August 18, 2011

‘Coffee Ring Effect’ Could Help Form New Techniques For Ink

Researchers are using the "coffee ring effect" to turn it into new techniques or formulations for ink and paints.

The "coffee ring effect" takes place when a drop of coffee on a surface dries, leaving the color of the edges of the ring more distinct as the coffee dries.

According to the researchers, as liquid in a droplet evaporates the edges remained fixed, so as the volume decreases fluid flows outward from the middle of the droplet to its edges.  This carries particles to the edges, and round particles at the edge will pack closely.

Once all the liquid in the droplet evaporates, most of the particles will be at the edge, producing the coffee ring effect.

"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," Peter Yunker, a graduate student working on the research, said in a press release.

The team found that elongated particles in liquid behave differently than round ones because of the way they are affected by the surface tension of the air-liquid interface.

"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," Yunker said in a press release.

The clumping changes the way the particles distribute themselves within the droplet.  The loosely packed clumps eventually spread to cover the entire surface.

"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 Arjun Yodh, lead author of the study, said in a press release.

The team said they will explore drying and deposition of different types of fluids in future studies.  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," Mary Galvin, program director for the division of materials research at the National Science Foundation, said in a press release.

Their research is published in the journal Nature on August 18.


Image Caption: 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


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