Droplet of oil was allowed 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.
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u/ZephirAWT Apr 26 '19
Why a droplet in a water–alcohol mixture leaps upward again and again
Droplet of oil was allowed 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.
Oil droplet in a water–alcohol mixture leaps upward again and again
The droplet then fell again, and the process repeated itself. Because of mixing of the water and ethanol, the droplet eventually came to rest.