New Discoveries in Liquid Drop Impacts.

Abstract

In this dissertation, I presented experimental studies on several topics of the problem where a spherical liquid drop impacts onto a liquid layer.

Two unique liquid jets are produced during the impact: the thin, fast and early-emerging ejecta; and the thick, slow and late-emerging lamella. We clarified the identities of the ejecta sheet and the lamella, which were not sharply distinguished in the literature. In particular, we found that for low Reynolds number the ejecta sheet and the lamella merge into a single continuous jet, and for high Reynolds number two jets are separate. Different fates of the ejecta sheet were determined; we also studied the emergence time, position and velocity of the ejecta sheet as a function of the impact velocity and the viscosity of the liquid.

As a simplest type of splash, the crown splash is caused by an instability developed on the rim of the lamella. By performing many experiments for the same parameter values, we measured the spectrum of small-amplitude perturbations growing on the rim. We showed that for a range of parameters in the crown splash regime the generation of secondary droplets results from a Rayleigh-Plateau mechanism of the rim, whose shape is almost cylindrical.

We investigated the dynamics of the lamella with an X-ray phase contrast imaging technique that enabled a complete detection of the interface between liquid and air. We acquired reliable experimental data of the lamella for multiple parameter sets, and compiled them into a database that is used to validate numerical simulations. We used the same X-ray technique to observe other features of the problem, such as the vortices generated inside the liquid and the air bubble trapped underneath the drop.

Liquid drop impact is a two-phase flow problem involving both the liquid and the surrounding gas, however the properties of the latter were often ignored in conventional views of the problem that resulted in the so-called conventional parametrization. We studied the influence of the surrounding gas; our results showed that the density and the viscosity of the gas change the behavior of the impact, and proved the conventional parametrization insufficient.