Superheated metal moves liquid uphill

I’ve been reading the latest issue of American Scientist in my spare time lately. A really cool article caught my eye, and I thought I’d pass it on for the more geeky, science-loving types in my audience. While I think half my readership isn’t into the science stuff I enjoy, I believe that other reader might be. For him, I pass along this cool story of liquid moving in the wrong direction.

Going Against the Flow
Sometimes particles prefer to propel themselves uphill
Fenella Saunders

Particles strive for the life of a couch potato—sinking into a spot that has the least energy, where gravity can’t pull them down any farther and movement is at a minimum. Getting a particle moving requires keeping it off kilter, out of equilibrium. But particles in such a state tend to bounce all over; harnessing their movement in a single desired direction is the goal of many nanoscale devices.

One way to do this is with a ratchet effect—a mechanism that uses spatial asymmetry and energy gradients to make movement easier in one direction than another. It turns out that in some cases, ratchets not only control movement, but can also move particles in unexpected directions—away from a minimum energy state, the molecular equivalent of a creek climbing uphill under its own power.

. . .

If a skillet is heated to an extremely hot temperature, between 200 and 300 degrees Celsius, drops of water flicked into the pan will skitter across the surface, remaining intact for a minute or so. A surface not quite so hot will boil away the water droplets instantly, but the superheated surface instead instantly turns the bottom of the droplets into a layer of steam. Vapor is a poor heat conductor, so the steam insulates the drops from further boiling. It also provides them with a means of movement: The water drops bounce around like hovercraft on a cushion of air.

Linke and his colleagues did not use a smooth metal surface, but one covered with a sawtooth pattern. The teeth inclined more steeply in one direction than the other—an asymmetrical surface, and therefore a ratchet mechanism. Millimeter-sized water droplets piped onto the superheated sawtooth surface zip off in one direction like airport passengers on a moving walkway, reaching speeds of up to 5 centimeters per second, even if the surface is tilted so that the droplets have to climb uphill. As the investigators reported in the April 21 issue of Physical Review Letters, the phenomenon works for many other liquids, such as ethanol and liquid nitrogen, although the temperature at which the Leidenfrost effect kicks in varies from 50 to 150 degrees above the boiling temperature of the liquid.

More details on the science behind this in the full article. Also, access the full issue online, where many of the articles are freely readable. Some of the really juicy stuff requires membership ($28-$70 for 1-3 years in the US), but after reading a single issue, I’m considering a subscription.

[tags]American Scientist, Liquid moving uphill, Cool geeky science stuff[/tags]