  When an immiscible oil drop is immersed in a stably stratified ethanol-water
mixture, the Marangoni flow on the surface of the drop can experience an
oscillatory instability, so that the drop undergoes a transition from
levitating to bouncing. The onset of the instability and its mechanisms have
been studied previously, yet the bouncing motion of the drop itself, which is a
completely different problem, has not yet been investigated. Here we study how
the bouncing characteristics (jumping height, rising and sinking time) depend
on the control parameters (drop radius, stratification strength, drop
viscosity). We first record experimentally the bouncing trajectories of drops
of different viscosities in different stratifications. Then a simplified
dynamical analysis is performed to get the scaling relations of the jumping
height and the rising and sinking times. The rising and sinking time scales are
found to depend on the drag coefficient of the drop $C_D^S$ in the stratified
liquid, which is determined empirically for the current parameter space. For
low viscosity (5 cSt) oil drops the results on the drag coefficient match the
ones from the literature. For high viscosity (100 cSt) oil drops the parameter
space had not been explored and the drag coefficients are not readily
available. Numerical simulations are therefore performed to provide external
verification for the drag coefficients, which well match with the experimental
results.
