Acknowledgements
6.2 Desert Beetle-Inspired Surface with Patterned Wettability for Fog Collection
6.2.2 Traditional Lithographic Methods for the Fabrication of Biomimetic Patterned Surfaces for Fog Collection
the most commonly used strategy for the fabrication of a surface with pat- terned wettability is the mask-based photolithographic method. photo- lithographic methods all share the same working principle: exposure of an appropriate material to electromagnetic radiation (UV, dUV, eUV, or X-ray) introduces a pattern into the material as a result of a set of chemical changes in its molecular structure.34–36 When masks are used, the lithographic pro- cess yields a replica of the pattern on the mask. Figure 6.2 shows a typical photolithography-based fabrication of the patterned wetting surface by Zahner et al.37 First, a superhydrophobic, microporous poly(butyl methacry- lateco-ethylene dimethacrylate) (bMa-edMa) film was prepared by UV-initi- ated radical polymerization of a prepolymer mixture containing monomers, cross-linkers, porogens, and a UV initiator onto a glass plate. then, the superhydrophobic bMa-edMa film was wetted with a mixture composed of a hydrophilic methacrylate monomer (e.g., [2-(methacryloyloxyl)ethyl]
Figure 6.2 Schematic illustration of typical preparation of patterned wetting surfaces by a photolithographic method. (a) Making superhydropho- bic porous polymer films on a glass support and (b) creating super- hydrophilic micropatterns by UV-initiated photografting. (adapted with permission from d. Zahner, J. abagat, F. Svec, J. M. J. Fréchet and p. a. Levkin, a Facile approach to Superhydrophilic–Superhydrophobic patterns in porous polymer Films, Adv. Mater., 2011, 23, 3030–3034.
Copyright © [2011] John Wiley and Sons.)
trimethylammonium hydrochloride (Meta) or 2-acrylamido-2-methyl-1-pro- panesulfonic acid), benzophenone as the initiator, and a mixture of tert-bu- tanol/water, and then was irradiated with UV light through a photomask. the photografting of the hydrophilic polymer only occurred at the area exposed to the UV light, and after mask removal and washing with solvents, hydrophilic patterns were obtained on the superhydrophobic surface. this method can be used to create superhydrophilic patterns on the superhydrophobic back- ground with feature sizes as small as a few micrometers.
based on this photolithographic method, garrod et al. first fabricated patterned a superhydrophobic–superhydrophilic surface to mimic the des- ert beetle’s back for fog collection.38 as shown in Figure 6.3a, they created patterned hydrophilic polymers on a superhydrophobic background using plasma-chemical deposition through patterned grids as mask. they explored the effect of various hydrophobic–hydrophilic surface functionalities and pattern sizes on water-collection efficiency and found that hydrophilic spots with a size of about 500 µm exhibited optimal water collection performance in the study, whereas spots exceeding 1200 µm showed little to no condensa- tion (Figure 6.3b). For the larger spots (exceeding 1200 µm), it was observed that a critical size for droplet detachment could not be reached: the rate of water loss through processes such as evaporation exceeds the rate of fog droplet condensation. this means that the hydrophilic domains are so large that they act in a way similar to that of purely hydrophilic surfaces. in addi- tion, the authors also revealed there is a minimum hydrophilic spot diame- ter required to facilitate micro-condensation: below 400 µm, droplets were seen to form and quickly reach critical size. however, they lacked sufficient mass to overcome the surface tension, and thus remained attached to the surface. these results are consistent with the Stenocara beetle’s back, which has bumps approximately 600 µm in diameter and 500 µm to 1.5 mm apart.
by using a combination of microcontact printing lithography techniques and surface modification, Varanasi and coworkers fabricated two different hybrid surfaces: flat surfaces with repeating hydrophobic and hydrophilic
Figure 6.3 (a) Schematic illustration of micropatterning a hydrophilic plasma polymer onto a superhydrophobic background. (b) Water condensation study for different sized arrays. (reprinted with permission from Lang- muir, 2007, 23, 689–693. Copyright 2007 american Chemical Society.)
regions, and textured surfaces consisting of pillars that had hydrophobic sidewalls and hydrophilic top surfaces.39 their results also demonstrated that textured hydrophobic surfaces with hydrophilic tops promote nucle- ation and growth of Cassie-type droplets and therefore exhibited superior droplet shedding properties under condensation.
also by using the photolithography-based method, Chen and coworkers pre- pared hierarchical nanograss micropyramidal architectures with hybrid wet- tability.40 globally, the structured surfaces were superhydrophobic, favoring the departure of the condensing droplets (Figure 6.4). Locally, they had wetta- ble patches that facilitated droplet growth and preferential coalescence. With such 3d structured hybrid surfaces, they showed that the number density of droplets was enhanced by approximately 65% and the spontaneous removal volume of droplets was dramatically increased by approximately 450%.
recently, bai and coworkers reported the fabrication a novel kind of surface with star-shaped wettability patterns.41 in their study, a super- hydrophilic surface was first fabricated by depositing tiO2 slurry onto a bare glass slide via a spin-coating method. then the film is treated with a
Figure 6.4 (a) Schematic drawing showing different surface roughness on the smooth sidewalls of the pyramids, nanograssed sidewalls, and nanograssed floor. (b) SeM image showing the as-fabricated nanograssed micropyra- mid arrays on a Si wafer. (c) drop departure dynamics on surfaces with hierarchical nanograssed micropyramid arrays. (d) Schematic drawing of drops sitting on the nanograssed micropyramid arrays. the synergy between the micro- and nanoscale roughness features resulted in a sta- ble Cassie state and upwards surface tension force, which assisted in drop departure. adapted with permission from X. Chen, J. Wu, r. Ma, M. hua, n. Koratkar, S. Yao and Z. Wang, nanograssed Micropyrami- dal architectures for Continuous dropwise Condensation, Adv. Funct.
Mater., 2011, 21, 4617–4623. Copyright © [2011], John Wiley and Sons.
heptadecafluorodecyl-trimethoxysilane (FaS) to change the wettability from superhydrophilic (Figure 6.5a) to superhydrophobic (Figure 6.5b), where fog droplets hardly wet the surface and remain in a spherical shape (Figure 6.5b). in the next step, photomasks with a circle-shaped pattern or 4-, 5-, 6-, and 8-pointed star-shaped patterns were used to obtain features of wetta- bility patterns via selective illumination of UV light, which decomposed the FaS monolayer (Figure 6.5c) by the underneath photocatalytic tiO2. On the surfaces with star-shaped wettability patterns, fog droplets could be direc- tionally collected toward more wettable regions (Figure 6.5c). they found that such surfaces with star-shaped patterns realize more efficient water collection compared to other surfaces that are uniformly superhydrophilic, uniformly superhydrophobic, or even circle patterned. the improved water collection by the star-shaped pattern is because the tips of the star generate a Laplace pressure gradient from the shape gradient, which further enhances this directional movement of water droplets toward the center of the stars.