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. Experiments are conducted on two double-roughness surfaces with different surface roughness, achieved by varying the size of the microscale roughness features. The surfaces are characterized in terms of static contact angle and roll-off angle and compared with surfaces consisting of only single-tier microscale roughness. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces are more resistant to wetting than the single roughness surfaces and show higher coefficients of restitution for droplets bouncing off the surface. The droplet dynamics upon impingement are explored.
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