Drops of coffee on a table dry into dark rings, but fluids with
less-spherical particles suspended in them dry as filled-in
circles. The research team that discovered this distinction now
demonstrates a new surprise—that the motion of the particles
during drying provides the first easily-tuned example of an
equation that appears in phenomena as diverse as bacterial
growth and burning paper. In Physical Review Letters they
describe watching drying drops of fluid under a microscope and
measuring the distribution of the particles during the process.
The experiments provide a rare direct test of the so-called KPZ
equation, which has been difficult to verify in other phenomena.
The brown outlines left behind by spilled drops of coffee were
explained in 1997 . As water at the drop edges evaporates,
remaining water flows outward from the center to replace it,
carrying the brown coffee particles and leaving a dark ring when
the liquid is gone. Last year, Arjun Yodh of the University of
Pennsylvania and his colleagues