Thermocapillary actuation of binary drops on solid surfaces

On hydrophobic solid surfaces, aqueous drops are typically not conducive to thermocapillary actuation. This letter reports thermal mobilization of water drops by encapsulating them with a long-chain alcohol. On a parylene-coated silicon substrate, a water-heptanol binary drop can assume a unique shape: the dome-shaped water drop is capped by a layer of heptanol, and the heptanol cap protrudes through a foot to a precursor film. For intermediate drop diameters, the speed of the binary drop is linearly proportional to its diameter and the imposed temperature gradient, with an offset accounting for the hysteresis force. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3632041]