Multi-scale simulation of droplet–droplet interaction and coalescence

Highlights: • A multiscale method for simulating droplet collisions efficiently. • Reynolds equation used to model the thin-film evolution. • Novel coupling via pressure between thin film model and surrounding flow. • Good agreement with experimental observations under different collision conditions. In this work, droplet–droplet interaction is modelled using a multi-scale approach which couples multiphase flow simulation using a volume of fluid method to a surface thin film model operating on the sub-grid scale. The volume of fluid model is based on a multiple marker method with a smoothed surface tension calculation, and a thin film model is derived to simulate film drainage using a Reynolds equation approach. A novel method of coupling the two allows for the prediction of coalescence or rebound of colliding droplets essentially from first principles, relying only on a critical film thickness parameter. The model is implemented using the open source tool set OpenFOAM and tested against experimental results of colliding hydrocarbon droplets from the literature. It is found to produce accurate interface deformation results for the duration of the collision, and to consistently predict the outcome of the collision process.