  When an oil droplet is placed on a quiescent oil bath, it eventually
collapses into the bath due to gravity. The resulting coalescence may be
eliminated when the bath is vertically vibrated. The droplet bounces
periodically on the bath, and the air layer between the droplet and the bath is
replenished at each bounce. This sustained bouncing motion is achieved when the
forcing acceleration is higher than a threshold value. When the droplet has a
sufficiently low viscosity, it significantly deforms : spherical harmonic
\boldmath{$Y_{\ell}^m$} modes are excited, resulting in resonant effects on the
threshold acceleration curve. Indeed, a lower acceleration is needed when
$\ell$ modes with $m=0$ are excited. Modes $m \ne 0$ are found to decrease the
bouncing ability of the droplet. When the mode $\ell=2$ and $m=1$ is excited,
the droplet rolls on the vibrated surface without touching it, leading to a new
self-propulsion mode.
