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Biomimetic Superhydrophobic Silicon Surfaces

Liangliang Cao1, Hsin-Hua Hu2, and Di Gao1. (1) Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, (2) Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15261

Superhydrophobic surfaces are often found on plant (notably Lotus) leaves in nature. Water on these surfaces beads up with a contact angle of greater than 150º and drips off rapidly when the leaves are slightly inclined. This phenomenon, known as the “Lotus effect”, has been attributed to the hydrophobic wax as well as the surface roughness resulted from the hierarchical protrusions found on the leaves. This observation has stimulated extensive research to fabricate artificial superhydrophobic surfaces with self-cleaning properties, by tailoring the roughness of hydrophobic surfaces with intrinsic water contact angles (the water contact angle of a flat surface, Θflat of greater than 90º). A question of significant interest is whether one is able to make a superhydrophobic surface using materials with a Θflat of less than 90º, because this implies the possibility of making intrinsically hydrophilic materials superhydrophobic. Here we show examples of such superhydrophobic surfaces made of silicon with a Θflat of ~70º. The superhydrophobicity is induced by the fabricated overhanging structures present on the surface. Our results provide a new explanation to the “Lotus effect” which agrees with the observations that some leaves with absence of waxes also exhibit superhydrophobicity as well as recent findings that the wax on Lotus leaves has a Θflat of ~74º (in contrary to the expected value of greater than 90º). Furthermore, the fabricated surfaces that are superhydrophobic to water droplets are completely wetted when immersed in water. This phenomenon also resembles what has been observed on Lotus leaves, which however can not be understood by the previous explanations of the “Lotus effect”. Our experiments provide an answer to this problem.