Researchers in the US are first to show how a melting ice disc can quickly propel itself across a patterned surface in a manner reminiscent of the Leidenfrost effect. Jonathan Boreyko and colleagues at Virginia Tech demonstrated how the discs can suddenly slingshot themselves along herringbone channels when a small amount of heat is applied.
The Leidenfrost effect is a classic physics experiment whereby a liquid droplet levitates above a hot surface – buoyed by vapour streaming from the bottom of the droplet. In 2022, Boreyko’s team extended the effect to a disc of ice. This three-phase Leidenfrost effect requires a much hotter surface because the ice must first melt to liquid, which then evaporates.
The team also noticed that the ice discs can propel themselves in specific directions across an asymmetrically-patterned surface. This ratcheting effect also occurs with Leidenfrost droplets, and is related to the asymmetric emission of vapour.
“Quite separately, we found out about a really interesting natural phenomenon at Death Valley in California, where boulders slowly move across the desert,” Boreyko adds. “It turns out this happens because they are sitting on thin rafts of ice, which the wind can then push over the underlying meltwater.”
Combined effects
In their latest study, Boreyko’s team considered how these two effects could be combined – allowing ice discs to propel themselves across cooler surfaces like the Death Valley boulders, but without any need for external forces like the wind.
They patterned a surface with a network of V-shaped herringbone channels, each branching off at an angle from a central channel. At first, meltwater formed an even ring around the disc – but as the channels directed its subsequent flow, the ice began to move in the same direction.
“For the Leidenfrost droplet ratchets, they have to heat the surface way above the boiling point of the liquid,” Boreyko explains. “In contrast, for melting ice discs, any temperature above freezing will cause the ice to melt and then move along with the meltwater.”
The speed of the disc’s movement depended on how easily water spreads out on to the herringbone channels. When etched onto bare aluminium, the channels were hydrophilic – encouraging meltwater to flow along them. Predictably, since liquid water is far more dense and viscous than vapour, this effect unfolded far more slowly than the three-phase Leidenfrost effect demonstrated in the team’s previous experiment.
Surprising result
Yet as Boreyko describes, “a much more surprising result was when we tried spraying a water-repellent coating over the surface structure.” While preventing meltwater from flowing quickly through the channels, this coating roughened the surface with nanostructures, which initially locked the ice disc in place as it rested on the ridges between the channels.
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As the ice melted, the ring of meltwater partially filled the channels beneath the disc. Gradually, however, the ratcheted surface directed more water to accumulate in front of the disc – introducing a Laplace pressure difference between both sides of the disc.
When this pressure difference is strong enough, the ice suddenly dislodges from the surface. “As the meltwater preferentially escaped on one side, it created a surface tension force that ‘slingshotted’ the ice at a dramatically higher speed,” Boreyko describes.
Applications of the new effect include surfaces could be de-iced with just a small amount of heating. Alternatively, energy could be harvested from ice-disc motion. It could also be used to propel large objects across a surface, says Boreyko. “It turns out that whenever you have more liquid on the front side of an object, and less on the backside, it creates a surface tension force that can be dramatic.”
The research is described in ACS Applied Materials & Interfaces.
