  It is demonstrated that non-coalescent droplets of acetone can be formed on
liquid substrates. The fluid flows around and in an acetone droplet hovering on
water are recorded to shed light on the mechanisms which might lead to
non-coalescence. For sufficiently low impact velocities, droplets undergo a
damped oscillation on the surface of the liquid substrate but at higher
velocities clean bounce-off occurs. Comparisons of experimentally observed
static configurations of floating droplets to predictions from a theoretical
model for a small non-wetting rigid sphere resting on a liquid substrate are
made and a tentative strategy for determining the thickness of the vapor layer
under a small droplet on a liquid is proposed. This strategy is based on the
notion of effective surface tension. The droplets show self-propulsion in
straight line trajectories in a manner which can be ascribed to a Marangoni
effect. Surprisingly, self-propelled droplets can become immersed beneath the
undisturbed water surface. This phenomenon is reasoned to be drag-inducing and
might provide a basis for refining observations in previous work.
