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

## TL;DR

This study investigates the self-propelling behavior of oil droplets in a stratified ethanol/water mixture, revealing a repeated bouncing mechanism driven by Marangoni stress and explaining the phenomenon's sudden cessation.

## Contribution

It provides experimental and numerical insights into droplet bouncing driven by Marangoni effects in a stratified liquid, a phenomenon not previously characterized in detail.

## Key findings

- Droplets bounce repeatedly with increasing distance
- Marangoni stress causes exponential growth in propulsion
- Droplet bouncing ceases suddenly after about 30 minutes

## Abstract

Droplets can self-propel when immersed in another liquid in which a concentration gradient is present. Here we report the experimental and numerical study of a self-propelling oil droplet in a vertically stratified ethanol/water mixture: At first, the droplet sinks slowly due to gravity, but then, before having reached its density matched position, jumps up suddenly. More remarkably, the droplet bounces repeatedly with an ever increasing jumping distance, until all of a sudden it stops after about 30 min. We identify the Marangoni stress at the droplet/liquid interface as responsible for the jumping: its strength grows exponentially because it pulls down ethanol-rich liquid, which in turn increases its strength even more. The jumping process can repeat because gravity restores the system. Finally, the sudden death of the jumping droplet is also explained. Our findings have demonstrated a type of prominent droplet bouncing inside a continuous medium with no wall or sharp interface.