Computational Modelling of Drop-Drop Collisions in the Presence of Gas Microfilms: When Do Drops Bounce?

Collisions and impacts of drops are critical to numerous processes, including raindrop formation, inkjet printing, food manufacturing and spray cooling. We will see that with increasing speed, drop collisions undergo multiple transitions: from merging to bouncing and then back to merging, which were recently discovered to be surprisingly sensitive to the radius of the drops as well as the ambient gas pressure. To provide new insight into the physical mechanisms involved and as an important predictive tool, we have developed a novel, open-source computational model for the collision and impact of drops, using the finite element package oomph-lib. This uses a lubrication framework for the gas film, incorporating micro and nano-scale effects into an interfacial flow. Our simulations show strong agreement with experiments of impacts and collisions, but can also go beyond the regimes considered experimentally. We will show how our model enables us to explore the parameter space, probe different regimes of contact and gas film behaviour, with the aim of predicting the minimum film height and the critical impact speed for contact to occur. Beyond this, we can extend with novel lubrication models to consider Leidenfrost conditions and impacts of drops onto liquid films.