Study of impact velocity and curvature ratio on the dynamic characteristics of double droplets impacting super-hydrophobic tubes

Abstract

In this paper, the impact of successive double droplets on a super-hydrophobic tube surface is numerically studied using a three-dimensional model by the coupled level set and volume of fluid method. The effect of impact velocity on double droplets impact under different curvature ratios is studied. With the increase in impact velocity, two kinds of impact models (out-of-phase and in-phase impact) are obtained, and the coalescent liquid film presents rebound, breakup-rebound, and splash-rebound. With the increase in the curvature ratio, the spread of the liquid film along the circumferential direction is promoted, and the rim of leading liquid film up-warp can be observed earlier. The breakup of an extending liquid film is discussed in detail under the curvature ratio of 1, which is mainly influenced by the combined effects of local negative pressure, air flow motion, surface tension, and gravity of the gathering liquid. The air entrainment occurs for the impact velocity between 0.75 m/s (We = 15.4) and 1.25 m/s (We = 42.9), while no air entrainment occurs for the impact velocity exceeding 1.5 m/s (We = 61.7). The entrapped air bubbles would cause a center breakup of the liquid film or escape from the liquid film. The escaping direction is mainly determined by the pressure distribution in the liquid film.