These water droplets are travelling uphill.
The track is heated and covered with tiny asymmetric ridges. Together, heat and texture propel the drops.
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Water droplets that travel uphill
On metal hot enough, a water droplet rides on a cushion of its own vapour.
This is the Leidenfrost effect—the same reason droplets can skate across a very hot pan.
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Water droplets that travel uphill
Add a carefully shaped surface and the vapour flow can drive droplets along a chosen route.
Bath researchers used this principle to build tracks and a maze for water.
