U. PENN (US) — By changing the shape of particles, physicists are able to disrupt a common phenomenon known as the “coffee ring effect“— the ring-shaped stain left after coffee drops evaporate.
Understanding the impact of particle shape on drop drying could have applications in printing and painting. The principles could also be relevant in biological and medical contexts. The journal Nature published the findings last week.
“There are a lot of situations where you want uniform coatings,” says Peter Yunker, a doctoral candidate in physics at the University of Pennsylvania. “This work will stimulate people to think about new ways of doing it.”
“The coffee ring effect is very common in everyday experience,” adds Yunker. “To avoid it, scientists have gone to great lengths designing paints and inks that produce an even coating upon evaporation. We found that the effect can be eliminated simply by changing the shape of the particle.”
The edges of a water drop sitting on a table or a piece of paper, for example, are often “pinned” to the surface. This means that when the water evaporates, the drop can’t shrink in circumference but instead flattens out. That flattening motion pushes water and anything suspended in it, such as coffee particles, to its edges.
By the time the drop fully evaporates, most of the particles have reached the edge and are deposited on the surface, making a dark ring.
University of Chicago physicists Sidney Nagel, Thomas Witten, and their colleagues wrote an influential paper about this process in 1997, which focused mainly on suspended spherical particles. Recent experiments by Arjun Yodh, professor of physics at the University of Pennsylvania, and his team led to the surprising discovery that suspended particle shape played a major role.
Yodh’s team used uniformly sized plastic particles in their experiments. These particles were initially spherical but could be stretched into varying degrees of eccentricity, to ensure the experiments only tested the effect of the particle’s shape on the drying pattern.
The researchers were surprised at how big an effect particle shape had on the drying phenomenon.
“Different particle geometries change the nature of the membrane at the air-water interface,” Yodh says. “And that has big consequences.”
Spherical particles easily detach from the interface, and they flow past one another easily because the spheres do not substantially deform the air-water interface. Ellipsoid particles, however, cause substantial undulation of the air-water interface that in turn induces very strong attractions between the ellipsoids.
Thus the ellipsoids tend to get stuck on the surface, and, while the stuck particles can continue to flow towards the drop’s edges during evaporation, they increasingly block each other, creating a traffic jam of particles that eventually covers the drop’s surface.
“Once you stretch the spherical particles by about 20 percent,” Yunker says, “the particles deposit uniformly.”
After experimenting with suspended particle shape, the researchers added a surfactant, essentially soap, into the drops to show that interactions on the drop’s surface were responsible for the effect. With the surfactant lowering the drop’s surface tension, ellipsoid particles did not get stuck at the interface and flowed freely to the edge.
They also tested drops that had mixtures of both spherical and oblong particles. When the spheres were much smaller than the ellipsoids, the spheres flowed to the edge, but, at a certain size, they became similarly trapped.
“We were thinking it would be useful if you could just sprinkle in a few of these ellipsoid particles to remove the coffee ring effect,” Yodh says, “and we found that sometimes this idea works and sometimes it doesn’t.”
This research was supported in part by the National Science Foundation and NASA.
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