Laser-driven Marangoni flow and vortex formation in a liquid droplet

Abstract

We present a systematic study of the laser-driven Marangoni flow and curvature induced vortex formation in a copper sulfate pentahydrate solution, visualized by dispersed carbon nanotube (CNT) bundles. The experiments are carried out using different objectives of numerical aperture (NA) in the range of 0.1–0.6 to investigate the effect of focusing on the flow dynamics. The flow velocities measured (for 0.1 NA) are in the range of 2 mm/s–5 mm/s depending on the size of CNTs. Both primary and secondary vortices are observed in our experiment. In the primary vortex, with a sixfold increase in NA, a tenfold increase in the angular velocity of CNTs is measured. We also discuss the important role played by the curvature of the droplet in the vortex formation. The numerical simulations carried out for flow velocity are in agreement with the experimental values.