Droplet formation caused by laser-induced surface-tension-driven flows in binary liquid mixtures

Abstract

Experimental and numerical study of the droplet formation mechanism in thin layers of water-isopropanol mixtures under the laser heating is presented. It is shown that after the beginning of the irradiation the thermocapillary convection dominates that result in the thinning of the layer in the heated area due to the flow of liquid into the cold area. In a few seconds, the loss of isopropanol in the heated area due to evaporation become significant that produces solutocapillary convection with the opposite directed flows. As a result, a droplet appears in the laser beam and continuously grows owing to the solutocapillary liquid flow through the wetting film between the droplet and the receding layer. The experimental results indicate that an increase in the initial concentration of isopropanol in mixture leads to a decrease in the droplet growth rate at a given power of the laser beam. A numerical model of the droplet formation process is proposed. The evolution of the liquid-gas interface is modelled in the frame of lubrication theory including the dependency of surface tension on both temperature of mixture and concentration of volatile component. The model takes into account the dynamics of the vapor in the gas phase including the convective and diffusion mechanisms. A reasonable agreement between the numerical and experimental results is shown.