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Coffee drinkers are familiar with the ring-shaped stains that result
from spilled drops that have dried, in which the brown stain is not
evenly distributed, but instead concentrated at the edge. Now, a team
led by Gerard Wong, a professor of materials science and engineering,
physics, and bioengineering at the University of Illinois at
Urbana-Champaign has found the same "coffee-ring" formation in drying
drops of DNA.
To gain insights into the physics behind the ring
phenomenon, Wong's team experimentally studied the dynamics of drying
DNA droplets on glass surfaces. They report their findings in a paper
accepted for publication in the journal Physical Review Letters, and
posted on its Web site.
"As the droplet evaporated, DNA chains were transported outward by
water flow to the drop's perimeter," Wong said. "At the droplet edge,
the DNA became increasingly concentrated and formed a liquid crystal
with concentric chain orientations. (Liquid crystals are materials that
flow like a liquid, but can align in a preferred direction like a
crystalline solid.) During the final stages of drying, stresses
propagated from the rim inward through the liquid crystal, creating
cracks that formed a periodic zigzag pattern."
To examine the
structure and behavior of the DNA liquid crystal, the researchers used
a relatively new imaging technique developed at Kent State University.
Called fluorescence confocal polarizing microscopy, the technique
imaged the DNA in the drying droplet in three dimensions.
DNA alignment parallel to the droplet's edge was counterintuitive,"
Wong said. "We had expected the DNA to extend along the direction of
flow, which was mainly in the radial direction. But, instead of
resembling the spokes of a bicycle wheel, the transported DNA resembled
the rim of a bicycle wheel."
Since nearly all the DNA is
concentrated in a narrow ring with almost no DNA in the rest of the
stain, these effects should be accounted for in the design of arrays in
which DNA droplets are sequentially deposited onto a glass surface for
hybridization studies, the researchers report.
optimization of the wetting conditions, it is possible to miss all the
DNA in the ring stain of a dried droplet, resulting in false
negatives," Wong said. "We need to think of strategies to minimize this
The co-authors of the paper are postdoctoral research
associate Ivan Smalyukh, graduate students Olena Zribi and John Butler,
and professor Oleg D. Lavrentovich, director of the Liquid Crystal
Institute at Kent State. University of Illinois at Urbana-Champaign. May 2006.
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