Just as powdery snow supports a slaloming snowboarder, a track of material as fluffy as goose down could support a speeding train. Or so say biomedical engineers who discovered the underlying physical principle while studying blood cells gliding through capillaries.
Blood vessels are lined with a compressible gel that slowly moving blood cells easily slide over. When the cells stop, they sink into the gel, only to pop out when they get going again. To explain this phenomenon, biomedical engineer Sheldon Weinbaum of The City College of New York argued in 2000 that a moving blood cell rides on the fluid trapped in the gel, which cannot seep through the material fast enough to get out of the way. When the cell comes to a halt, however, the fluid has time to ooze out from under it, so the cell bogs down. In the same way, a snowboarder rides the air trapped in fresh snow--not the snow itself--Weinbaum's calculations suggested.
Now, Weinbaum and colleagues Qianhong Wu and Yiannis Andreopoulous have backed up their analysis with experiments. First, they measured the ability of air in snow to support weight by squeezing snow in a piston with mesh sides, as they report in a paper published in the 5 November issue of Physical Review Letters. Transducers tracked the air pressure in the snow as the researchers dropped a weight on the piston's plunger. As expected, the pressure rose quickly and then slowly decreased as the air seeped out through the mesh. It remained high long enough to easily support a snowboarder moving at 35 kilometers per hour.
Next, the researchers tried a new material: goose down. It didn't hold air as well, but it still sustained a sizable pressure surge for roughly a fifth of a second. Scaling up from the data, the researchers argue that, when placed in a channel with airtight sides, a material like goose down traps air well enough to support a 25-meter-long, 50-metric-ton train car moving 70 kilometers per hour or faster.
"It's surprising that such great loads can be support by just air and a porous medium," says Timothy Pedley, a fluid dynamicist at the University of Cambridge, U.K. Richard Post, a physicist at Lawrence Livermore National Laboratory in California and an expert on magnetically levitated trains, or maglevs, says he doubts porous materials will ever be used to bear the full weight of a train. However, Post says, they might be used to guide a maglev down its track and prevent it from swaying side to side.