WHAT do red blood cells and skiers have in common? They both overcome friction by exploiting the unusual properties of porous surfaces, an idea that could help reduce friction between the moving parts of machines.
The idea that skiers and red blood cells move in a similar way occurred to Sheldon Weinbaum, a biomedical engineer at The City College of New York when he noticed that a skier made deeper tracks in powdered snow when she was standing still than when she was moving. He says that when she moves, the skier’s weight is supported not just by the particles of ice in the snow, but also by trapped air. The mechanism works only as long as the skier is moving more quickly than the air can escape from the snow.
The same process explains how red blood cells are able pass through capillaries that are barely bigger than they are, says Weinbaum. These vessels are lined with a layer of glycoproteins called the glycocalyx, which is up to 400 nanometres thick. The puzzle is how the cells can squeeze past this layer without destroying it. “It occurred to me that red blood cells were skiing,” he says, except on plasma rather than air.
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Weinbaum backed up the idea by watching how cells passed through capillaries at different speeds. At speeds above 20 micrometres per second, the cells appeared to float over the glycocalyx, but at slower speeds tended to crush it. When moving quickly, he says, the cells are supported not by the vessel walls but by blood plasma trapped in the glycocalyx. At slower speeds, however, the plasma is squeezed out.
“A 50-tonne train could glide on a track made of goose down provided it moved at more than 36 kilometres an hour”
The same mechanism could radically change the way modern machines are built, says Weinbaum. For example, he and his colleagues Quianhong Wu and Yiannis Andreopoulos, have shown that it is possible for a train to travel on a track made of goose feathers without crushing it.
The team tested the idea by filling a porous cylinder with goose down, dropping a piston onto the feathers and measuring how long the device could support the weight before the air escaped. They calculated that a train 25 metres long, 2 metres wide and weighing 50 tonnes could glide along a track made of goose down provided the train was travelling faster than 36 kilometres per hour (Physical Review Letters, vol 93, number 19, p 194,501).
“It’s beyond most people’s imagination that we can support something that weighs over 50 tonnes on something as soft as a pillow,” says Weinbaum. Roger Kamm, a biomedical engineer at the Massachusetts Institute of Technology says the idea has potential. “It’s a new form of lubrication,” he says.
But building a train track using this principle would be a lot of work for little reward, since the only benefit would be a smoother ride. A better application, he says, would be to use the idea to help reduce friction between moving engine parts. The technique could help to reduce jarring, which can be a major cause of metal fatigue and breakdowns.