Trending science: Evaporating water powers mini car. The process of evaporation has hidden ‘power’ that we have not appreciated until now, as researchers from at Columbia University in the US have recently demonstrated.
Led by Ozgur Sahin, the Columbia team has developed a miniature car that harnesses the power of evaporation to drive itself forward, as well as an evaporation-driven generator that powers a flashing LED lamp.
The study, which was published in Nature Communications this week, details engines developed by the team which start and run autonomously when placed at ‘air–water interfaces’. According to the abstract, ‘They generate rotary and piston-like linear motion using specially designed, biologically based artificial muscles responsive to moisture fluctuations.’ Using these engines, the team demonstrates an electricity generator that rests on water while harvesting its evaporation to power a light source, and a miniature car (weighing 0.1 kg) that moves forward as the water in the car evaporates.
The Guardian quotes study leader Sahin, who noted that the engines were cheap and could draw energy from water as it evaporates continuously from the surfaces of lakes and oceans: ‘Water wants to evaporate. It has a desire to evaporate. If you make a surface wet, it will dry up, that’s the natural course. What we did was find a way to channel that desire into doing some useful work.’
The basis of the research is work that Sahin previously carried out on the bacteria Bacillus, microorganisms commonly found in soil. In humid conditions the spores absorb moisture from the air, expanding up to 40 percent in volume. In dry conditions, the process reverses. Sahin deduced that expanding and contracting spores can act like a muscle, pushing and pulling other objects: ‘We noticed that we could harness the motion of spores and convert it to electrical energy.’ And this is what the team has done with these two new engines.
The Guardian details the construction of one of the engines: ‘To make one of the machines, a floating piston engine, Sahin and his colleagues glued a line of spores to each side of a thin, plastic tape. The spores were spaced out and arranged so that those on one side overlapped with the gaps between spores on the other.’
‘When the tape is exposed to dry air, the spores shrink and the tape retracts like a spring. In moist air, the tape extends as the contraction is released. The result is an artificial muscle powered by differences in moisture. The scientists call them hygroscopically driven artificial muscles, or hydra.’
Peter Fratzl, a materials scientist at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, commented on the research in Science magazine, ‘These are fun demonstrations, but they prove the principle. Researchers are constantly looking for sources of energy, even if they’re small. It makes sense to use these gradients [in humidity], because they’re everywhere and they’re free.’
The researchers hope that evaporation-driven engines may find applications in powering robotic systems, sensors, devices and machinery that function in the natural environment.
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