Light Moves Water Droplets

A team at Arizona State University in Tempe reported in the Aug. 26 issue of Journal of Physical Chemistry B that a nanostructured surface morphology similar to that of lotus leaves imparts the ability to move 15-µl droplets of water using light. Silicon surfaces roughened with 20- to 50-nm-diameter nanowires and coated with photochromic spiropyran displayed a lower water contact angle under UV than under visible-wavelength illumination, so that water droplets on the surface moved toward the UV end of an applied radiation gradient. The effect may have applications in the design of microfluidic systems for pharmaceutical development and for environmental and biochemical analysis. In this image, the drop on the left sits on the coated nanowire surface, and the one on the right is on similarly coated flat silicon. The droplets are treated with a fluorescent dye. Courtesy of Arizona State University.