Harnessing the energy of evaporating water

Guardian : Researchers at Columbia University have developed two tiny machines that are powered by the evaporation of water. Both use artificial muscles created by attaching common soil bacteria spores—which swell when exposed to moist air and shrink in dry air—to strips of thin, plastic tape. By enclosing dozens of those hygroscopically driven artificial muscles, or hydra, in a plastic case with shutters on top, the researchers have created a miniature piston engine. When the device is placed on water, the humidity causes the hydra to elongate and open the shutters, which then allows moisture to escape, causing the hydra to contract and close the shutters. As the cycle repeats, enough electricity is generated to power an LED bulb. The researchers’ other tiny machine, called a moisture mill, consists of a plastic wheel covered with hydra. Half the wheel is exposed to dry air, and the other half to moist air. As the hydra curl up and straighten out, they cause the wheel to spin, which can create enough power to drive a small toy car.