Dynamics of Drop Impact on Liquid Film

Abstract

Drop impact on the surface of a liquid film or pool is ubiquitous in many natural and industrial applications. However, despite of being a classical problem that has been extensively studied, the detailed dynamics is still not fully understood, especially when the liquid film thickness is neither too small, relative to the drop size, for impact to resemble that on solid surface, nor too large such that the impact is free from effects of the bottom substrate.

This dissertation provides a comprehensive study on the dynamics of drop impact on a liquid film with thickness comparable to the drop radius, and investigates various aspects of the impact process to analyze the interplay of the controlling mechanisms such as the impact inertia, surface tension, liquid viscosity, and the resistance from the bottom solid substrate.

The dissertation first presents and discusses the global impact outcomes including bouncing and merging in a regime diagram of Weber number, We, which characterizes the effect of impact inertia and surface tension, and liquid film thickness normalized by drop radius, H∗. It is found that, in general, across all film thicknesses high We impacts exclusively exhibit merging outcomes, while low We impacts result in bouncing. However, for intermediate We, a non-monotonic transition is observed when the impact outcome non-monotonically transitions from merging, to bouncing, to merging, and to bouncing again as the film thickness is increased from very small values; with the merging time in the extended merging regime at intermediate film thickness being characteristically longer than those in other regimes. In addition, for liquids with different viscosities, the regime diagram is found to maintain this general feature but with shifts in the transition boundary. Higher viscosity is found to render merging more difficult and as such the extent of the merging regimes shrink. The controlling physics of each transition boundaries including the role of viscosity are identified and discussed, leading to derivation of the scaling laws for the transition boundaries which are found to agree well with the experiments.

After identifying the bouncing and merging outcomes in different regimes, specific dynamics pertaining to each outcome are explored. For the bouncing outcomes, from the bottom view, the thickness of the micron-scale interfacial gas layer trapped between the drop and liquid surface, hence preventing them to coalesce, is measured through color interferometry and analyzed for different conditions. It is found that for thin films where the bottom substrate has the most significant influence, the gas layer behavior resembles that of the drop impact on a solid surface. For deep pools where the liquid surface can deform freely without any resistance from the solid substrate, the effect of the surface capillary wave dominates the gas layer dynamics. At intermediate film thicknesses, the gas layer dynamics exhibit a mixed feature of the thin film and deep pool, with nonmonotonic variation in the center gas layer thickness. Furthermore, the dynamics of the drop shape oscillation is also studied from the top view for various impact speed, liquid viscosity and liquid film thickness. The dynamics are analyzed in the form of three key features, namely the maximum spreading diameter of the drop, which is found to have smaller value compared to the drop impact on solid surface owing to reduced resistance from the solid substrate; the shape oscillation time scale, which is found to be controlled by the capillarity of the drop; and the damping of the oscillation, which has been correlated with the viscosity and the film thickness.

Finally, for impacts that result in merging, the trajectory of the drop inside the liquid film after merging is measured and analyzed using laser induced fluorescence. Three stages are identified. The first stage, when the drop just merges with the liquid surface, is found to be mostly inertia controlled, and thus shows a linear penetration process. At the second stage, the drop penetration shows a combined effect of capillarity of the deformed liquid surface and viscous drag from the ambient liquid, demonstrating an oscillation behavior. At the final stage the drop is far from the liquid surface to be affected by capillary such that viscous drag continuously slows down the drop as it penetrates through the liquid pool. The scaling of the drop motion is obtained for all three stages, which agree well with the experiments.