Physicists have known how to levitate water for more than 260 years. They also just figured out how to push the tape up.
By pushing a well-known physical phenomenon known as the Leidenfrost effect to the extreme, a team of researchers at Virginia Tech make frozen ice discs float on hot aluminum surface. Outside the lab, they say, engineers could exploit this effect to create stronger metals or even forest fires. The results were published in Physics review liquid.
The Frost effect named after Johann Gottlob Leidenfrost, a German physician who described the phenomenon in 1756. He noticed that when a drop of water was placed on a very hot surface, the liquid did not evaporate immediately. , as he had predicted, which evaporated around like a white scarab before finally being burned to steam. Upon investigating, Leidenfrost discovered that an extremely small layer of air cushion – about a millimeter thick – had formed below the water’s surface. This airbag acts as insulation, evaporating the droplet and keeping it in a liquid state for longer.
Virginia Tech researchers wanted to see if they could achieve the same effect using ice. “It just started out as a kind of curious experiment,” said Jonathan Boreyko, a mechanical engineer at Virginia Tech and lead author of the study.
To the team’s surprise, the ice didn’t soar at 150 degrees Celsius (about 300 degrees Fahrenheit), the hot dish’s temperature causing the water to float. And it didn’t go up when the scientists raised the sheet metal’s temperature to 300 degrees Celsius, the upper limit of most Leidenfrost experiments. “These authors went further,” said Felipe Pacheco-Vazquez, a physicist at the Autonomous University of Pueblo in Mexico who was not involved in the study.
In the end, the researchers increased the heat to 550 degrees Celsius (1022 degrees Fahrenheit). Only then did the ice begin to hover over the disk. “We couldn’t make it any hotter because it would start to melt or warp the aluminum,” Boreyko said.
The 400 degrees Celsius difference between the temperatures needed to push water and ice up is confusing, especially since it’s four times the distance between water’s freezing point (0 degrees Celsius) and its boiling point (100 degrees Celsius). C). When the team took a closer look, they discovered a layer of molten water separating the ice sheet from the superheated plate. Beneath the water is a classic Leidenfrost air pocket, resulting in all three water phases stacking up like a sheet of suspended parfait. The researchers hypothesized that the meltwater acts as a buffer for heat, dissipating nearly 70 percent of it when caught between the boiling point below and the cold ice above. Therefore, a much larger amount of energy would be required to trigger the large-scale vapor-producing boiling pattern, said Mojtaba Edalatpour, a doctoral student at Virginia Tech and lead author of the study.
“Clearly, there is an impact on engineering applications,” says Pacheco-Vazquez. Water is commonly used in metallurgy to cool hot iron and steel, making them harder by rapidly locking the metal atoms into a hard lattice. In theory, cooling with ice would cool the metals more quickly, making this atomic lattice more stable – although this effect requires very precise temperature control to prevent fracturing due to stress. internal capacity.
Boreyko and Edalatpour suggest that this effect could even be useful in fighting fires. Wildfires — such as the Marshall Fire that recently ravaged Boulder, Colo. — usually explodes at 550 degrees Celsius or hotter, vaporizing water on contact. But if firefighters were to drop ice from helicopters rather than water, the energy needed to turn it into vapor could draw more heat from the flames. The surface texture and unpredictable temperature of a wildfire can limit its effectiveness, as can the physical difficulty of holding ice long enough to drop it. But if those obstacles can be overcome, this approach holds firefighting promise.
“It was great,” Edalatpour said, “literally.”
https://www.scientificamerican.com/article/how-to-levitate-ice-with-science/ How to Fly on Ice – With Science