In Associate Professor Jonathan Boreyko's Nature-Inspired Fluids and Interfaces Lab, Ph.D. student Jack Tapocik watched a disc-shaped chunk of ice resting on an engineered metal surface. As the ice melted, the water formed a puddle beneath.
Even after many seconds of melting, the ice disk remained adhered to the engineered surface. At first, Tapocik was tempted to conclude that nothing would happen, but he waited. His patience paid off. After a minute, the ice slingshot across the metal plate he designed, gliding along as if it was propelled supernaturally.
The results are important because they have a host of potential applications. The methods team members developed lay the foundation for rapid defrosting and novel methods of energy harvesting. Their work has now been published in "ACS Applied Materials & Interfaces."
Looking to Death Valley
The team was inspired by a naturally occurring phenomenon at Racetrack Playa in Death Valley, California. This dry lake bed in Death Valley National Park is host to a ghostly display of boulders the size of watermelons that make long trails in the cracked earth, left behind by their apparent movement. Because the ground was level and many of the rocks were flat, the reason for their migration remained a mystery until recently.
Harvard University Professor Richard Norris found out why this happened in 2014. Rather than a supernatural occurrence, it was a combination of hard ground, rain, ice, and wind. Rainfall covered the ground with water, but the hardness of the ground prevented the water from being absorbed. This water froze when the temperature dropped, and as it later started to melt, the resulting ice rafts drifted in the breeze for the rocks to "sail" across the meltwater. Some of the rocks have been seen traveling together, which gave rise to the "racetrack" mythos.
While Norris solved a mystery of the rocks, Boreyko's team was seeking something new: Its members wanted to create a surface that would propel melting ice all by itself, without any wind required. They wanted to harness the science of the racing rocks.
Building the track
Initially conceived by Boreyko and former graduate student Saurabh Nath back in 2019, the experiments took three years to complete with two more years needed for the model.
Boreyko's team cut asymmetric grooves into aluminum plates. This herringbone pattern, which looks like a series of arrowhead-shaped channels, causes the underlying meltwater to flow in one direction.
"This directional flow of meltwater carried the ice disk along with it," said Tapocik. "A good analogy is tubing on a river except here, the directional channels cause the flow instead of gravity."
Out of curiosity, team members tried coating the aluminum herringbones with a water-repellant spray. They expected to simply see a faster version of the disk getting propelled by the flow, but surprisingly, the disk stuck to the surface instead. This is what led to the discovery of the slingshot effect.
"On a waterproof surface, the excess meltwater above the channels gets squeezed out very easily," Boreyko said. "This makes the ice disk stick to the surface's ridges. The meltwater is still flowing along the channels, but the ice can't ride along anymore. The fun trick here is that as the meltwater flows beyond the front edge of the ice disk, it creates a puddle. 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."
By comparison, the movement of the "racing" rocks in Death Valley is quite slow, and they don't shoot like slingshots. It's not likely to become an official competition, but Boreyko's surfaces are winning over the Racetrack Playa for having the fastest ice on earth.
Boreyko has already imagined one of the most high-impact applications: energy harvesting. In that scenario, he brings back the idea of the racing rocks.
"If the surface structure were patterned in a circle rather than a straight line, the melting object would continually rotate," he said. "Now imagine putting magnets on top of the ice, rather than boulders. These magnets would also rotate, which could be used for power generation."
In addition to Boreyko and Tapocik, other research colleagues were involved in this project:
- Saurabh Nath, a graduate student at the beginning of the project, recently hired into a tenure-track position at the University of Pennsylvania
- Sarah Propst, an undergraduate researcher at the beginning of the project, now a Ph.D. student at Johns Hopkins University
- Venkata Yashasvi Lolla, a graduate student who is now a postdoctoral research associate at UC Berkeley
John R. Jones III and the John Jones Faculty Fellowship supported this work.
