Front-Tracking Simulation of a Drop Actuation Utilizing Temperature Dependency of Wettability and of Surface Tension

Abstract

A drop actuation utilizing the wettability change by laser beam irradiation was studied by numerical simulations. The drop is actuated because surface tension force acts on the irradiated side more strongly in the wall parallel direction due to the smaller contact angle on that side. On the other hand, the temperature gradient due to laser irradiation causes surface tension gradient, then so-called Marangoni flow is caused. Each flow direction due to the wettability effect and the Marangoni effect is opposite. Those effects were evaluated by numerical simulations by a front-tracking method, which can properly evaluate the surface tension effect even on the solid surface and even with spatial gradient. As the results, we found that a simple model considering the macroscopic force balance could predict the moving condition due to wettability change fairly well although it could not predict the condition of movement caused by Marangoni effect. It is shown that the present numerical approach is very useful to predict such complex interfacial phenomena.