The little blobs that could.
In a modern twist to “The Little Engine that Could,” researchers have developed a way of making a single water droplet behave as an “engine,” while also carrying cargo several times its own mass from one location to another.
Syuji Fujii of the Osaka Institute of Technology in Japan, and his colleagues, developed the droplets after being inspired by Stenus beetles, which have the ability to propel themselves across ponds by secreting a substance called stenusin. This substance lowers the surface tension of the water behind them, creating an imbalance called Marangoni flow — pulling the beetles forwards.
To accomplish this, the team coated millimeter-sized droplets of water in a nanometer-scale powder of polypyrrole, which is a plastic that heats up when illuminated, but is also hydrophobic — meaning it repels water. The powder coating turned the drop into a liquid marble, trapping the fluid inside.
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The team then placed one of the marbles into 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, and amazingly, a single 9-milligram droplet was able to pull a boat loaded with cargo totaling 1.4 grams (seen in the video below). If two marbles were being used, the boats could be steered 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,” said Fujii to New Scientist.
The team also found that if you shine enough light on the marble, it will eventually burst, releasing the liquid inside.
According to Fujii, this could be useful for delivering liquids to certain locations. “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 explained. “This should have potential applications in light-controlled micromachinery, microfluidics, pollution detection and drug delivery systems.”
The paper was published in the journal Advanced Functional Materials.
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