United through separation: a study of the coalescence behaviour of separating micro-droplets

Abstract

The coalescence of two droplets suspended in a viscous fluid is the focus of this study. When two droplets that are suspended in a shearing flow come in contact with each other, it is currently uncertain if the contact would result in coalescence. To be able to make such predictions, it is necessary to study the effects of the system parameters, such as the angle of collision between the droplets and the properties of the flow and the two fluids, on the process of coalescence. This necessitates the need for a robust experimental setup in which such studies can be performed.

The characteristic of a systematic study is repeatability and control over experimental conditions. In coalescence studies, the impact angle between the droplets is a parameter that has proved difficult to control. In this study, a microfluidic device is developed that uses the concept of surface energy wells to achieve repeatability in the impact angles of droplet collisions. Using the device, droplet coalescence experiments were performed. An interesting observation was made from the experiments which guided the further course of this research work. It was seen that the droplets did not coalesce upon approach, but coalescence was driven by the separation of the droplets. In the literature, this phenomenon is referred to as 'separation-driven coalescence'. Furthermore, it was suspected that an experimental condition could be defined based on a non-dimensional parameter, namely, the Capillary number Ca, such that for Ca > Ca_cr, separation of droplets ceases to trigger coalescence.

The effect of the system parameters, namely the impact angle, θ_i, and the viscosity ratio, λ, over the Ca_cr was investigated. In this study, the presence of a Ca_cr for separation-driven coalescence is confirmed, both experimentally and through a scaling argument. The results of the study indicate that the thickness of the film on the onset of separation influences the Ca_cr. However, a dependency between Ca_cr and λ was not found experimentally. Large experimental uncertainties prevent any further conclusions to be made regarding the Ca_cr.

In this thesis, a framework is developed for the investigation of the Ca_cr for separation-driven coalescence. With more experimental data, a deeper understanding of separation-driven coalescence can be obtained.