COMPUTATION ON HEAD-ON COLLISION OF 2 IDENTICAL DROPLETS WITH CONSTANT SURFACE-TENSION COEFFICIENTS

A computational model to simulate head-on collision of two identical d roplets with constant surface tension coefficients was constructed. A potential flow was postulated inside each droplet with a thin film vis cous air flow between the two droplets. The potential flow was solved by the BEM, whose tangential velocity on the collision surface was emb edded into the analytic thin film solution to determine the minimum th ickness of the air film. Bouncing takes place when the minimum thickne ss is larger than the effective distance of the molecular force, while coalescence occurs on the contrary. Detailed information on droplet d eformation and the thin film air flow was obtained for different surfa ce tension coefficients. It is found that the collision process progre sses in a way that the droplet is first deformed to a flat one and the collision area increases in time. As the curvature increases at the e dges, the droplet recovers from its flat shape and the collision area begins to shrink. The distance of the droplet mass center, the air fil m thickness and the pressure on the collision surface first decrease, approach their minimums in turn, and then increase in time. In the ear ly stage of collision, the pressure at the center of the collision sur face is lower than those at the droplet edges due to large curvatures there. The tangential velocity on the collision surface initially dire cts outward and then inward, with maximum slopes occurring at a small distance from the droplet edges. The air film thickness exhibits a W-t ype distribution in most of the time and changes to a U-type at the mi nimum pressure instant. Consequently, the air film approaches its mini mum thickness at the center of the collision surface, where a physical coalescence may start off. The mechanism of the head-on collision of the liquid droplets is therefore revealed.