  We investigate the effect of electrical charge on collisions of
hydrodynamically interacting, micron-sized water droplets settling through
quiescent air. The relative dynamics of charged droplets is determined by
hydrodynamic interactions, particle and fluid inertia, and electrostatic
forces. We analyse the resulting relative dynamics of oppositely charged
droplets by determining its fixed points and their stable and unstable
manifolds. The stable manifold of a saddle point forms a separatrix that
separates colliding trajectories from those that do not collide. The
qualitative conclusions from this theory are in excellent agreement with
experiments.
