This illustration represents a how a dried drop would appear if it contained round particles (red) or elongated particles (blue). When a drop of coffee or tea dries, its particles (which are round) leave behind a ring-like stain called the “coffee ring effect” (upper left). But if you change the shape of the particles, the coffee stain behavior changes too. Elongated particles (blue) do not exhibit the coffee ring effect, rather they are deposited across the entire area of the drop, resulting in a uniformly dark stain (lower right). Credit: Felice Macera, University of Pennsylvania |
If
you’ve ever spilled a drop of coffee on a surface, you might have
noticed the curious way the color concentrates at the edges when the
coffee dries. This is known as the “coffee ring effect,” and recently,
researchers have determined that the shape of the particles in the
liquid is an important factor in creating this pattern. The research
results could eventually translate into new techniques or formulations
for product coatings, or better inks and paints.
This work, published in the August 18 issue of the journal Nature was performed by Arjun Yodh and colleagues at the University of Pennsylvania.
“We
found that if you change the shape of the particles in the solution,
the coffee ring effect goes away, and you end up with a uniform
coating,” said Peter Yunker, a graduate student in Yodh’s lab.
First,
a little fluid dynamics: As the liquid in a droplet evaporates, the
edges remain fixed, so as the volume decreases, fluid flows outward from
the middle of the droplet to its edges. This flow carries particles to
the edges, and round particles at the edge will pack closely. By the
time all of the liquid in the droplet evaporates, most of the particles
will be at the edge, producing the coffee ring effect.
Both
the shape that liquid droplets take, and the way the shape changes as
the droplets evaporate, is greatly influenced by surface tension at the
air-liquid interface. This tension is a property of the interface, based
on how the molecules in the liquid interact with one another versus the
air. For example, liquids with a high surface tension, like water, may
form a raised droplet, because the molecules are very attracted to one
another and not so attracted to the air. In contrast, liquids with lower
surface tension, like alcohols, are more likely to form flat spots
instead of curved droplets.
The
Yodh group found that elongated particles in a liquid behave
differently than round ones because of the way they are affected by the
surface tension of the air-liquid interface. The forces at work are even
observable in a common breakfast cereal.
“If
you make the particles elongated or ellipsoidal, they deform the
air-water interface, which causes the particles to strongly attract one
another. You can observe this effect in a bowl of cheerios-if there are
only a few left they clump together in the middle of the bowl, due to
the surface tension of the milk,” explained Yunker.
This
clumping changes the way the particles distribute themselves within the
droplet. Even if the clumped ellipsoidal particles reach the edge of
the droplet, they do not pack as closely as round particles. The loosely
packed clumps eventually spread to cover the entire surface, filling it
so an even coating of particles is deposited when evaporation is
complete.
“This
work gives us a new idea about how to make a uniform coating,
relatively simply. If you change the particle shape, you can change the
way a particle is deposited. You can also make mixtures. In some cases,
even just a small amount of ellipsoids can change the way the particles
deposit when they dry,” said Yodh.
In
future studies, the research team will explore drying and deposition of
different types of fluids. They will also investigate different
particle sizes and shapes, and the interplay of particle mixtures.
“This
is an exciting scientific result with potential commercial
applications, which was in part enabled by support of the Materials
Research Science and Engineering Center at the University of
Pennsylvania,” said Mary Galvin, program director for the division of
materials research at the National Science Foundation, which partially
funded the research. The centers program, recently renamed Materials
Research Centers and Teams, provides support for interdisciplinary
materials research and education while addressing fundamental problems
in science and engineering.
Video that explains coffee ring effect
Suppression of the coffee-ring effect by shape-dependent capillary interactions
