Characterization of Ultrahydrophobic Hierarchical Surfaces Fabricated Using Single-Step Fabrication Methodology

Hydrophobic surfaces with microscale roughness can be rendered ultrahydrophobic by the addition of sub-micron scale roughness. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. Using this method the complexities generally associated with fabrication of two-tier roughness structures are eliminated. Three two-tier roughness surfaces with different roughness parameters are fabricated and tested. The surfaces are characterized in terms of static contact angle and roll-off angle and are compared with surfaces consisting of only single-tier microscale roughness. The evaporation characteristics of a sessile droplet on the hierarchical surfaces is also assessed relative to comparable single-roughness surfaces. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces exhibit very high contact angle and lower contact angle hysteresis compared to the single roughness surfaces and are more resistant to wetting. The energy loss during impact on the surfaces is quantified in terms of the coefficient of restitution for droplets bouncing off the surface.