Light-actuated water droplet motions on ZnO nanorods

Water droplets were either pushed or pulled with an ultra-violet (UV) light on vertically aligned ZnO nanorods. Steric acid-immobilized ZnO nanorods grown on quartz substrates exhibit a hydrophobic surface possessing high contact angles between water droplets and the substrates. Exposure of UV onto droplets on ZnO NRs led to reduction of contact angles and resulted the internal circulating flows inside the droplets. Droplets located at different sites under the spot of the UV light created different magnitudes of contact angle changes and the internal circulating flows which allowed us to push the droplets away or pull the droplets toward the centre of the UV spot.