Superhydrophobic hierarchical structures can not only enhance stability against wetting transition but, if well designed, also produce a larger slip compared to single-scale microstructures. Selective, rather than unilateral, roughening of the microstructure surfaces is desirable to realize increased slip and to study the role of nanoscale roughening; our superhydrophobic hierarchical structures are microscale posts and grates whose top surfaces are kept smooth while sidewall surfaces are etched into nanostructures. To obtain such two-scale micro-nano structures, we have combined two recent microfabrication techniques — gold coating by galvanic displacement and gold-assisted porous etching of silicon. As a result, we have obtained a slip length as large as ∼400 μm on micro-nano structures, which is a ∼100% increase from the previous record of ∼200 μm obtained on micro-smooth structures. Preservation of the microscale geometric parameters was what differentiated our hierarchical samples from others, which showed enhanced stability but would not result in an increased slip. The paper presents the process details developed to fabricate the micro-nano hierarchical structures.

This content is only available via PDF.
You do not currently have access to this content.