WASHINGTON - Sprinkle water droplets on a flat surface and often they'll slowly trickle in different directions. Now engineers have figured out how to force droplets to zip along at amazingly fast speeds, a discovery that could help industries from heating to microelectronics.
The key is simple steam, chemical engineers from Lehigh University report in Friday's edition of the journal Science.
It may sound a tad arcane, but lots of processes, such as printing, depend on how fast liquid droplets can spread over a horizontal surface, thus unaided by gravity.
A 19th-century scientist first discovered that imbalances in surface tension, usually from temperature, cause droplets on that surface to move, but incredibly slowly.
Harnessing that energy to speed droplet flow would help industrial manufacturing, especially increasingly small-scale technology like microchips or miniature fuel cells that need to prevent liquid buildup, explained Lehigh lead researcher Manoj Chaudhury.
He succeeded in forcing water droplets to race at speeds hundreds of times faster, reaching over a meter a second.
First, Chaudhury coated a typical copper block with a mixture of silane and silicon. The actual spot where the two chemicals mixed was very water repellent; surfaces farther away became gradually less water repellent. The imbalance enticed droplets to move from the more-repellent to less-repellent spots - but still not fast enough.
Then Chaudhury passed steam over the copper block. The resulting condensation helped small droplets merge into bigger droplets that have more energy to zip away from the water-repellent spots. High-speed video measured droplets zooming around at over a meter a second.
It's "a clever manipulation of the forces acting on the droplet" that "holds great promise for practical applications," Howard Brenner of the Massachusetts Institute of Technology wrote in an accompanying review.
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