  When a liquid drop falls on a solid substrate, the air layer in between them
delays the occurrence of liquid--solid contact. For impacts on smooth
substrates, the air film can even prevent wetting, allowing the drop to bounce
off with dynamics identical to that observed for impacts on superamphiphobic
materials. In this article, we investigate similar bouncing phenomena,
occurring on viscous liquid films, that mimic atomically smooth substrates,
with the goal to probe their effective repellency. We elucidate the mechanisms
associated to the bouncing to non-bouncing (floating) transition using
experiments, simulations, and a minimal model that predicts the main
characteristics of drop impact, the contact time, and the coefficient of
restitution. In the case of highly viscous or very thin films, the impact
dynamics is not affected by the presence of the viscous film. Within this
substrate--independent limit, bouncing is suppressed once the drop viscosity
exceeds a critical value as on superamphiphobic substrates. For thicker or less
viscous films, both the drop and film properties influence the rebound dynamics
and conspire to inhibit bouncing above a critical film thickness. This
substrate--dependent regime also admits a limit, for low viscosity drops, in
which the film properties alone determine the limits of repellency.
