Aug. 31, 2000 Liquid jets a few nanometers 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 August 18 issue of the journal Science, Georgia Institute of Technology researchers suggest that jets as small as six nanometers 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 nanometers in diameter, Landman and collaborator 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.
"A key to the formation of these jets is fighting certain phenomena that occur upon exit of the jets from the nozzle," Landman noted. "Films condensing on the nozzle exterior surfaces start to thicken and eventually block further outflow from the nozzle. Such films are not of great concern on the macroscopic scale, but become key to the ability to form nanoscale jets."
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.
* 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 nanometers, in contrast to results of deterministic Navier-Stokes calculations predicitng 500-nanometer-long jets. These observations matched the researchers' predictions based on modifications of the hydrodynamic equations to include the effect of stress fluctuations.
* 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."
In a second phase of their work, the researchers attempted to reconcile the predictions of traditional fluid dynamics equations -- that is, Navier-Stokes -- to their observations. They found that these equations did not account for the effect of thermally-induced fluctuations which significantly affect the stability of nanojets. These fluctuations are of much less importance at larger size scales.
"At the small length scale that we are dealing with here, fluctuations become amplified," Landman said. "There are always fluctuations or noise in all natural phenomena. But as the scale of the physical system decreases, then the amplitude of the relative effect of such fluctuations becomes stronger and stronger."
Reformulating the hydrodynamic equations allowed Landman and Moseler to properly describe what they had observed in their atomistic simulations. Their modified equations can now be used by other researchers to predict the behavior of nanojets of other materials under other conditions.
"With this fluctuating description, hydrodynamics becomes valid even on the molecular scale," Landman said.
In certain real-world applications, the jets would be formed under conditions where they may collide with molecules in the ambient environment. Such collisions may somewhat reduce the length of the jets, and cause additional perturbations, he added.
As a next step, the researchers would like to create nanojets experimentally and use them to apply patterns that could replace current lithographic processes in the manufacture of nanoscale miniaturized circuits. They could potentially also be used as "gene guns" to insert genetic materials into cells without causing damage.
"They could be very economical and perhaps allow one to achieve things that are not available technologically any other way," Landman added.
The work is supported by the U.S. Department of Energy, the U.S. Air Force Office of Scientific Research and in part by the Deutsche Forshungsgemeinschaft.
Graphics to illustrate this story are available at: http://gtresearchnews.gatech.edu/nanojets.html
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