  The Leidenfrost effect describes liquid drops under gravity levitating on a
vapour cushion, which is sourced at the liquid-vapour interface from
evaporation caused by the hot substrate below. It has been experimentally
observed that when two or more Leidenfrost drops approach one another, there is
a short-range repulsion that can prevent coalescence. In the present work, we
investigate the coalescence process of initially momentum-less Leidenfrost
drops. This is carried out by numerically simulating two-dimensional systems of
liquid and vapour using the recently developed dynamic van der Waals theory, in
which the two-phase hydrodynamics is coupled with liquid-vapour transition. It
is confirmed that Leidenfrost drops experience a remarkably strong short-range
repulsion, due to a build-up of vapour in the common space under both drops.
This resistive force scales in a similar manner to the pressure-sourced force
from the vapour layer under a single drop, in concurrence with intuition, with
good agreement seen in the simulation results. A threshold drop size therefore
exists in order for an applied horizontal acceleration to overcome the
repulsion, while only very small drops are predicted to experience significant
viscous effects.
