Numerical simulations on the self-motion of droplets in hydrophobic microchannels driven by wettability gradient surfaces

Abstract

In this work, numerical simulations were performed to investigate the self-motion behaviors of droplets in microchannels driven by wettability gradient surfaces. The effects of the surface gradient, wetting condition, and channel width on the droplet movement were analyzed and evaluated. As the wettability gradient was established onto a channel wall, the driving power provided by the three-phase contact lines (CLs) enabled the spontaneous droplet movement toward the high-wetting side in a range of several to more than 10 cm/s. The droplet was found to move faster at a larger wettability gradient under an identical initial wetting condition, as well as at a smaller initial contact angle under the same wettability gradient. Additionally, the droplet moved faster and more stably with the increase of the channel width due to the reduction of flow resistance and the greater driving force. During the self-motion process, the droplet velocity initially increased dramatically and then tended to a constant value accompanied by local fluctuations. Vortexes generated by the velocity difference between the CL and the droplet body were primarily observed around the CLs inside the droplet, which was also responsible for the droplet movement.