A bouncing oil droplet in a stratified liquid and its sudden death*

A millimeter-sized oil droplet is released in a stratified liquid consisting of ethanol carefully deposited on water. The droplet slowly sinks due to gravity, but before reaching the density matched position, it suddenly jumps up by ~ 5 mm, and then settle again for a minute or so, after which the sudden jump occurs again. This process can repeat 30 to 40 times, with the jumping height even increasing, but then the drop all the sudden falls dead. We identify the Marangoni flow at the interface between the oil drop and the stratified liquid as the driven force of the jumping: It exponentially builds up, because it pulls down ethanol-rich liquid, thus increasing the Marangoni force more and more, up to a time that the drop is pulled upwards. We also explain why the jumping expires all the sudden.