Oil droplet bouncing in a stratified ethanol-water mixture

An oil droplet bounces up and down though a water–alcohol mixture. Credit: Yanshen Li/Physics of Fluids Group/Univ. Twente

Physics

How a droplet in a tube leaps upward again and again

Surface tension between an oil droplet and surrounding liquid helps droplet to defy gravity.

An oil droplet added to a water–alcohol mixture repeatedly jumps toward the surface before it finally drops dead.

Detlef Lohse of the University of Twente in Enschede, the Netherlands, and Xuehua Zhang at the University of Alberta in Edmonton, Canada, and their colleagues allowed a droplet of oil to fall through a tube filled with water and ethanol. This mixture formed a ‘concentration gradient’ in which ethanol concentrations were highest at the tube’s top, and water was most concentrated at the bottom.

An oil droplet in ethanol generates less surface tension than it does in water. As a result, the top of the droplet falling through the tube experienced less surface tension than the bottom. The difference in the surface tension across the droplet’s surface caused a circulation in the surrounding fluid that brought ethanol-rich liquid from above the droplet to the droplet’s top , increasing the gradient further. This feedback loop eventually created a flow that was strong enough to pull the droplet upwards.

The droplet then fell again, and the process repeated itself. Because of mixing of the water and ethanol, the droplet eventually came to rest.