  Under shock loading, metals have been found to melt and with reflection of
the shock wave from the material free surface, cavities nucleate and grow. This
process is referred to as micro spall and has been studied experimentally with
analytical models describing debris sizes. Measurements during the cavity
growth phase are not possible at present and we present here the Direct
Numerical Simulation of an idealized problem where we assume an inviscid,
incompressible liquid subject to a constant expansion rate with cavities at a
vanishing vapour pressure.
  In order to allow for a time-varying gas volume a free-surface interface
condition has been implemented in an existing incompressible multiphase
Navier-Stokes solver, PARIS Simulator, using a volume-of-fluid method. The gas
flow remains unsolved and is instead assumed to have a fixed pressure which is
applied to the liquid through a Dirichlet boundary condition on the arbitrary
liquid-gas interface. Gas bubbles are tracked individually, allowing the gas
pressure to be prescribed using a suitable equation of state.
  Simulations with hundreds of bubbles have been performed in a fixed domain
under a constant rate of expansion. A bubble competition is observed: larger
bubbles tend to expand more rapidly at the demise of smaller ones. The time
scale of competition is shown to depend on a modified Weber number.
