Dynamic behaviors of the coalescence between two droplets with different temperatures simulated by the VOF method

The photothermal effect of a laser can be advantageously used to manipulate the droplets coalescence, which usually encounters the coalescence between two droplets with different temperatures. In this study, the dynamic behavior of the coalescence between two droplets with different temperatures in oil is simulated using the volume of fluid (VOF) method. The continuum surface stress (CSS) model is used to describe the interfacial tension force. It is found that the temperature difference between two droplets can cause the self-propelled migration of the coalesced droplet as a consequence of the pressure difference and the induced thermocapillary flow. Particularly, in the cases of large temperature difference, low thermal conductivity and viscosity of the oil phase, the high-temperature region easily shift from one side to another side because of strong thermocapillary flow. Therefore, not only the migration velocity is increased but also the migration direction can change. These results will be helpful for the future operation encountered in the coalescence of two droplets with different temperatures towards the applications of the droplet-based microfluidics.