Narrow Escape: Sharp nanogutters hustle out wetness

Researchers in the Netherlands have found that the shapes of nanoscale channels can dramatically affect how quickly a fluid flows through the extremely fine conduits.

WICKED FAST. Water races along the knife edge of a nanochannel with a novel profile. Micrograph (bottom) shows a 70-nanometer-high channel with a 7° knife-edge angle. Both the diagram and micrograph show the channel’s right half. Eijkel et al./Physical Review Letters

Jan C.T. Eijkel of the University of Twente in Enschede and his colleagues found that newly developed nanoscale channels with knife-edged profiles boost water flows and drying speeds.

Nanochannels, which typically have circular or rectangular cross sections, are a type of capillary. As open-ended channels of this kind dry out, water flows as a thin film along the inner walls to the opening.

As the new channels dry, however, most of the water runs in the blade-edge section, enhancing the flow, the scientists report in the Dec. 16 Physical Review Letters.

These channels’ novel geometry might provide a way to improve the performance of water-containing conduits called heat pipes, used to cool power-hungry micro-chips, the Twente team proposes. Manufacturers mount such chips on metal plates riddled with these heat pipes.

In another potential application, garments, medical dressings, and other items designed to wick away liquid might benefit from fibers etched with sharp-edged grooves, Eijkel suggests.

The team, led by Albert van den Berg, etched troughs only tens of nanometers deep into glass plates and created channels by heat fusing flat glass pieces on top. The channels’ unusual profiles had initially resulted from accidental overetching, Eijkel says.

The researchers found that the loss of water occurred hundreds of times as fast as well-known effects such as evaporation of water along the channel could explain. Moreover, the drying rates showed remarkable indifference to ambient humidity.

The high drying rates occur because of the channels’ geometry, Eijkel says. As the main body of the channel dries out, its knife-edge section remains filled, serving as a conduit for water to continue to flow to an open end. In a channel of more conventional shape, by contrast, water doesn’t collect and flow in one part of the channel.

Bruno Michel of IBM Zurich in Rüschlikon, Switzerland, suggests an alternative reason that the new channel shape might prove useful in microchip cooling: The geometry creates a large amount of exposed liquid surface, he says, leading to a high rate of evaporation.

However, he notes, scaling down chip-cooling heat pipes, which are tens of micrometers in diameter, would cause other problems, such as a leap in resistance to the flow of both liquids and gases.

At the nanoscale, the new work helps unravel drying mechanisms, comments Albert B. Frazier of the Georgia Institute of Technology in Atlanta. “Use of this knowledge could lead to smarter nanochannel designs,” he says.

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