Experiments on Drop-impact Splashing, Singular Jets and Coalescence in Emulsions

Abstract

This dissertation describes experiments on drop dynamics. It is split into two main parts: The first investigates the breakup of liquid during the impact of a drop on a pool surface, with focus on the smallest and fastest splashed satellite droplets. The second part studies the much slower coalescence of two minute water droplets in oil inside a micro-channel, with applications to separation of water droplets from crude oil emulsions. First, we study drop-on-liquid impacts in high-speed experiments with extreme time and spatial resolutions using up to 5 million frames-per-second video imaging. This is used to identify and explain two primary mechanisms which produce the smallest and fastest splashed secondary droplets, i.e. ejecta sheets and singular jets. Using a novel 25-m-tall vacuum tube we generate very large impact velocities, to reach regimes in parameter-space never studied before. During the earliest stage of the impact a fast-moving horizontal ejecta sheet emerges from the neck between drop and pool. The breakup of this sheet forms a myriad of micron-size droplets. The ejecta bending is dominated by air resistance, which we investigate under reduced ambient pressures and successfully model based on Bernoulli suction which pulls down the ejecta to hit the pool surface. The ejecta can initially bend up or down depending on the relative viscosities of the drop and pool, bending up if the pool is less viscous. Singular jets are produced by the collapse of drop-impact craters for deep pools, when a dimple forms at the bottom of the crater focusing the energy into a micron-sized region, with jetting velocities over 100 m/s. We use Gerris to study the fine details, obscured in the experiments. In the second part, we study the coalescence of water droplets inside an oil emulsion, developing an empirical relation between the coalescence interaction time tc and the modified shear-rate. This is done by tracking 3-D drop trajectories inside a microchannel, with two perpendicular high-speed cameras. For droplets in crude oil, we implement near-infrared visualization in an innovative device to quantify demulsifier efficiency, using mono-disperse micro-droplets.