No matter who ends up at the top of the podium, one thing all gold medalists in the Winter Olympics have in common is their mastery of friction. The defining characteristic of winter sports is sliding. And whether it’s over snow or ice, on skis, boards, skates or sleds, winning means imposing just the right amount of control over a slide.
While technology and technique have evolved quite a bit since the world first gathered to compete in the Winter Olympics in 1924, the strategies of friction management have remained largely the same. Hisham Abdel-Aal, PhD, an associate teaching professor in Drexel’s College of Engineering, whose research focuses on the applications of tribology, the study of friction, recently shed some light on the slippery subtleties that are crucial for success in two Winter Olympic sports.
We often hear skiers talk about the differences in snow — mainly how nice it is to ski on “powder” — what else can affect the speed of a skier’s trip down the mountain?
Sliding on ice, in general, is as complex a phenomenon as it comes. This is because of all the variables — from the inner composition of the ice particles and water molecules to the temperature to the pressure exerted by the sliding surface.
Moving over snow is even more complex, because of the many different snow conditions that are possible.
While our knowledge of friction on ice is pretty settled, our scientific knowledge about friction on snow is still relatively limited because of the complexity of the system. But there is plenty of “alternative science” about how best to slide on snow — and the best kinds of snow to slide on. Some of that aged wisdom works, other has no root in scientific fact.
The snow-ski interaction is a complex system because of the number of factors that affect the coefficient of friction and the varying nature of the interaction between ski, skier and snow.
Some of the factors that can affect the speed of descent down a ski slope are the skier’s technique – the plowing or carving action; the contamination of the snow — mixing of finer snow particles and melted snow with debris from the skis.
How does ski design affect the speed of a skier?
The primary factor driving the evolution of ski technology, in addition to making them more durable, has been ensuring that the ski surface does not absorb water — a characteristic called hydrophobicity.
Hydrophobicity has to do with the angle of contact between a droplet of water and the ski surface. The goal is to decrease that angle so that the water droplets from the thin water layer under the ski, roll freely away from the surface. This allows the skier to glide, free of surface traction.
Back when skiers wore wooden skis, they applied pint tar and pitch to the bottom as a way of water-proofing the running surface.
By the 1970s the need to water-proof skis was eliminated with the advent of plastic skis. Early plastic skis were treated with sandpaper, steel scraping and stone grinding to ensure a smooth surface that accepts any wax or applied lubricant.
Today the best racing skis are made from a material called ultra-high molecular weight polyethylene (UHMWPE), which is actually the same lightweight, durable polymeric material used in making hip and knee replacements.
Why is waxing the skis so important?
The degree of hydrophobicity is a function of the micron-sized roughness of the ski surface, so surface preparation is very important — this is where the wax technicians come in. Waxing works to increase the hydrophobicity of the skiing surface and to repel contaminants from the surface.
Waxing is not all about reducing friction, however. The friction requirements depend on the position on the ski. A skier normally needs traction near their foot, to enhance control, so the wax’s composition has to be adjusted for grip in that area.
The composition of the wax also varies by event. In cross country skiing, for example, the wax must last much longer than alpine skinning, so a more durable wax is used.
Tribologists have conducted many studies about the physics of friction on ice, which, not surprisingly, originally had nothing to do with curling. Most of it was related to moving military equipment on icy lakes and terrain.
That said, since the 1930s there has actually been a good bit of research on curling and even more after it was reintroduced as an Olympic sport in 1998.
How does the friction at play in the surface-to-surface interaction of a frozen granite stone with the ice affect the way it moves?
On a curling sheet the ice is not flat or ultra-smooth, it actually has topography. It is covered with fine droplets of water that freezes to form 1/32-inch protrusions called pebbles.
A stone’s path curves, or “curls,” due to an asymmetry of friction caused by the slight spin of the stone imposed by the curler upon release, and the interaction between the granite and the pebbled surface of the ice.
If the stone is sliding at a low speed, the protrusions of the ice surface resist the movement of the stone, like a piece of gum stuck to one’s shoe — they deform under its weight but never fully break down and without rapid movement to create heat they will also begin to adhere to the slider and slow it down.
A curling stone will not curl without redistribution of the friction forces to cause asymmetry of friction. Recent research on the subject out of Sweden suggests this asymmetry is caused by the interaction of the roughness of the granite stones with the scratches induced on the ice due to motion.
This asymmetry bears a great similarity to the slithering movement created by the surface topography of snake skin — which is something we’re studying at Drexel.
So what does all that sweeping (and yelling) do?
The sweeping produces heat, like rubbing your hands on a cold day. That sweeping causes heat, which in turn raises the surface temperature of the ice. Locally, thereby, heat melts a thin layer of water, which acts as a lubricant that reduces the coefficient of friction between the stone and ice. This lubricating action increases the distance the granite stone can slide.
Much research was done into figuring out the most effective sweeping techniques, but it boils down to this: sweep vigorously with maximum pressure above the brush head and as close to the stone as possible. The goal is to create a lubricating, watery trail just in front of the stone as it moves.
A stone moving at a decent speed — about 4 inches/second — over ice that’s about 15 degrees Fahrenheit can be induced to slide in a straighter line along a water-lubricated path created by the sweeping-induced heat.
To guide the stone to the house you need to be skilled in controlling the evolution of its friction path. The “in-flight” correction of the stone’s trajectory depends on the ability to modify the friction and also the ability to precisely judge when to apply the correction.
In addition to is impact on winter sports, tribology research has contributed to the development of many things designed not to slip, snow tires and boots, for example; many that are quite slippery, such as pistons and non-stick pans; and some that need to slip and stick at just the right time, such as prosthetic joints, or factory conveyors. Abdel-Aal’s research looks at what we can learn about tribology from nature and how it can be applied to custom engineering of surfaces.
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