Temperature variations move microdroplets
By Steven Schultz
When Sandra Troian, associate professor of
chemical engineering, published a paper in December
describing a novel way of moving minute amounts of
liquid across very small surfaces, it guaranteed
that her e-mail and phone would be jumping.
More than just a neat trick of microscopic
tinkering, Troian's discovery could prove useful in
a range of industries. Chemists and biologists are
increasingly able to shrink experiments that used
to require large arrays of test tubes down to
microdroplets of liquid. The result is the
so-called lab-on-a-chip, which marries the silicone
chips of electronics with chemistry and
biotechnology.
Such a hybrid chip, for example, could be
"programmed" to test thousands of chemical
combinations for their potential to become a new
drug. One problem, however, has been how to move
the molecules from place to place on the chip.
Liquid racetracks
In the December 16 issue of Nature,
Troian and graduate student Dawn Kataoka showed
that they could coax microscopic amounts of liquid
to move across a surface by creating slight
temperature variations on a chemically patterned
surface. Working with films of liquid less than one
thousandth of a millimeter in thickness, they found
that the liquid flowed from warmer spots to cooler
ones. They also found they could precisely control
the direction of the flow by laying down tiny lines
of water-attractive coating, which acted like
racetracks for the liquid.
Previous techniques for moving small amounts of
liquid have involved either miniscule pumps, which
clog easily, or large voltage supplies, which
require bulky hardware. Troian's approach has the
potential to be cheaper and more flexible.
These advantages have attracted interest among
researchers in academics and industry, many of whom
have requested more information about Troian's
experiments. Her work has been featured in news
articles in Nature, New Scientist and
Physics Today, among other journals. She
filed an invention disclosure, which she hopes to
patent this year; and several biotechnology
companies in New Jersey and California have
expressed interest in licensing the method. On
campus Troian is assembling a group interested in
microfluidics to participate in a broader
micro-engineering project under way in the
Electrical Engineering Department.
Why fluids extend fingers
Troian came to her discovery through a series of
coincidences. First, as a postdoctoral fellow at
Exxon Corp., she became interested in the way that
fluids flow across a surface in drips, or fingers,
like wet paint running down a wall. She developed
mathematical explanations for why fluids extend
fingers rather than flowing in a straight-edged
sheet.
Then, working in France, she was introduced to
researchers who were using temperature gradients to
move liquids. Troian showed that the liquids moved
with the same fingering effect she had studied
before. These temperature-induced flows were not
very useful, however, because there was no way to
control the flow precisely.
The final piece came together at Princeton with
a question from an undergraduate. Electrical
engineering major Gwendolyn Barriac '99 came to
Troian for advice on how to print microliquid
patterns from one surface to another. Working with
the student, Troian learned about photolithography
techniques common in electrical engineering for
making microscopic patterns on silicone wafers. She
immediately realized such patterns could be used to
channel and control minute volumes of liquid.
She and Barriac made wafers with alternating
strips of two materials, each 50 microns wide. The
first stripe attracted the fluid; the next repelled
it. When Troian applied the temperature gradient,
the stripes imposed order on the fingers of flowing
liquid by directing them along the attractive
lanes.
Congenial collaboration
With seed money from the Princeton Center for
Complex Materials, Troian is now working with Anton
Darhuber, a research fellow from Austria, and
professors Sigurd Wagner of Electrical Engineering,
Robert Austin of Physics and Roberto Car of
Chemistry to develop a working chip.
Although she is wary of diving into a field as
hotly competitive as the lab-on-a-chip business,
Troian has enjoyed the opportunities for
collaboration.
"I simply could not have found better colleagues
with whom to work," she says. "For four lonely
years it was just Dawn and me working on the theory
and experiments, not realizing that there was this
really neat application just around the corner.
When we found the application, it suddenly became
less monastic and a whole lot more fun."
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