Impact of water droplet on oil films suspended over water bath: Crater and jet dynamics

Abstract

The water droplet impact dynamics on an oil film over the water bath are studied based on extensive experiments, with a focus on the effects of key parameters (droplet size, impact velocity, and oil-film thickness) on the crater growth and the jet formation. A multi-interfaces theory, considering the surface-energy variations of the deformed droplet–air, oil–droplet, and oil–water interfaces, is proposed to describe the total change in the surface energy during the impact, leading to an effective surface tension and Weber number for the following analysis. The maximum crater depth follows a scaling $d_e^{\prime }\sim \left(F{r}^{1/4}/{\rho }^{\prime }\right)$, consistent with classical theory, although the crater depth is slightly overestimated for thick oil films due to an underestimation of viscous dissipation. The total crater energy accounts for approximately 27∼35% of the initial droplet impact energy for$\text{Fr}\ge 100$. Furthermore, the maximum jet length scales as $l^{\prime }\sim \text{We}$, with jet pinch-off occurring when $l^{\prime }\frac{>}{\sim }1.24$, demonstrating the capability of the proposed multi-interfaces theory to capture crater and jet dynamics. The measured maximum jet length also falls into the classical scaling relationship with the effective Weber number. Finally, two main impact regimes are identified using the effective Weber number $W{e}_{e\_h}$ in combination with the product of the Reynolds and Froude numbers $ReF{r}_h$. Generally, the developed multi-interface theory can correctly estimate the effect of the oil layer on the impact dynamics.