Photon-Modulated Wettability Changes on Spiropyran-Coated Surfaces

A coating of photoresponsive spiropyran molecules covalently bound to a glass surface along with a mixture of organosilanes to control the surface environment was prepared. The relatively nonpolar spiropyran can be reversibly switched to a polar, zwitterionic merocyanine isomer that has a much larger dipole moment by UV light, and back again by visible light. The contact angle was 11°−14° lower for dry, spiropyran-coated surfaces after irradiation with UV than that for dry, spiropyran-coated surfaces after irradiation with visible light. The light-induced changes observed in the surface energy were correlated to the switching of the surface-bound spiropyran molecule between polar and nonpolar forms by means of fluorescence spectroscopy and epifluorescence microscopy. Water in capillary tubes coated with the photosensitive layer was observed to rise when the wavelength of incident light was switched from visible to UV. The UV-induced rise was of the order of 2.8 mm for a 500 μm diameter capillary. This microfluidic actuation of water in an enclosed capillary or microchannel using light is termed "photocapillarity". Contact angle hysteresis prevented the water from flowing back down the capillary when the light was switched from UV back to visible.