Liquid Drops Defy Gravity, Travel Uphill
A droplet moving up an inclined surface. Shaking moves the drop in such a way that it forces it to defy gravity and move upward.
Credit: Philippe Brunet

Gravity-defying drops of liquid can travel uphill if the surface they are sitting on is shaken up and down vigorously enough, a new study finds.

If you've ever looked out a window on a rainy day, you've probably noticed that small drops of water tend to stay where they fall, while big ones roll down the glass. Small drops stay still because the surface tension holding them up is balanced against the force of gravity pulling them down. But for bigger drops, gravity overwhelms the surface tension force and causes the drop to slide down.

Mathematicians at the University of Bristol have shown that not only can small drops withstand the force of gravity, they can completely defy it. Even on an incline as steep as 85 degrees (almost perpendicular to the ground), small drops can travel uphill if the surface vibrates strongly enough.

"As the shaking plate rises, the drop is compressed, while it bulges upward as the plate falls," explained researcher Jens Eggers. "If the shaking is vigorous enough to overcome the surface tension experienced as the drop is compressed, the drop will tend to lean forward, producing a net force which drives the drop uphill."

Drops must be just the right size, as large drops will tend to break apart under the forces applied to them. Similarly, the liquid's viscosity is important: Pure water droplets aren't strong enough to hold together and would break apart, and liquids that are too thick will move too slowly. The results will be published online this week in the journal Physical Review Letters.

Scientists have previously used extremely hot surfaces to force water to move up an incline.

Understanding how to move these tiny droplets could help improve certain processes, like arranging pieces of DNA, said Egger's colleague Philippe Brunet.