Charged surface-based droplet pipette

Four water droplets acting as wheels on a cargo carrier whiz along a negatively charged surface. Credit: Xu Deng/Zuankai Wang/Hans-Jürgen Butt

Materials science

How to make water flow uphill

Water droplets skitter across a surface ‘printed’ with patterns of negative charge.

Droplets of water can propel themselves up a vertical surface against the pull of gravity.

Water droplets will travel in a desired direction if placed on a surface with an uneven coating of water-repellent material: the most thickly-coated areas repel the droplets. But the droplets usually move slowly, and stop where the coating ends.

Xu Deng at the University of Electronic Science and Technology of China, Chengdu, and his colleagues have designed a system in which water drips from a pump onto a highly water-repellent surface. Upon impact, some of the water droplets’ negatively charged electrons are transferred to the surface.

The team varied certain factors, such as pump position, to layer an uneven distribution of electrons on the surface. Areas that were thus ‘printed’ with an extra helping of negative charge tugged on the water droplets, which whisked the droplets quickly along the surface. The technique allowed water droplets to flow uphill and climb walls.

This approach could control the flow of water, blood and other fluids in medical devices, the authors write.