metals and inorganic materials (e.g., silica, titania and alumina) that can be vapor deposited at a low temperature using techniques in addition to sputtering, including plasmaenhanced chemical vapor deposition (PECVD)31 and atomic layer deposition (ALD). As a proof-of-concept, we also fabricated tilted Janus pillars (Figure 3) by sputter coating ∼20 nm gold−palladium alloy (60/40 wt %) on deformed pillars.
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RESULTS AND DISCUSSION SMPs are unique polymers whose shape can be programmed by heating above the glass transition temperature (Tg) or melting temperature (Tm). The temporary shape can be fixed by cooling below the transition temperature. When reheated, the original shape of the SMPs can be recovered due to release of the elastic energy stored in the deformed state.28,29 Previously, we altered the surface wettability of the SMP pillars for directed water shedding between two states, namely, the original straight state (water repellent state) and the fully deformed state (water pinning state).14,23 Since SMPs can be fixed in any temporary shape, it is possible to fine-tune the tilting angle of pillars during recovery to improve our control of directional wetting11,13 and adhesion.12 Here, we show that it is feasible to access partially deformed states by manipulating the recovery constraint in the SMP pillars. To achieve this, a thin layer of gold was sputtered onto the fully deformed SMP pillars, followed by reheating the whole system. The pillars were partially recovered to an equilibrium position as shown in Figure 1a, where the pillar
Figure 1. Control of the tilting angle (θ) of SMP pillars coated with a thin layer of metal. (a) Schematics of metal coating. Top-view (b) and side-view (c) SEM images of SMP pillars tilted at different θ depending on the thickness of Au coating, which is increased by ∼6.44 nm/increment from left (0 nm) to right (25.8 nm).
Figure 3. Tilted (left) and fully recovered (right) SMP pillars coated with ∼20 nm Au/Pd (60/40 wt %).
tilting angle θ was fine-tuned by the gold film thickness (see Figure 1b). When the gold film thickness was increased from 0 to 25.8 nm with an increment of 6.44 nm, θ increased from 0 to 22.8° (see Figure 2). The deformed SMP pillars tend to recover to their original straight state to release the stored elastic energy. However, deposition of a much stiffer metal layer (∼80−100 GPa) on the fully deformed pillars (∼3 MPa) would
More interestingly, we found that the gold layer coated on the SMP pillars not only restricted the recovery behavior but dramatically changed the surface chemistry and, thus, the liquid spreading characteristics on the tilted pillars. As seen in Figure 4a, when water was continuously added to the existing droplet sitting on the pillars, the wetting front of the droplet proceeded 9524
DOI: 10.1021/acs.langmuir.5b02622 Langmuir 2015, 31, 9523−9526
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Figure 4. (a−c) Anisotropic water spreading on gold-coated, tilted SMP pillars. (a) A series of optical images of anisotropic liquid spreading (to the left) on the partially recovered SMP pillar (Au thickness ∼22.8 nm, tilting angle θ ∼ 19.8°). The water volumetric increment is ∼4 μL. (b) Schematic for liquid triple phase line pinned on the coated pillar surface, with pinning angle θp, advancing angle θa, and actual contact angle θ. (c) Optical images of the pinned triple phase line on the tilted pillars in the tilting direction (left) and the reverse direction (right). (d) Water traveling uphill on the Au/Pd-coated tilted SMP pillar array. The pillar bending angle θ is ∼28°.
on one side continuously, while the other side was fully pinned. A closer look suggested that the liquid spread toward the opposite side of the pillar tilting direction, which was completely different from the observation reported in the literature,11,13,15 where the entire surface of the tilted pillars is conformably coated with metal or polymers. Therefore, the anisotropic wetting behavior is determined solely by surface topography. In our system, the metal was coated only on one side of the deformed SMP pillars. Once the triple phase line reached the tips of the tilted pillars, water would be pinned both at the bottom of the shadowed area and on the tips of the partially coated pillars (see schematic in Figure 4b and optical image in Figure 4c, left panel). As the liquid front is pinned on the bent pillars, there are two types of contact between the liquid front and the underlying surface. In the first type, the liquid front on the flat metal-coated surface in between the pillars should follow the advancing contact angle θa of the liquid on the surface. In the second type, the liquid front on top of the tilted pillars should conform to the effective contact angle (θa − θ). The composite pinning angle θp of the liquid front should be between these two values. Experimentally, θa is measured as ∼87°, θ ∼ 19°, and θp ∼ 82° for the gold-coated surface shown in Figure 4a. As predicted, (θa − θ) < θp < θa. In the opposite direction of tilting pillars, the rim of the spreading liquid front can make preferential contact with the pillar tip and propagate downward along the coated surface to next pillars, making it much easier to spread in a stick-and-slip manner as shown on the right panel of Figure 4c. Supporting this, we observed that water spreading uphill on the Au/Pd coated, tilted SMP Janus pillar array (Figure 4d), again proving the robustness of this mechanism. The unidirectional wetting exhibited here can be potentially used in microfluidics and for water shedding,14 while the hydrophobicity/hydrophilicity contrast on different sides of the pillars will be useful for fog harvesting.32 Importantly, surface topography and surface chemistry in our Janus pillars can be independently controlled without altering the materials intrinsic properties of the original pillars. Here, surface topography can be tuned by selecting the pillar bending direction and the
thickness of the thin coating. Surface chemistry, on the other hand, can be modified by the choice of thin coating material, which can be further modified, for example, by SAMs of different chain length and end groups and polymer brushes to further enhance the hydrophobicity/hydrophilicity contrast and introduce responsiveness.33,34 We believe that the insights of creating tilted Janus pillars can also be applied to other applications such as actuators, dry adhesives, and optical windows.24,35,36
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Laboratory for Research on the Structure of Matter (LRSM, Penn MRSEC, NSF/DMR-1120901) and Nanoscale Characterization Facility (NCF) are acknowledged for the access to the SEM.
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REFERENCES
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