Collisions of viscous droplets

From industrial food and drug processing to car and jet engines, the interactions between small liquid droplets have a surprisingly large effect on our lives. Wherever they occur, the interactions are complex—involving multiple phases and multiple time and length scales. And as shown in the figure, collisions can have different outcomes, including reflexive separation (left), stretching separation (right), and coalescence (center). Although numerical, theoretical, and experimental efforts over the past few decades have explored the underlying fluid dynamics, a detailed understanding, especially concerning the role of viscous forces, has yet to emerge. For water droplets, viscous forces are usually much less relevant than surface tension, but in spray drying and other industrial applications they can be substantial and lead to significant energy loss as the droplets merge and deform. To better understand the effect of droplet viscosity, a team of Dutch researchers led by Hans Kuipers (Eindhoven University of Technology) has analyzed the influence of viscous energy dissipation on the outcomes of 116 simulated droplet collisions. Based on their results, the researchers derive a phenomenological model that captures the dependence of the collision outcome on the droplets’ viscosity, impact parameter, and ratio of kinetic energy to surface tension. The model may help improve the production of milk powder, infant formula, and many other goods. (G. Finotello et al., Phys. Fluids 29, 067102, 2017, doi:10.1063/1.4984081 .)