Coalescence-Induced Bubble Departure: Effects of Dynamic Contact Angles.

Coalescence-induced bubble departure is a common phenomenon in boiling and gas evolution reactions, which has significant impacts on the heat/mass transport. In this work, we systematically investigate the effects of dynamic contact angles on the coalescence and departure processes of two equal-sized bubbles. A critical contact angle (θ) of 76° is determined for an ideal surface on the basis of a surface energy analysis, beyond which the coalesced bubble does not depart from the wall. Using 3D multi-relaxation-time (MRT) lattice Boltzmann simulations, we demonstrate that the advancing contact angle mainly governs the movement of the outer side of the contact lines, and the increase of the advancing contact angle may delay or even prevent the departure of the coalesced bubble. On the other hand, the receding contact angle dominates the motion of the inner side of the contact lines, and the decrease of the receding contact angle facilitates the departure of the coalesced bubble. We identify a regime map for the coalescence-induced bubble departure with respect to the contact angles, which includes four regions: the all-departure region, the advancing contact angle dominated region, the receding contact angle dominated region, and the nondeparture region. Numerically simulated critical contact angles that separate the above-mentioned regions agree well with theoretical analyses. The results of this study will contribute to the manipulation of bubble behaviors and the optimal design of working surfaces in a variety of energy systems involving boiling and gas-evolving reaction processes.