Source: Georgia Institute Of Technology
(http://www.gtri.gatech.edu/rco.html)
Date: Posted 8/31/2000
Research Shows Potential Of "Nanojets" For Smaller Circuitry & Injecting
Genes
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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