Inertial stretching separation in binary droplet collisions

Binary droplet collisions exhibit a wide range of outcomes, including coalescence and stretching separation, with a transition between these two outcomes arising for high Weber numbers and impact parameters. Our experimental study elucidates the effect of viscosity on this transition, which we show exhibits inertial (viscosity-independent) behaviour over an order-of-magnitude-wide range of Ohnesorge numbers. That is, the transition is not always shifted towards higher impact parameters by increasing droplet viscosity, as it might be thought from the existing literature. Moreover, we provide compelling experimental evidence that stretching separation only arises if the length of the coalesced droplet exceeds a critical multiple of the original droplet diameters (3.35). Using this as a criterion, we provide a simple but robust model (without any arbitrarily chosen free parameters) to predict the coalescence/stretching-separation transition.

Automated summary

Binary droplet collisions can result in coalescence or stretching separation, with the transition between these outcomes dependent on high Weber numbers and impact parameters. The effect of viscosity on this transition is revealed in an experimental study, showing an inertial behavior over a wide range of Ohnesorge numbers. Compelling evidence is provided to show that stretching separation only occurs when the length of the coalesced droplet exceeds a critical multiple of the original droplet diameters.