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Scientists Levitate Cart with Water Vapor

By Ross Pomeroy

A group of mechanical engineers from Washington State University have declared war... on friction.

"Approximately one-third of the world's energy resources are consumed by friction in one form or another," they say. "A recent estimate shows that the annual friction-induced costs in the U.S. are roughly $270 billion to $800 billion."

The engineers believe that a possible way to reduce these astronomical costs is via levitation. Maglev trains are commonly used, for example. But another potential solution may be via the often overlooked, yet elegant, Leidenfrost effect.

Though the term may be foreign to you, you've almost certainly witnessed the Leidenfrost effect on countless occasions. It's what happens when a liquid is placed on a surface that's much hotter than the liquid's boiling point. An insulating vapor barrier is rapidly produced, causing the liquid to float above the surface and evaporate much more slowly. If you've ever seen water or oil skitter across the surface of a scalding hot skillet, then you've witnessed the Leidenfrost effect.

In a study just published to Scientific Reports, the Washington State engineers describe -- for the first time -- how the Leidenfrost effect can be harnessed to levitate a cart, enabling it to move almost frictionlessly.

To carry out the experiment, the researchers simply heated a small brass plate to 140 degrees Celsius. On it, they placed a solid, square cart with 5 drops of water on its underside.

The researchers found that a cart weighing up to 40 grams could be levitated with 0.4 ml of water on a flat surface. The mass was dependent upon the temperature of the plate and the thickness of the vapor film, which, for water, appears to be pretty small. (Above: The green squares represent the trials where near frictionless levitation was attained.)

Furthermore, the researchers discovered that the levitated cart could be made to slide down a gentle decline of a few degrees, and that the weight of the cart could be increased as high as 100 grams in this setting (due to reduced weight pressing directly against the incline). They also found that the cart could propel itself across a flat, ratcheted surface.

Could this method of levitation be used in any practical setting? Certainly not at this time. Water requires a significant amount of energy in the form of heat to create the vapor film necessary for Leidenfrost levitation, and other, specially designed liquids that don't require as much energy aren't feasible due to their high production costs. Moreover, much heavier objects seem to be out of the question.

But, in this setting, mechanical engineers have struck a decisive victory against friction. The war, however, will prove much more difficult to win.

Source: Ali Hashmi, Yuhao Xu, Benjamin Coder, Paul A. Osborne, Jonathon Spafford, Grant E. Michael, Gan Yu, & Jie Xu (2012) Leidenfrost levitation: beyond droplets. Scientific Reports 2, Article number: 797, doi:10.1038/srep00797

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