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Left: A conventional electrohydrodynamic (EHD) jet
- a stream of electrically charged liquid forced from a nozzle -
which whips uncontrollably.
Right: A stabilized jet produced by Princeton
University engineers. The long-sought achievement has many
possible uses in electronics and other industries.
Credit: Princeton University
Sibel Korkut, a graduate student in chemical
engineering, discovered how to control electrically charged jets
of liquid and print super-thin lines - just one ten-thousandth of
a millimeter wide. Here she adjusts a high-speed camera she used
to analyze the jets.
Photo by Frank Wojciechowski
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�It is a liquid delivery system on a micro
scale,� said Ilhan Aksay, professor of chemical engineering. �And it
becomes a true writing technology.�
Aksay and graduate student Sibel Korkut published the results Jan. 25,
2008, in Physical Review Letters. The paper also includes as a
co-author Dudley Saville, a chemical engineering professor who
initiated the project but died in 2006. The research was funded by
grants from the Army Research Office, the National Science Foundation
and NASA.
Developing a deep understanding of the fundamental physics behind the
process rather than building highly specialized equipment, the
researchers were able to use a nozzle that is half a millimeter wide,
or 5,000 times wider than the lines it produced.
The key to the process is something called an �electrohydrodynamic (EHD)
jet� - a stream of liquid forced from a nozzle by a very strong
electric field. Such jets were first investigated in 1917 and are now
commonly used in a variety of industrial processes. However, one of
the main features of EHD jets is that the stream of liquid becomes
unstable soon after it leaves the nozzle and either whips around
uncontrollably or breaks up into fine liquid drops. Engineers have
used these effects to their advantage in spinning fibers and in
industrial electrospray painting, but the reason for the whipping
instability, and thus any hope of stopping it, has been a
long-standing problem.
In the early part of this decade, two researchers working
independently -- Princeton graduate student Hak Poon and Cornell
University physicist Harold Craighead -- found that the jet was stable
for a very short distance after leaving the nozzle, but the result was
still not practical and the reasons were still elusive.
�To understand how to control the jet in any engineering application
we had to understand why this was happening,� Aksay said.
Korkut took up the challenge and worked for nearly six years to nail
down the mechanisms at play. In the end, she found that a key factor
was that the liquid jet was transferring some of its electrical charge
to the surrounding gas, which breaks into charged particles and
carries some of the electrical current. Korkut�s predecessors and
other scientists had looked only at the density of the electrical
charges on the surface of the liquid jet.
Expanding her view of the system led Korkut to a simple way to control
the stability of the jet by changing the gas and the amount of water
vapor. She was able to produce an extremely straight and stable jet
more than 8 millimeters from the nozzle.
The result is highly practical not only because of the fineness of the
stream but also because the large size of the nozzle and the distance
from the nozzle to the printed surface will prevent clogs or jams.
Aksay said a chief use for the technique could be in printing
electrically conducting organic polymers (plastics) that could be the
basis for large electronic devices. Conventional techniques for making
wires of that size (100 nanometers) require laboriously etching the
lines with a beam of electrons, which can only be done in very small
areas. The new technique can lay down lines at the rate of meters per
second as opposed to millionths of a meter per second.
Another application would be to use a liquid that solidifies into a
fiber for making precise three-dimensional lattices. Such a product
could be used as a scaffold to promote blood clotting in wounds and in
other medical devices.
Princeton University has filed for a patent on the discovery and has
licensed rights to Vorbeck Materials Corp., a specialty chemical
company based in Maryland.
�Electronics is a huge potential application for this discovery,� said
John Lettow, president of Vorbeck and a 1995 chemical engineering
alumnus of Princeton. �The printing technique could greatly increase
the size of video displays and the speed with which high performance
displays are made.� Lettow said the technique also could be used in
creating large sensors that collect information over a wide area, such
as a sensor printed onto an airplane wing to detect metal fatigue.
For Korkut, publishing the results in the premier physics journal
marks a gratifying conclusion to years of painstaking work that
offered no guarantee of a practical answer. �You are digging into a
hole and you don�t know if you will hit the bottom,� Korkut said. �You
could just keep on digging.�
Even though she began to see improved stability of the jet after five
years, she still did not have a precise handle on the causes. Aksay
and Saville pressed her to have a deeper understanding before
publishing the results.
�It took more than a year after we saw the clues. We had to look at
many possibilities,� Korkut said.
Aksay said Korkut succeeded because of her persistence. �If you give
up too soon, you can�t come up with a breakthrough.�
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