Analysis and optimisation of mixing in binary droplet collisions

Abstract

The collision of binary droplets plays a key role in several industrial, chemical and biological processes. In these processes, the quality of the desired outcome is strongly dependent on the mixing of the liquid droplets as they collide in mid-air. In this work, multiphase direct numerical simulations based on the volume-of-fluid method have been used to investigate the process of mixing and analyse the effects of parameters such as injection velocity, timing and collision angles. The evolution of mixing due to convection and irreversible diffusive processes has been quantified by means of the segregation parameter. To synthesise the outcome of a collision, the impact parameter has been redefined to account for the collision of non-spherical droplets. It has been found that the optimal mixing does not occur for symmetric head-on collisions, but rather at moderately asymmetrical configurations. This behaviour has been explained by analysing the velocity gradient tensor. It has been demonstrated that by breaking the symmetry, the local topology of the flow is altered and the resulting convective flows increase the contact area between the liquids, thereby augmenting the mixing process. However, it was also observed that lateral misalignment transforms the initial kinetic energy into the spinning of the merged droplets, thus preventing an enhanced mixing.