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.
- Received 27 November 2018
- Revised 12 February 2019
DOI:https://doi.org/10.1103/PhysRevLett.122.154502
© 2019 American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Drops Bounce Inside Liquids
Published 17 April 2019
Oil drops falling through a layered liquid with ethanol at the top and water at the bottom repeatedly jump inside the fluid.
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