Laser-Driven Steering of Liquid Flows in Sessile Droplets

We studied excitation of fluid flows in sessile liquid droplets by short laser pulses. It is shown that the observed flows are Marangoni thermal flows caused by the temperature gradient of the surface tension coefficient and are excited due to the thermal effect of laser pulses on the liquid in the droplet. The cases of pinned and unpinned contact line, as well as different contact angles and different localization of the irradiating laser pulse within the droplet are investigated. The possibilities of both excitation of internal vortex flows in a stationary droplet and movement of the entire droplet on the substrate are demonstrated. The proposed technique can find application to speed up mixing of multicomponent droplets, as an additional mechanism for tailoring liquid flows in evaporating droplets to control the distribution of impurities deposited on the substrate after droplet drying, as well as for laser-assisted control of droplet motion on superhydrophobic substrates in microfluidics and lab-on-chip systems.