Blending different metals together produces an alloy, but layering them instead creates a bimetal that often has unique properties. In this case, bimetal feet allow a tiny robot to walk across hot surfaces, without the need for any other power sources.
To realise the Thermobot, researchers at the University of Tokyo took advantage of the fact that different metals expand at different rates when heated. And when multiple metals with different expansion properties are layered together, the resulting object will warp and deform as heat is applied.
Thermobot's curved feet are made of just that kind of deforming bimetal, and are weighted more towards the back. When the bottom of the feet touch a hot surface — one much hotter than the room's ambient temperature — the feet warp, causing the robot's weight to shift and it to lean, which in turn causes the legs to swing forward, facilitating locomotion.
The walking motion requires everything to be perfectly calibrated — the weights, the curves of the feet, and the amount of metal deformation — but in theory the tiny robot could wander around a frying pan, or a grill, forever, as long as the heat is left on.
Do we want to turn every footpath and road into red hot surfaces to power the robots of tomorrow? Probably not. But the researchers are working to make the robot function on surfaces that are far less scorching. And the technique used for the locomotion could also be used to power other moving mechanisms without the need for batteries or power tethers.