Design of bio-inspired surface topographies via polymer bilayer wrinkling superposition

Naturally occurring patterns on surfaces have evolved to form topological micro- and nano-structures with a variety of functionalities, such as drag reduction, tuneable wetting, anti-microbial resistance and optical effects, often found in insect wings and plant leaves. Wrinkling of bi- and multi-layered materials provides a powerful, versatile, large-surface area patterning methodology to fabricate complex periodic structures with features ranging from 10s of nm to 100s of μm. Here, we investigate the formation of analogous 2D wrinkling patterns in artificial soft materials, built as a superposition of single frequency features. The pattern superposition is achieved sequentially, starting from single frequency wrinkles and generated by stretching a cross-linked elastomer slab that is simultaneously plasma oxidized. The wrinkled surface formed upon the strain release is replicated, then stretched at a specific angle to the first wrinkling step and plasma treated simultaneously to yield a prescribed wrinkling second generation. Specifically, an orthogonal superposition generates checkerboard patterns. Oblique superposition, gives rise to wavy or sand dunes-like patterns with structural characteristics that are independently tunable via changing the plasma experimental parameters.