Numerical investigation of the forward and backward thermocapillary motion of a water droplet in a microchannel by two periodically activated heat sources

The main objective of this paper is to numerically investigate the transient forward and backward thermocapillary motion of a water droplet in a microchannel occupied by the hexadecane oil. The top and bottom walls of the microchannel are kept at the ambient temperature. Two heat sources activated periodically are put on the front side and rear one of the droplet in a microchannel, respectively. When the heat source is activated, two vortices are formed inside a water droplet and the oil flow passes over the droplet in the microchannel. The forward and backward thermocapillary migration caused by two periodic heat sources results in the thermocapillary stress gradient along the fee interface which drives a water droplet to move to the cold side of the open channel. The mechanism of a droplet migration behavior is consistent with the previous experimental observation. The water droplet first accelerates rapidly, and then decelerates significantly at various intervals. The dynamic contact angle is significantly altered owing to the pressure change exerting on the hexadecane/water interface during the actuation process. The velocity of a water droplet is enlarged by a higher heating power and a larger microchannel height.