Effect of Liquid Transparency on Laser-induced Motion of Drops

Abstract

An Experimental investigation of the role of liquid transparency in controlling laser-induced-motion of liquid drops is carried out. Droplets with diameters of 1 4 mm were propelled on a hydrophobic substrate using pulsed-laser beam (532 nm, 10 Hz, 3-12 mJ/pulse) with 0.9 mm diameter fired parallel to the substrate. The test liquid was distilled water whose transparency was varied by adding different concentration of Rhodamine 6G fluorescent dye. Motion of the drops was observed under a camera. Measurements include direction of motion, distance traveled before the drops come to rest, and drop acceleration at the start of the motion. The motion of both transparent and opaque drops was dominated by thermal Marangoni effect. The present results show that direction of motion depends on the drop transparency; opaque drops moved away from the laser beam, whereas transparent drops moved at small angles toward the laser beam. This is plausible because the laser beam was absorbed near the front face of opaque drops, whereas the laser beam was focused near the rear face of transparent drops. Energies lower than 3 mJ were incapable of moving the drops and energies higher than 12 mJ shattered the drops instead of moving them. The distance the drops move follows a log-normal profile, with most of the drops moving about 5 drop diameters. A phenomenological model is developed for the drop motion which explains the physics behind the phenomenon. The entire process of drop motion has the potential of being automated.