  We report an experimental observation of an instability in gas of constant
density (air) with an initial non-uniform seeding of small droplets that
develops as a planar shock wave passes through the gas-droplet mix. The seeding
non-uniformity is produced by vertical injection of a slow-moving jet of air
pre-mixed with glycol droplets into the test section of a shock tube, with the
plane of the shock parallel to the axis of the jet. After the shock passage, we
observe development of two counter-rotating vortices in the plane normal to
that axis. The physical mechanism of the instability we observe is peculiar to
multiphase flow, where the shock acceleration causes the second (embedded)
phase to move with respect to the embedding medium. With sufficient seeding
concentration, this leads to entrainment of the embedding phase that acquires a
relative velocity dependent on the initial seeding, resulting in vortex
formation in the flow.
