The first example of evaporation-driven engines that can power common tasks like locomotion and electricity generation is reported this week in Nature Communications. The authors demonstrate that these engines can drive a mini car and power LEDs, highlighting the untapped capability of water in the environment to supply useful levels of power.
Evaporation is a ubiquitous phenomenon and a dominant form of energy transfer in the Earth’s climate. Engineered systems rarely use evaporation alone as a source of energy, despite many examples of such adaptations in the natural world. Ozgur Sahin and colleagues now create “hygroscopy driven artificial muscles”, or HYDRAs, by attaching bacterial spores to 8 μm-thick polyimide tape. Inside the bacterial spores, water is confined to nanoscale cavities and depending on the humidity, large pressure changes are induced. This means that the tape changes its curvature in moist and dry conditions and when multiple tapes are assembled in parallel they can lift weight against gravity.
The authors then use these HYDRAs to create rotary and piston-like, evaporation-driven engines that start and run autonomously when placed at air-water interfaces. They designed an electricity generator that rests on water while harvesting its evaporation to power an LED and a miniature car (weighing 0.1 kg) that moves forward as the water in the car evaporates. Evaporation-driven engines may find applications in powering robotic systems, sensors, devices and machinery that function in the natural environment.