Ever watch what happens to the last bunch of Cheerios left floating in your breakfast bowl? The oat rings tend to cluster on the milk’s surface as if they had a magnetic attraction.
Known as the “Cheerios effect,” the clustering is caused by the weight of each ring denting the surface of the milk without being heavy enough to overcome the surface tension and sink. As one Cheerio floats near another, they slide into each other’s dents and cluster together.
Now, the humble bowl of cereal has inspired a new technique for designing and controlling small aquatic robots, which could be used to monitor environmental conditions or spread chemical treatments in a lake.
“We thought let’s utilize this logic to create new robotic devices and make them move around,” said Jackson Wilt, a PhD student at Harvard University and the lead author of a paper exploring how to control floating 3-D-printed robots using the “Cheerios effect.”
The research, which got started while Wilt was at the University of Amsterdam two years ago, also took inspiration from the “cocktail boat,” a bar trick that uses a little bit of alcohol to propel a floating gel or candy across the surface of a drink.
Known as the “Marangoni effect”, the stream of alcohol emitted by a cocktail boat reduces the surface tension of the liquid behind the boat. With higher tension in front of the boat and lower tension at the back, the boat shoots forward. Insects skim the surface of a pond by releasing fatty acids that also lower the surface tension of the water behind them.
In essence, Wilt and other researchers at University of Amsterdam used 3-D printers to build devices in the shape of Cheerios containing a tiny reservoir of alcohol for propulsion. Then they tweaked the shapes and the amount of alcohol to dictate how the devices moved around.
The resulting robots aren’t programmed by software but controlled by the materials and shapes. Higher concentrations of alcohol, for example, prevented the bots from clustering for longer periods of time while bots with different designs could be combined to move in specific patterns. “We were able to tune how they behave through a materials lens, rather than programming,” Wilt said.
Further research will be needed to design more useful robots. Ultimately, the small bots could carry environmental sensors to monitor water quality or a chemical treatment being applied to a lake, Wilt said. The robots could easily be made of fully biodegradable materials.
Wilt also hopes the research will inspire students around the world to 3-D print their own surface skimming robots. He has already made available online the files needed to 3-D print the robots. “Pretty much anybody with a printer can create these,” he said. “Anyone can, which is really nice.”
Aaron Pressman can be reached at aaron.pressman@globe.com. Follow him @ampressman.