Reconfigurable Nanostructures from Wrinkling of Soft Skin Materials

Rhee, Dongjoon

doi:https://doi.org/10.21985/n2-5qd6-3g78

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Reconfigurable Nanostructures from Wrinkling of Soft Skin Materials

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Surface patterns that can reconfigure under external stimuli are important for tailoring diverse properties such as surface adhesion, optical transmittance, and wettability depending on the feature size and orientation. Wrinkling a stiff skin layer on a pre-strained elastomer substrate has emerged as a method to create responsive structures without using complex lithographic processes involving multistep masking and etching. Most efforts to fabricate wrinkles used metal or silica films as skin layers with Young’s moduli much larger than the substrate. This hard-skin-based approach, however, has limited control over functional responses of the patterned surface because (1) the skin/substrate system cannot readily access nanoscale textures and (2) cracks form at random locations in the skin layer and prevent precise tuning of properties. Although cracks can be suppressed by using softer polymer skin layers, solution-based coating techniques commonly used to form these layers have limited control over the film thickness and the resulting wrinkle wavelength. This dissertation presents mechano-responsive wrinkles based on chemical vapor deposition of polymer skin materials. Soft fluoropolymer layers resulting from fluorine-containing plasma treatment allowed for switching of wrinkle orientation without crack formation or delamination at high tensile strains. The plasma process featured nanometer-scale tunability in skin thickness necessary for engineering the wrinkle wavelength both in nano- and micrometer regimes. Because the soft skin layer formed a robust interface with two-dimensional electronic materials, the composite skin exhibited conformal wrinkles that can switch orientation with suppressed crack formation. By patterning local thicknesses of soft skin layers or applying sequential wrinkling processes, we could design structural hierarchy of the patterns in-plane or out-of-plane. With control over characteristic feature sizes and orientations at multiple length scales, our strategy can serve as a promising bottom-up strategy to support a range of functional responses for applications in water harvesting systems, antibacterial fouling surfaces, nanoelectronics, and nanophotonics.

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