A scientist at the Universidad Complutense de Madrid believes he’s solved a longstanding mystery about ice and the different shapes ice crystals take, which has perplexed researchers for decades.
According to researcher Luis MacDowell, a thin layer of liquid water on the surface of frozen water ice, called a “premelting film,” causes ice crystals to form at different rates, resulting in ice crystals taking various shapes depending on their growth rates.
MacDowell hopes to apply the theory to the study of ice dynamics in Earth’s atmosphere. The Spanish researcher also suggests his concept could unravel another longstanding mystery about ice: how ice-skating works.
In a statement detailing the proposal, MacDowell separates ice into several main categories. For example, ice crystals that form in a home freezer are structurally different from those that join to form snow, or from the layer of ice that can form atop a frozen pond.
Still, scientists have been unable to explain why some ice crystals are shaped like prisms, stocky hexagonal, or flat plates, while others can be shaped like Grecian columns. According to MacDowell, when researchers first observed these stark differences, they suggested that physicist Michael Faraday’s theoretical predictions were correct. Even ice below its melting point will still support a microscopically thin layer of liquid water on its surface.
Unfortunately, scientists who have designed experiments to confirm or refute the premelting film theory have found contradictory evidence. For example, some studies reported a single thickness measurement of the water layer on ice crystals, while others reported a different thickness. Another set of studies found no evidence of a premelting film.
To resolve this longstanding mystery about ice, MacDowell started with a traditional phase diagram of ice depicting how vapor, liquid water, and ice exist across a wide range of temperatures and pressures. Specifically, the researcher focused on the point where all three phases meet, are equally stable, and exist in perfect equilibrium, called the triple point.
When MacDowell modeled this triple point in computer simulations to visualize the movement of molecules on the ice’s surface, a nanometer film of liquid water suddenly appeared. Still, previous experiments had returned different thicknesses, leaving the longstanding mystery about ice unsolved.
In his new paper, MacDowell suggests that his model is likely accurate and that real-world studies that reported different thicknesses of premelting film were simply unable to achieve true phase equilibrium.
“Equilibrium is a point,” the researcher explained. “You are as close as you can be, but never just there. Just a tiny deviation can become sufficiently out of equilibrium, making it very difficult to measure these things.”
In MacDowell’s solution, the premelting film is restricted to a specific thickness when the phases approach equilibrium due to water’s “unusual density properties.” Specifically, the researcher said that solid ice is an “energetically preferable state” to liquid H2O. When the film of liquid water is at this precise thickness, it directly affects the growth rate of the crystals forming below.
“This sequence of transitions in the shape of the snow crystals is related to changes in premelting film thickness that occur at the surface of ice,” MacDowell explained. “It exhibits surface phase transitions, and at each transition, you have a sudden change of the properties and of the growth rate of the faces.”
As a result, different growth rates on the various sides of the ice yield different final shapes. Ultimately, MacDowell’s theory aligns with Faraday’s, providing a firmer foundation for the concept of a premelting film as the solution to the longstanding mystery of ice.
When discussing possible applications of his solution, the researcher suggested his concept could be applied to atmospherics and friction science. As for his own plans, MacDowell is preparing to investigate how friction influences ice slipperiness and how impurities in the ice can affect the thickness of the remelting film. The researcher said he also plans to use his theory to potentially unravel how ice-skating works, thereby solving another longstanding mystery about ice.
The study “The key physics of ice premelting” was published in The Journal of Chemical Physics.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.