Syuji Fujii, who spearheaded the research, was inspired by the movement of the Stenus beetle, a tiny insect that navigates its way across waterways by secreting stenusin, a compound that reduces the surface tension of water. When the surface tension behind a beetle is less than that ahead of the beetle, it creates a phenomenon known as Marangoni flow, which pulls the beetles along the water.
To emulate this natural marvel, Fujii and his team took millimeter-sized water droplets and coated them in polypyrrole, a type of plastic that gets hot in the presence of light, and also happens to be water-repellent. This coating turned an average water droplet into a water-filled marble, and a very powerful one at that.
When the researchers placed the marble in water and shone light upon it, the same surface tension imbalance appeared — with less tension on one side of the marble, it moved across the water effortlessly. More impressive still, the team found that when these liquid marbles were attached to plastic boats, the 9mg water droplet was able to pull a load of 1.4 grams.
“One liquid marble can produce enough power by light irradiation to pull the larger objects, which have more than 150 times its own weight,” Fujii told the New Scientist. And if enough light was trained on the marble for a sufficient amount of time, the structure would actually burst, allowing the contained water to escape.
“Our approach makes it possible to not only transport the materials encapsulated within the liquid marble but also release them at a specific place,” Fujii said. “This should have potential applications in light-controlled micromachinery, microfluidics, pollution detection and drug delivery systems.”
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