AVS 51st International Symposium
    Surface Science Friday Sessions
       Session SS1-FrM

Paper SS1-FrM9
Light-Induced Contact Angle Switching on Nanowire Surfaces

Friday, November 19, 2004, 11:00 am, Room 210B

Session: Hydrated Surface Phenomena
Presenter: S.T. Picraux, Arizona State University
Authors: S.T. Picraux, Arizona State University
R. Rosario, Arizona State University
T. Clement, Arizona State University
J.L. Taraci, Arizona State University
J.W. Dailey, Arizona State University
D. Gust, Arizona State University
A.A. Garcia, Arizona State University
M. Hayes, Arizona State University
Correspondent: Click to Email
We combine monolayer surface chemistry with silicon nanowire substrates to create a lotus leaf like surface, and for the first time demonstrate the amplification of light-induced water contact angle switching. Si nanowires are grown by the vapor-liquid-solid growth technique and the air oxidized surfaces are functionalized with tert-butyldiphenylchlorosilane and perfluorooctyltrichlorosilane, followed by 3-aminopropyldiethoxymethylsilane, to which a photochromic spiropyran molecule was attached. Measurements of the contact angle for water on both the smooth and nanowire surfaces allow direct estimation of the effects of surface morphology on hydrophobicity. Functionalized nanowire surfaces with contact angles above 90° on the smooth surface exhibit superhydrophobic behavior, whereas those with smooth surface angles below 90° exhibit superhydrophilic behavior. Spiropyran-functionalized surfaces show reversible photoswitching of the contact angles. When irradiated with UV light (366nm) the spiropyran is converted from a closed, nonpolar form to a highly polar open form. Visible light (450-550 nm) irradiation of the spiropyran coating yields a relatively hydrophobic surface (higher contact angle) that can be reversibly converted into a more hydrophilic surface (lower contact angle) with UV light irradiation. The nanowire surfaces are observed to exhibit a significant amplification in the contact angle change over that for smooth surfaces (from 12° to 23°). The roughness induced amplification of contact angle switching was accurately predicted using a Wenzel model for contact angles on fractal surfaces. These results, based on a biomimetic approach to nanotechnology, have wide ranging implications for the design of microfluidic systems.