The physics of droplet movement is well established but coalescence is less well understood. The velocities of the drops when far apart and isolated is a function of the density difference, droplet diameters, viscosity and surface tension of the interface. In a cloud of bubbles this does not work, and the interactions between droplets becomes significant. In a well dispersed, low droplet population system, when the two drops are close enough to interact the pressure and velocity fields around the duplet have to be computed, this used to be done by potential flow analysis, nowadays by CFD models. This allows calculation of the approach velocity but not the coalescence.