A novel experimental approach to study drop-particle collisions

Free drop-particle collisions occurring in air are experimentally produced by combining a stream of drops and a stream of particles, which results from the selective and ultra-fast hardening of another regular drop stream. The set-up offers the possibility to accurately vary the drop and particle diameter, the collision eccentricity and the relative drop-particle velocity. First observations obtained with drop Weber numbers ranging from 30 to 300, drop Reynolds number between 390 and 4600, and with typical equilibrium contact angle of 70° evidence the existence of full deposition, separation and bouncing events. For off-center separation, a liquid ligament forms between the particle and the outlying drop cap that fragments due to excessive stretching, a phenomenon similar to what can be observed for drop-drop collisions. In contrast, for head-on collisions and intermediate inertia, a lamella first forms, whose constrained recoil leads to liquid protuberance(s) that eventually pinch(es)-off. These outcomes can be distinguished using a bi-dimensional regime map built on the impact parameter and the drop Weber number. Despite remarkable similarities with binary drop collisions, important differences are observed especially for low and moderate eccentricities.