New technique makes water droplets sprint

It’s been more than 8 years since researchers found a way to alter a surface so that water would crawl uphill. Now, expanding on that work, they have made individual water drops race across flat, level surfaces at a brisk clip of 3 miles per hour.

Within 0.033 seconds, drop 1 (A) speeds outward from near the hydrophobic center of a modified silicon wafer and is visible as a white streak that stretches toward the top of the disk (B). At 0.466 seconds, drop 2 drifts off center (C) and quickly races outward to the bottom left (D). Science

Payoffs of the new capability could include improved industrial coatings, printing processes, and cooling technologies, comments Darsh T. Wasan of the Illinois Institute of Technology in Chicago. A water-moving coating might whisk away liquid, rather than simply repel it as ordinary waterproofing does, adds Manoj K. Chaudhury of Lehigh University in Bethlehem, Pa., whose team reported its results in the Jan. 26 Science.

In the earlier experiments, he and his colleagues chemically altered the lower parts of an inclined surface to make them hydrophobic, or water-hating, so that droplets would creep upward toward more friendly territory (SN: 6/13/92, p. 391). The newly developed process encourages drops at the hydrophobic center of a wafer to speed outward to the water-friendly edge, says Chaudhury.

To create the water-propelling platform, his current team modified the surface of silicon wafers to have a gradient of water compatibility. The group started with hydrophilic, or water-loving, wafers and deposited a layer of hydrophobic alkyltrichlorosilane molecules on each one, with the molecules becoming less concentrated toward the edge of each wafer.

Each water drop deposited near the altered wafer’s center experiences different surface tensions at its head and tail, resulting in a force coercing it toward the water-friendly edge.

The chemical gradient alone couldn’t propel water droplets to the striking speeds that the team captured on video, says Chaudhury. The drops would have crept at a glacial pace as they did in the experiments 8 years ago, he explains.

However, in the new work, Chaudhury and his colleagues used steam to deposit energetic, hot water droplets over the wafer’s hydrophobic center. A complex set of physical interactions among the surface, condensing drops, and drops already on the surface cranked the rate of water movement to a sprint. Stopping the flow of steam immediately resulted in a dramatic slowing of water movement, Chaudhury says.

“It is a very interesting set of phenomena the authors have observed,” says Wasan. “It’s very intriguing, and they combined several ideas in a clever way.”

Chaudhury suggests that one of the first uses of the new technology will be in improving heat transfer in heating and cooling systems, such as those in power plants. The technique could also benefit heat pipes that control temperatures in microelectronic and microfluidic devices, he says. Eventually, the team could reverse a wafer’s gradient and thereby the flow of water, he says. Then, for example, researchers could use the device as a microreactor on which small amounts of chemicals added to the outer edges will speed inward and mix themselves.

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