Engineers have designed a flat metallic surface that allows ice to zip across it without a push to get it going. The team wasn’t inspired by a river’s rapids or ocean currents, however. The Virginia Tech researchers facilitated the seemingly supernatural movement after examining a famous phenomenon at one of the driest places on Earth. According to their study published August 14 in the journal ACS Applied Materials & Interfaces, the results may have surprisingly major ramifications for everything from rapid defrosting techniques to green energy harvesting.
Death Valley’s Racetrack Playa is home to some very strange rocks. First documented during the early 1900’s, the dry lake bed contains countless “sailing stones.” The rambling rocks appear to travel across the sand on their own, leaving clear dirt trails in their wake. Apart from a wide range of implausible and otherworldly explanations, researchers have spent decades trying to figure out the exact mechanics behind the odd behavior.
In 2014, a group led by Harvard paleoceanographer Richard Norris finally found an answer.
Racetrack Playa’s sailing stones only move under very specific conditions. Specifically, the ground must be dry enough so that what little rainfall it receives doesn’t penetrate the hardened surface. Next, the temperature must drop below freezing so that ice forms atop the desert floor. Once it inevitably begins melting, small breezes give the remaining ice rafts the momentum they need to start moving. And sometimes, stones hitch a ride on top of these small makeshift vessels.
After learning about the sailing stone mechanics in 2019, a team led by Jonathan Boreyko at Virginia Tech’s Nature-Inspired Fluids and Interface Lab wondered if they could manufacture a surface that also moved ice along a level, horizontal path. But, unlike the Desert Valley rocks, they wanted to do it without any wind power. After three years of experimentation and another two for fine-tuning the design, Boreyko and colleagues had their answer.
It takes a few moments, but the results are clear: their novel surface allows a small ice disk to begin moving without any external instigator. To make it happen, researchers constructed aluminum plates etched with asymmetric, arrowhead-shaped grooves resembling a herringbone pattern. As the ice melts, the water is directed through those grooves to chart a path forward.
“This directional flow of meltwater carried the ice disk along with it,” study co-author Jack Tapocik explained in a statement. “A good analogy is tubing on a river except here, the directional channels cause the flow instead of gravity.”
For curiosity’s sake, the team coated their aluminum plates with a water-repellant spray. While they assumed the ice disk would simply move faster from the very beginning, the result was more complicated. Once waterproof, the surface actually sticks to the disk as the melting water is squeezed out along the channels. At a certain point, however, the disk suddenly zooms forward as if making up for lost time.
“The fun trick here is that as the meltwater flows beyond the front edge of the ice disk, it creates a puddle,” said Boreyko. “Having a flat puddle on one side of the ice creates a mismatch in surface tension, which dislodges the ice and causes it to shoot across the surface.”
These dynamics can hypothetically improve defrosting techniques for various materials. According to Boreyko, however, his team’s discovery may lead to something more electrifying. Picture the aluminum herringbone surface, but instead of a two-directional square or rectangle, it’s a circle. And remember those sailing stones?
“Now imagine putting magnets on top of the ice, rather than boulders,” he said. “These magnets would also rotate, which could be used for power generation.”
While more research and experimentation is needed to determine the design’s efficacy and scalability, it’s still a promising possibility for future sustainable energy advancements—all thanks to a bunch of rocks.
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