Universal, geometry-driven hydrophobic behaviour of bare metal nanowire clusters.

A parallel array of isolated metal nanowires is expected to be hydrophilic. We show, however, that a clustering of such nanowires brought about by vacuum drying produces a 'dual-scale roughness' and confers a strongly hydrophobic property to the surface. The mean size of the nanowire clusters as well as the contact angle are both found to be related to the wire length, and the critical wire length above which the surface becomes hydrophobic is ≈10 µm. Surface roughness is generally known to enhance water-repellent properties, but this is the first report of roughness-induced hydrophobicity on a bare (uncoated) metallic surface.