Bouncing dynamics of oil droplets in strongly stratified liquids*

Remarkably rich physics is involved when an oil droplet is submerged in a stably stratified ethanol-water mixture. Whereas some droplets may levitate above their density matched position due to a stable Marangoni convection along its surface, others may show an oscillatory bouncing motion. This transition is due to a Marangoni instability that happens either when the stratification is strong enough (low viscous oil) or when the droplet is large enough (high viscous oil) [Li, et al., PRL 126, 124502 (2021)] [Li, et al., JFM, Under Review (2021)]. In the present work, we experimentally study the bouncing dynamics of silicone oil droplets of different viscosities and characterize how the jumping characteristics (jumping height and amplitude, rising and sinking time) depend on the control parameters (droplet radius, stratification strength, viscosity). In addition, we performed numerical simulations with the boundary element method, allowing for one-to-one comparisons between numerical and experimental results. In general, we find good overall agreement, apart from a small over-prediction of the Marangoni force, which we ascribe to surfactants in the experiments. The present study helps to better understand the rich phenomenology of physicochemical hydrodynamics out of equilibrium.