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 of a UV-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.