Numerical investigation on spontaneous droplet/bubble migration under thermal radiation

The spontaneous migration of droplet/bubble has widely application in manipulating droplet/bubble in microgravity. In this study the droplet/bubble migration due to thermal capillary force under uniform incoming thermal radiation is numerically investigated. The numerical model is based on the transient two-dimensional axisymmetric model with a level set method. After validation with the analytical solution under low Reynolds numbers and Marangoni numbers, the study is extended to the droplet/bubble migration under high Reynolds numbers and Marangoni numbers. The ratios are defined as the values in the outer continuous phase over those in the inner discrete phase. The ratios of dynamic viscosity, thermal conductivity, density and specific heat on the migration velocities of droplet/bubble are investigated under all Reynolds numbers. It is found that the droplet/bubble can migrate spontaneously under thermal radiation due to thermal capillary effect, the driving force is from the induced pressure drop in the middle region of droplet/bubble. Under low Reynolds numbers the migration velocities depend only on the ratios of dynamic viscosity and thermal conductivity, while under high Reynolds numbers they also depend on the ratios of density and specific heat. Under all Reynolds numbers the migration velocities are the lowest when the dynamic viscosity or thermal conductivity in the continuous phase is equal to that in the discrete droplet/bubble phase. With the increasing difference between the continuous phase and discrete phase, the migration velocities are increased. This study might be helpful for manipulating droplet/bubble through thermal capillary force with incoming thermal radiation.