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Microscopic jets, diamonds unlikely on Uranus, and amazing mosquito legs

18.07.2007
News from the American Physical Society
Microscopic Polystyrene Balls - now Jet-propelled!
J. Howse, R. Jones, A. Ryan, T. Gough, R. Vafabakhsh, R. Golestanian
Physical Review Letters (forthcoming, advance copy available)
A collaboration of British and Iranian physicists has created an armada of self-propelled polystyrene balls about as wide as a strand of your hair. Their efforts are moving toward self-propelled nanoswimmers that could navigate narrow channels such as the human circulatory system.

The researchers, led by Ramin Golestanian of the University of Sheffield, coated one side of each polystyrene ball with a thin layer of platinum before dropping them into a solution of hydrogen peroxide and water. This metal catalyzes a reaction in which hydrogen peroxide breaks into oxygen and water. Because the reaction spits out three molecules for every two that it consumes, the polystyrene ball is pushed from the platinum side.

Objects as small as these polystyrene balls naturally wander about randomly, a phenomenon caused by jostling about among vibrating atoms and molecules. This "random walk" movement is called Brownian motion. To account for it, the platinum-coated balls were tested against polystyrene balls with no coating.

Over short distances, they found that the half-coated balls moved in a particular direction although their paths meandered over longer distances. Still, the wanderings of the coated balls were distinct from the Brownian motion of the uncoated balls. Their paths were a random walk with step sizes that depended on the concentration of hydrogen peroxide. The larger the hydrogen peroxide concentration, the larger the step.

Physicists have yet to devise a way to keep the balls heading in a particular direction, but chemical reaction catalysis may prove a useful method for propelling microscopic objects in liquids. - KM

Diamonds unlikely in gas giants like Uranus
L. M. Ghiringhelli, C. Valeriani, E. J. Meijer and D. Frenkel
Physical Review Letters (forthcoming, preprint available)
A new study finds that diamonds probably don't crystallize in the atmospheres of planets such as Uranus and Neptune. The conclusion is contrary to recent speculation that small diamonds would spontaneously form in carbon rich layers of the gas giant planets. White dwarf stars, according to the study, are veritable diamond factories.

Physicists at the Universtiet van Amsterdam and the FOM Institute for Atomic and Molecular Physics in the Netherlands performed a numerical analysis showing that at the temperatures and pressures in gas giant planets like Uranus, arrangements of carbon atoms would be much more suitable for creating tiny bits of graphite rather than diamond.

In white dwarfs, on the other hand, the simulation shows that the conditions would cause the carbon atoms to line up in configurations that are much more amenable for diamond crystallization. The conclusion is consistent with the 2004 discovery of a cooling white dwarf star that appears to have a solid diamond core 4000 kilometers across.

Although diamond formation in the atmospheres of gas giants is not strictly impossible, the Dutch physicists say that the odds are exceedingly slim that a diamond could have formed under the conditions that exist in Uranus in the entire lifetime of the universe. - JR

Miraculous Mosquito Legs
C. W. WU, X. Q. King, and Diane Wu
Physical Review E
Mosquitoes walk on water better than water striders, cling to smooth ceilings and walls as tightly as geckos, and clutch the skin of their victims with annoying tenacity in search of blood. Now a collaboration of physicists from Dalian University in China and Simon Fraser University in Canada are looking beyond the insect's pesky reputation to discover how the tiny creatures manage to be so comfortable on such a diverse range of surfaces.

Like flies, mosquito feet are equipped with hooked claws for clinging to skin. Like geckos, they have hairy pads on their feet that stick to nearly any smooth surface with a velcro-like grip. But it's their ability to walk on water that really makes mosquitoes stand out in the animal kingdom.

Both water striders and mosquitoes rely on superhydrophobic (extremely water repelling) legs to allow them to stand on pond surfaces. Water striders' legs do a pretty good job of it, repelling water well enough to support up to 15 times their body weight. Mosquitoes, however, can easily beat that. Experiments now reveal that they repel water so well that each of a mosquito's six legs could support 23 times the insect's weight. The physicists measured the water repellant ability of mosquito legs by attaching an amputated leg to the end of a needle and recording the force as they pushed it down into a container of water.

The secret to mosquito water walking appears to be feathery scales a few microns across that in turn are covered with nanoscopic ribbing, forming what the physicists have dubbed (in an apparent fit of excessive prefixing) a micronanostructure. So the next time a mosquito lands on your arm, take a moment to ponder its impressive and versatile leg adaptations -- then squish it before it sucks your blood. - JR

James Riordon | EurekAlert!
Further information:
http://www.aps.org

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