Potential of nanojets

LIQUID JETS a few nanometres in diameter could one day be used for producing ever-smaller electronic circuitry, injecting genes into cells, etching tiny features and even serving as fuel injectors for microscopic engines.

But on these smallest of size scales, physical processes are often different than at larger scales, forcing engineers to reconsider both their expectations of how such nanoscale devices would perform and the established physical equations governing them.

Writing in the journal Science, Georgia Institute of Technology researchers suggest that jets as small as six nanometres in diameter may be possible to produce, though these tiny devices would require special conditions to operate and be particularly sensitive to effects not of concern at more familiar size scales.

"We are now being driven by fundamental, technological and economical considerations to explore and evaluate systems that are smaller and smaller," explained Dr. Uzi Landman, director of Georgia Tech's Center for Computational Materials Science.

"We need to understand these systems, because basic physics issues are especially important to them. There is no point in trying to make devices of this size scale without knowing what their physical behaviors and fundamental limitations are going to be."

To study jets just a few nanometres in diameter, Landman and collaboraterstor Michael Moseler used molecular dynamics simulations to observe how some 200,000 propane (C3H8) molecules would behave when compressed within a tiny reservoir and then injected out of a narrow nozzle made of gold.

Operating on an IBM SP-2 parallel processing computer, the simulations recorded the dynamics of the fluid molecules on the femtosecond time scale over periods of several nanoseconds.

The researchers first faced problems producing extended jets from the propane reservoir, to which they had applied 500 MegaPascals (5,000 Atmospheres) of pressure.

Their simulations suggest that the jet would quickly clog as a film several molecules thick formed on the outer surface of the nozzle.

To counter formation of the films, the researchers heated the outer surface of the nozzle to evaporate the film. In real-world applications, it may be possible alternatively to apply a coating that would prevent the propane molecules from adhering to the outer surface. Once able to maintain the flow of propane, the researchers studied the properties of their simulated nanojets. Among the findings:

- Jets exiting from the nozzle into a simulated vacuum achieved a relatively high velocity of up to 400 meters per second.

Friction of the pressurized fluid moving through the nozzle heats the propane, turning it to "a very hot fluid." Upon exit from the nozzle, rapid evaporation of molecules from the surface cools the jets, reducing their diameter by about 25 percent. A

After exit from the nozzle, instabilities caused by thermal fluctuations affect the jets' shape. Each jet forms a series of "necks" that cause it to resemble "links of sausage connected to one another." Ultimately, one of the necks "pinches off" and a droplet of propane separates itself from the jet.

- The jets remain intact, propagating as a whole over shorter distances than would macroscopic jets under similar conditions. Landman and Moseler observed jets extending 150-200 nanometres, in contrast to results of deterministic Navier-Stokes calculations predicitng 500-nanometer-long jets.

- As they break up, the jets form droplets of remarkably uniform size. Noted Landman: "In applications such as fuel injectors, this is a very important aspect because of the issue of efficient burning of the droplets."

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Section  : Science & Tech
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