It’s the little blob that could. A drop of water can behave as an “engine”, lugging cargo many times its own mass (see video above).
Syuji Fujii of the Osaka Institute of Technology in Japan and his colleagues drew inspiration from Stenus beetles. They propel themselves across ponds by secreting a substance called stenusin, which lowers the surface tension of the water behind them. This creates an imbalance called Marangoni flow, pulling the beetles forwards.
For their version, the team coated millimetre-sized drops of water in a nanometre-scale powder of polypyrrole, a plastic which heats up when illuminated and also repels water. The powder coating turned the drop into a liquid marble, trapping the fluid inside.
The team put one of these marbles in a pool of water and illuminated the marble with a laser. As the polypyrrole warmed up, it changed the surface tension on one side of the marble, propelling it across the water just like the beetles.
Next, the team attached the droplets to tiny plastic boats. A single 9-milligram droplet pulled a boat loaded with cargo totalling 1.4 grams, and two marbles were able to steer it left and right.
“One liquid marble can produce enough power by light irradiation to pull the larger objects, which have more than 150 times its own weight,” says Fujii.
The team also found that shining enough light on a marble would eventually cause it to burst, releasing the liquid inside. That could be useful for delivering liquids to certain locations, says Fujii.
“Our approach makes it possible to not only transport the materials encapsulated within the liquid marble but also release them at a specific place,” he says. “This should have potential applications in light-controlled micromachinery, microfluidics, pollution detection and drug delivery systems.”