Lotus Effect Amplifies Light-Induced Contact Angle Switching

A rough surface morphology is shown to significantly amplify the light-induced change in water contact angle of a photoresponsive surface. Smooth Si surfaces and fractally rough Si nanowire surfaces grown on a Si substrate were studied, both coated with a hydrophobic monolayer containing photochromic spiropyran molecules. Under visible irradiation the spiropyran is in a closed, hydrophobic form, whereas UV irradiation converts the spiropyran to a polar, hydrophilic form, reducing the contact angle. The superhydrophobic nanowire surface both amplifies the light-induced contact angle change by a factor of 2 relative to a smooth surface and reduces the contact angle hysteresis. As a result the UV-induced advancing contact angle is lower than the receding contact angle under visible irradiation, allowing water drops to be moved solely under the influence ofaUV-visible light gradient. The amplification of the reversible light-induced wetting angle change was predicted using the Wenzel model for fractally rough surfaces. The model and amplification effects are expected to apply to other types of stimuli-induced contact angle changes such as that by heat or electrical potentials. The physics of scale require that microfluidic devices exploit new approaches to fluid movement because of an inherently large ratio of liquid surface area to volume. One promising method is the manipulation of surface energy by light t