Photocontrol of liquid motion on an azobenzene monolayer

This paper describes a novel strategy for the non-mechanical motion of a liquid droplet by photoisomerization of surface-immobilized azobenzenes. When a liquid droplet is placed on a substrate surface that has been modified with a monolayer of azobenzene-terminated calix[4]resorcinarenes, alternating irradiation with UV and blue light causes in situ changes in the contact angle of the droplet. The spreading/retraction motion of a liquid droplet is based on the reversible change in surface free energy of the photoresponsive surface due to the trans-cis isomerization of the azobenzene molecules. By measuring the hysteresis in contact angles of various liquids, we determined the principal requirement for the motion of a liquid droplet to occur: a receding contact angle on a trans-rich surface should be larger than an advancing contact angle on a cis-rich surface. Localized illumination of a liquid droplet of several millimeters in diameter causes it to move vectorially across the photoresponsive surface. The direction and velocity of the motion are tailored by manipulating both the direction and steepness of an applied light intensity gradient. Detailed profiles of a moving droplet support the assumption that the surface-assisted motion is ascribed to an imbalance in contact angles. The potential applicability of light-driven motion to lab-on-a-chip technology was presented by delivery of reactants for a chemical reaction on a photoresponsive surface by illumination.