Coffee spills are brewing a whole new frontier in coatings research.
Credit the so-called “coffee ring effect,” named for the humble stain that is inspiring scientists at the University of Pennsylvania’s Materials Research Science and Engineering Center.
Steaming Mug of Particles
Coffee and tea drinkers who slosh a cup may have noticed the curious way the color concentrates at the edges when the drips dry. This is known as the “coffee ring effect,” and researchers have traced the source of the pattern in large part to the shape of the particles in the liquid.
“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 Arjun Yodh’s lab at the University of Pennsylvania, where the work is underway.
Felice Macera, University of Pennsylvania
|The illustration represents a how a dried drop would appear, depending on whether it contained round (red) or elongated (blue) particles. Round particles clump together, while elongated particles are deposited across the entire area of the drop, resulting in a uniform stain.|
The research results could eventually translate into new techniques or formulations for coatings or better inks and paints, reports the National Science Foundation, which helped support the project and has made a video of the science behind the coffee ring.
The research was published in the Aug. 18 issue of the journal Nature.
Shaping Surface Tension
NSF explains the “coffee ring effect” this way: 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 the droplet evaporates, most of the particles will be at the edge, producing the coffee ring effect.
Surface tension at the air-liquid interface greatly influences the shape of the droplet and the way the shape changes during evaporation, NSF says. 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, water and other liquids with a high surface tension tend to form a raised droplet, because the molecules are more attracted to one another than to the air. In contrast, alcohols and other liquids with lower surface tension are more likely to form flat spots instead of curved droplets.
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, Yodh’s team found. The same is true of a bowl of 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,” said Yunker. “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.”
Clumps and Coatings
This clumping changes the way the particles distribute themselves within the droplet, NSF explains. Even if the clumped ellipsoidal particles reach the droplet’s edge, they do not pack as closely as round particles. The loosely packed clumps eventually spread to cover the entire surface, 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,” says Yodh. “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.”
Drying and Deposition
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,” said Mary Galvin, program director for the division of materials research at NSF.