Control of the Shape of Liquid Lenses on a Modified Gold Surface Using an Applied Electrical Potential across a Self-Assembled Monolayer

Electrochemical desorption of a patterned, self-assembled monolayer (SAM) of an organothiol on a gold surface can be used to control the shape of drops of liquid hexadeSanethio1 (HDT) on the surface. The contact angle of HDT drops on a thin, transparent film of gold (100 A thick) under an aqueous electrolyte solution could be varied from 128 f 2" when the gold was held at -1.7 V (vs Ag pseudoreference) to 37 ic 2" when the gold was held at 0.0 V. Switching between these two limiting contact angles could be performed rapidly, reversibly, and reproducibly using this system. These drops could act as lenses and could be used as an optical switch which focuses/defocuses light. An array of 1-nL drops could be formed by differential wetting of a gold surface patterned with methyland carboxylic acid-terminated organothiol, and the contact angles of drops in this array could be switched in ca. 50 ms. Alternatively, drops could be confined to an array of gold pads on a conducting glass and switched in a similar fashion.