An Experimental and Theoretical Study of Binary Droplet Collisions

Understanding the outcomes of binary droplet collisions is important to many areas of physics and technology. For a given system, the collision outcomes can be mapped, in the parameter space of impact parameter (offset between the droplets centres) and Weber number, with clear regimes boundaries. As Weber number increases, the collision outcomes are bouncing, coalescence, stretching separation at high offset and reflexive separation. This work, in collisions of miscible droplets, experimentally and theoretically investigates the role of the viscosity, viscosity difference, size and size ratio as they have impacts on the collisions’ outcomes and hence the position of the regime boundaries in the regime map. Increasing the viscosity, decreasing the size and decreasing the size ratio (d_s/d_l), qualitatively, have the same impact on the collisions outcomes, as both enhance the coalescence regime by suppressing the other regimes. Collisions of droplets with non-identical viscosities show the following behaviours. The bouncing boundary shows an intermediate position compared to the identical viscosity cases, however, at high viscosity difference, a new regime was observed, namely, partial bouncing which is bouncing with a very thin ligament between the droplets. Likewise, in equal-size collisions, and in unequal size collisions where the large droplet has the higher viscosity, the boundary of reflexive separation regime shows an intermediate position. However, in unequal-size collisions, if the larger droplet has the lower viscosity the boundary remains similar to the identical viscosity case of the low viscosity droplet. Finally, stretching separation always shows a boundary similar to that of the identical viscosity case of the low viscosity droplet. New models for the boundaries of the bouncing and the reflexive separation regimes were developed to consider the effects of the viscosity, the impact parameter and the size ratio.