A simulation study of the forward and backward thermocapillary migration of fluids in a microchannel

In this study, the forward and backward thermocapillary migration of fluids in a microchannel is numerically investigated. Both the upper wall and the lower wall of the microchannel are set to be an ambient temperature. Two 40mW heat sources activated periodically are placed on the left side and the right side of the droplet in a microchannel. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet. The isotherms inside the droplet are extremely distorted by the thermocapillary convection. The forward and backward thermocapillary migration results in the net thermocapillary momentum which drives a water droplet moves from the hot side of the open channel to the cold side. The temperature gradient at the free interface on the side of acting heat source is always smaller than that on the cold side. The actuation velocity of the liquid droplet first increases significantly, and then decreases continuously for various interval times. The dynamic contact angle of a water droplet is strongly affected by the forward and backward oil flow motion and the net thermocapillary momentum inside the droplet. It is alternated due to the pressure difference acting on the free interface between two immiscible fluids during actuation process.