Bouncing Oil Droplet in a Stratified Liquid and its Sudden Death

Yanshen Li, Christian Diddens, Andrea Prosperetti, Kai Leong Chong, Xuehua Zhang (Corresponding Author), Detlef Lohse (Corresponding Author)

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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.

Original languageEnglish
Article number154502
Number of pages6
JournalPhysical review letters
Volume122
Issue number15
DOIs
Publication statusPublished - 17 Apr 2019

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death
oils
ethyl alcohol
liquids
gravitation
sinks
gradients
water

Cite this

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AU - Li, Yanshen

AU - Diddens, Christian

AU - Prosperetti, Andrea

AU - Chong, Kai Leong

AU - Zhang, Xuehua

AU - Lohse, Detlef

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AB - 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.

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