  Two oppositely charged droplets of (say) water in e.g. oil or air will tend
to drift together under the influence of their charges. As they make contact,
one might expect them to coalesce and form one large droplet, and this indeed
happens when the charge difference is sufficiently small. However, Ristenpart
et al discovered a remarkable physical phenomenon whereby for large enough
charge differentials, the droplets bounce off each other as they make contact.
Explanations based on minimisation of area under a volume constraint have been
proposed based on the premise that consideration of surface energy cannot be
sufficient. However, in this note we explain that on the contrary, the bouncing
phenomenon can be completely explained in terms of energy, including an
accurate prediction of the threshold charge differential between coalescence
and bouncing.
