Why is ice slippery?

Whether you’re gliding serenely across an ice rink, or awkwardly down a footpath, the frictionless surface offered by ice can often be surprising. What makes it this way is still something of a mystery. “Ice formation is a very complex process with no definitive consensus as to what is really going on,” explains Ras, a professor of Soft Matter Physics at Finland’s Aalto University. “There’s a lot happening from the microscopic to the macroscopic scales, each requiring specialist study equipment.” Knowing the properties of ice is not enough – there is the confounding factor of water’s unique transformational ability, as it shifts between solid, liquid and gas. What’s more, water is weird: unlike most liquids, it expands as it freezes, a property that allows solid water – from icebergs to ice cubes – to float in liquid water.

What beer can teach us about ice

To understand ice, we first need to clear up some misconceptions. Most of us will remember from school physics that water freezes at its melting point of zero degrees centigrade. Not quite, says Ras. “You can add pure liquid water to a plastic bottle, put it in your freezer, take it out a few days later and pour it out, because water can remain liquid at temperatures down to -35 °C, a state called supercooled,” he explains. The reason is that ice needs nucleation sites to form – tiny particles such as minerals, dirt or a rough surface in the container wall. “Anyone who has ever drunk Duvel beer from a Duvel beer glass will know that at the bottom of the glass, bubbles form like those that rise from deep-sea vents. This is thanks to nucleation at the rough engraved glass surface,” says Ras.

Facing down the interface

The next point to make is that a surface can only be said to be slippery when something comes into contact with it. It seems that rather than becoming a victim of the slipperiness, this interface actually causes it. It was only recently that the mechanism responsible was outlined in detail: the friction generated by the contact between a boot and patch of ice creates a thin, slick layer of interfacial water on top of the ice. More viscous than normal water, and one hundredth the thickness of a strand of hair, it has friction properties between liquid and solid. Other research has found that water molecules at this interface are weakly bonded compared to ice, and so can freely move around – a bit like gas. “The water molecules seem to function a bit like ball bearings, allowing things to travel across the ice’s surface easily, sometimes too easily,” elaborates Ras. The third key ingredient for slipperiness is speed. When the object travelling over the ice does so at speed, the interface friction and pressure generate more heat, acerbating the production of interfacial water. Without which, ice skating would look quite a lot less … fluid.

Embracing slipperiness

Ice’s slipperiness helped inspire Ras’s European Research Council supported project, SuperRepel. His team developed a surface coating to repel a range of substances, from dust to snow. “Once treated with our coating, currently the most durable water-repellent in existence, the resulting hydrophobic surfaces will be easier to keep dry, clean and bacteria-free,” adds Ras. Their methodology won the team the Anton Paar Research Award. They are currently exploring a range of possible applications, from wind turbine blades to skis and packaging materials. “We got this far thanks to our interest in how water droplets behave. This has given us the expertise to now look more closely at ice, and other phenomena such as fogging, which could benefit a range of industries from aerospace to automotive,” notes Ras. All the better to protect us from chilly weather. As winter bites, let’s hope he gets his skates on.