Coalescence behavior and mechanism of autopoietic droplets on slippery liquid-infused porous surfaces (SLIPS)

Droplets coalescence is an important stage in the self-repair process of the SLIPS. To reveal the coalescence behavior and mechanism, a numerical model for the coalescence of microdroplets on SLIPS was established. The morphological changes and flow characteristics during droplets coalescence were analyzed. The result shows that a concave liquid bridge is formed when the spreading leading edges of the two droplets meet. And the pressure in the liquid bridge is negative. During the coalescence process, the absolute value of the negative pressure gradually decreased. After two droplets merge into a large droplet, the pressure in the large droplets is positive and far less than that in the droplets before coalescence. In the process of coalescence, a velocity gradient will appear on the liquid-gas interface, resulting in vortex rings in the liquid bridge. The vorticity of the main vortex ring is continuously released to produce many small vortex rings, which causes the liquid-gas interface to fluctuate until the vorticity is completely dissipated. With the increase of the inherent contact angle of the wall, the vorticity of the vortex ring near the liquid bridge is larger, and the height of the bridge increases rapidly. The existence of the vortex ring can promote droplet coalescence.