Collisions of micron-sized charged water droplets in still air

Abstract

We show that the relative dynamics of charged water droplets settling in still air contains fixed points determined by the combination of hydrodynamic interactions, particle and fluid inertia, and electrostatic forces. For droplets with large enough opposite charges, the stable manifold of a saddle forms a separatrix that divides colliding trajectories from those that do not collide. We analyzed experimental data of hydrodynamically interacting, micron-sized water droplets in still air with Lagrangian droplet tracking and with a numerical model including gravity, particle inertia, hydrodynamic interactions, fluid inertia, and Coulomb interactions. The predictions of the model are in excellent agreement with the experiments. A consequence of our findings is that, for large charges, collision outcomes are not sensitive to the breakdown of hydrodynamics at small distances.