Water droplets that travel uphill

Water droplets can travel uphill on a heated track with tiny asymmetric ridges: heat and texture propel them. On metal hot enough, a droplet rides on a cushion of its own vapour. This is the Leidenfrost effect, also seen when droplets skate across a very hot pan. Researchers at Bath used the principle to build tracks and a maze. A carefully shaped surface lets vapour flow drive droplets along a chosen route.

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