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.
"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.