Single-walled carbon nanotube pillars: a superhydrophobic surface.

A series of SWNT arrays have been constructed by disproportionation of carbon monoxide on Co-Mo catalyst films deposited on flat Si wafer substrates. The arrays include a two-dimensional SWNT thin film prepared by spraying a SWNT suspension, as well as random SWNT networks, and vertically aligned SWNT. In addition, a novel SWNT array has been developed and termed SWNT "pillars". These pillars are prepared by controlled synthesis of SWNT over a templated substrate. When water droplets are deposited on these surfaces, it is observed that the static contact angle changes dramatically with the type of SWNT array. The higher is the degree of surface roughness (at the nano and microscales) of these structures, the higher is the contact angle. The novel SWNT pillars exhibit superhydrophobicity (i.e., contact angle approximately 160 degrees). To quantify the effect of surface roughness, two simple models have been employed. The Wenzel model can be used to quantify the behavior of the SWNT thin films while the Cassie-Baxter model is used for the SWNT arrays with three-dimensional structure (grass, forest, and pillars).