We report a low-cost, scalable way of fabricating hierarchical micro/nano structures with re-entrant shapes for superhydrophobic surfaces with extremely low hysteresis. Superhydrophobic surfaces have numerous applications in self-cleaning, anti-fouling, anti-freezing and heat transfer enhancement. Surface nanofeatures reduce the actual contact area between a solid surface and liquid droplet�thus contact angle increases and sliding angle decreases. In contrast to most hierarchical geometries with simple micropillar structures that do not prevent water from falling into gaps, re-entrant structures minimize the droplet contact area by repelling water from contacting the micropillar sidewalls. We combined the hierarchical micro/nanostructuring with re-entrant profiles for the micropillars (Fig. 1a-d) to draw advantage of both effects. Re-entrant microstructures were produced using conventional silicon microfabrication, and silicon nanopillars (SiNPs) were patterned over these microstructures by metal-assisted-chemical etching (MACEtch). We have developed a unique method of treating rough sidewall profiles resulting from a DRIE process that enables complete coverage of SiNPs on patterned microstructures. Our results (Fig. 1e) indicate that tuning the re-entrant geometry (area fraction) drives hydrophobicity, which is further strengthened by the SiNPs that prevent hysteresis by reducing the sliding angle well below the 2� criterion for superhydrophobicity. We have demonstrated water contact angle of 170� and minimum hysteresis with a sliding angle as low as 0.5�.

Hierarchical and re-entrant micro/nanostructures for superhydrophobic surfaces with extremely low hysteresis

246th National Meeting (2013)

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