WATCHING 鈥榮 model boat glide silently across the still water with no propellers or sails, you鈥檇 be forgiven for thinking a phantom hand was drawing the vessel forward. The boat is actually being driven by water surface tension, the same force that allows some insects to skate across the surface of a pond.
The design is inspired by Pyrrhalta beetle larvae, which also use surface tension to propel themselves. Since it requires no moving parts, the method should be more robust than those involving propellers and may use just a hundredth of the power. This could be ideal for extending the working life of cheap, environment-sensing robots that roam the world鈥檚 oceans, says Cho, who is at the University of Pittsburgh in Pennsylvania. Normally, the surface tension of the water surrounding a floating object is equal on every side. To upset this balance and create a forward thrust, Cho鈥檚 team attached an electrode to the back of the boat that alters the curve of the water鈥檚 meniscus.
The electrode is coated in an insulating polymer and Teflon, which is normally hydrophobic. Applying a potential to the electrode changes the insulating layer鈥檚 charge distribution and makes the electrode more attractive to water 鈥� a process called electrowetting. This extra attraction changes the shape of the water鈥檚 meniscus so that it curves upwards rather than downwards at the back of the boat, changing the balance of forces and pushing the boat forward (see diagram).
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In tests to be presented later this month at the in Sorrento, Italy, the team propelled a 2-centimetre-long prototype at roughly 4 millimetres per second. 鈥淏y streamlining the boat and optimising the electrodes, we could achieve speeds of 10 centimetres per second,鈥� says Cho (a video of the boat in action is at www.snipurl.com/9ut8w).
from the University of Sheffield in the UK is impressed with the work. 鈥淚t鈥檚 a very clever trick that puts fundamental physics into practice,鈥� he says. He thinks it should be simple to manufacture robots with a series of these electrodes along their side to provide very precise motion control.
