Penn Physicists Conquer the ‘Coffee Ring Effect’

A team of Penn physicists have figured out how to conquer the “coffee ring effect”—that “O”-shaped stain of particles that is always left over after coffee drops evaporate — by changing the shape of the particles. The discovery provides engineers with a new set of tools that could have implications in various industrial settings, particularly printing and painting.

The research was conducted by professor Arjun Yodh, director of the Laboratory for Research on the Structure of Matter; doctoral candidates Peter Yunker and Matthew Lohr; and postdoctoral fellow Tim Still, all of the department of Physics and Astronomy in the School of Arts and Sciences.

“The coffee ring effect is very common in everyday experience,” Yunker says. “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, 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, but it was not until the Yodh team’s recent experiments that the surprising role played by suspended particle shape was discovered.

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 air-water interface, and they flow past one another easily because the spheres do not substantially deform the surface of the drop, where the water meets the air. Ellipsoid particles, however, cause substantial undulation of that air-water interface that in turn causes the ellipsoids to be strongly attracted to one another. As a result, the ellipsoids tend to get stuck on the surface in clumps, 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. Sometimes, even adding a small amount of the elliptical particles was enough to disrupt the coffee ring effect.

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.

“In many cases, the way we make coatings involves hazardous chemicals and if you need something that’s bio-compatible, it’s more difficult,” Yunker says. “There are a lot of situations where you want uniform coatings and this work will stimulate people to think about new ways of doing it.”

Text by Evan Lerner
Video by Kurtis Sensenig