Paper SE-TuM12
Tailoring the Wettability of Vertically Aligned Si Nanorod Arrays by Glancing Angle Deposition

Tuesday, October 16, 2007, 11:40 am, Room 617

We report a facile method to tailor the wettability of vertically aligned Si nanorod arrays by glancing angle deposition. As-prepared Si nanorod array surface is hydrophilic, while after coating a fluorocarbon layer, it becomes hydrophobic. For vertically aligned nanorod arrays, when increasing the nanorod height from tens of nanometers to several microns, the as-prepared samples become more hydrophilic while fluorocarbon coated samples become more hydrophobic. A wetting transition from a rough surface to a composite surface is observed at the same critical nanorod height (~ 150 nm for deposition angle of 86^o) for both hydrophilic and hydrophobic surfaces. This critical height decreases when increasing the sample deposition angle, i.e., reducing the surface coverage of the nanorods. With a deposition angle of 88^o, both superhydrophilic (contact angle = 0) and superhydrophobic (contact angle = 170^o, and contact angle hysteresis < 10^o) surfaces are obtained. For tilted nanorod arrays, strong anisotropic wetting behavior is observed in the plane formed by the vapor incident direction and the substrate normal. After hydrophobization, the difference between the left and right (nanorod tilting direction) contact angles of a water droplet could be as large as 30^o. Such a droplet is able to move along the nanorod tilting direction under external disturbances. This tilting nanorod array provides a new morphological manipulation method for liquid delivery in microfluidic or nanofluidic devices.