Particles, Drops, and Bubbles in Gradient Fields

Particles, drops, and bubbles submerged in a host liquid are omnipresent in nature and industry. Often, they are subjected to gradients in concentration or temperature (or both). These gradients might change locally or evolve over time, placing the system far from its equilibrium. This gives rise to extraordinary rich physics at the intersection of fluid dynamics, chemical engineering, and colloid and interface science.
In this thesis, we investigate the behaviour of particles, drops, and bubbles in applied gradient fields. We focus on small-scale, idealized table-top experiments closely combined with theoretical and numerical modelling to study these objects under conditions that are far from equilibrium. For the latter, we consider particles, drops, and bubbles at a water-ice interface during unidirectional solidification (Part I) and immiscible drops in density stratified ethanol-water mixtures (Part II).

In Part I we deal with the freezing of suspensions and oil-in-water emulsions in order to study the interaction between different objects and an approaching water-ice solidification front. To do so in a controlled manner, we apply a thermal gradient over our sample and ensure slow, uni-directional freezing. We then change the type of object near the front to add more and more complexity to the system, starting with solid particles (chapter 1) before moving on to drops (chapter 1-4) and eventually bubbles (chapter 5).

In Part II we study the dynamics of immiscible drops in a density stratified ethanol-water mixture. These studies further investigate the peculiar observation that these drops can show continuous bouncing, against gravity, caused by an oscillatory solutal Marangoni flow around the drop. In chapter 6 we look in depth into the onset of the bouncing instability and extend the experimental parameter space by changing the viscosity of the oil, in order to determine the different mechanisms that trigger it. Finally, in chapter 7, we dive further into the characteristics of the bouncing cycle through well-performed experiments and numerical simulations, aiming for a one-to-one comparison.