Light-Driven Motion of Liquids on a

The macroscopic motion of liquids on a ßat solid surface was manipulated reversibly by photoirradiation of a photoisomerizable monolayer covering the surface. When a liquid droplet several millimeters in diameter was placed on a substrate surface modiÞed with a calix[4]resorcinarene derivative having photochromic azobenzene units, asymmetrical photoirradiation caused a gra-dient in surface free energy due to the photoisomerization of surface azoben-zenes, leading to the directional motion of the droplet. The direction and velocity of the motion were tunable by varying the direction and steepness of the gradient in light intensity. The light-driven motion of a ßuid substance in a surface-modiÞed glass tube suggests potential applicability to microscale chemical process systems. A gradient in surface tension can induce a net mass transport of liquids, which affords a driv-ing force for the operation of microfluidic de-vices (1) and for biological cell motility in vitr