Droplet Dynamics under Thermocapillarity

Abstract

The coalescence of droplets evoked by a temperature gradient in a microgravity environment, and the underlying mechanism of this phenomenon are investigated in this thesis by using Direct Numerical Simulations (DNS) and the Phase Field method to capture the interface. As the interaction of flow, interface and temperature are intertwined and complex, insights into the interplay are presented in the following chapters. Due to the dependence of surface tension on temperature, which is variable along the interface, Marangoni stresses are generated. Such stresses can cause droplet migration, which further affect the locally induced Marangoni stresses and droplet coalescence. On the other hand, the Marangoni stresses can promote the drainage of the film separating the droplets, thus promoting coalescence. Furthermore, the droplets deform, as temperature reduces the surface tension. Deformation, in turn, influences coalescence. All these different effects and their interplay then define the outcome of the droplet interaction. The structure of the thesis is as follows: first, the complex droplet interactions are investigated based on the data obtained from the numerical simulations with focus on the influence of an evolving temperature gradient. Subsequently, models are developed to describe and predict droplet interactions.