Tikalon Blog by Dev Gualtieri

Coffee Rings (Part II)

August 25, 2011

Scientists will investigate anything that catches their interest, including something as seemingly mundane as coffee rings . I reviewed the science behind coffee rings in a previous article (Coffee Rings, December 10, 2010). Coffee is a colloid ; that is, it's a solution of solid particles suspended in a liquid . Coffee rings form because of the outward flow of the suspended particles.

Robert D. Deegan and his colleagues at the James Franck Institute and the University of Chicago were the first to investigate coffee rings scientifically, in 1997.[1,2] The Chicago team verified the coffee ring effect also with red wine , milk , tea and soup . It's been found experimentally that the size of the suspended particles is important. Colloidal solutions of 100 nm particles will only form rings if the coffee ring is at least 10 μm .[3]

How a materials scientist makes coffee

My trick for a good cup is to grind the beans just before brewing in a drip , or vacuum , coffee maker. The beans are stored in a freezer chest.

(Image via Wikimedia Commons).

Since particle size is important to coffee ring formation, the next question is whether the shape of the particles is important, also. That was the purpose of a study recently published in Nature by materials scientists at the Materials Research Science and Engineering Center at the University of Pennsylvania .[4-8] Their experiments show that ellipsoidal particles will not make coffee rings, and their research may have an application to paints and other coatings.

Intuitively, this seems sensible. Coffee rings arise from capillary fluid flow, caused by surface tension , outward from the drop's center that pushes the suspended particles to the edge. Ellipsoidal particles would first align with the flow direction, and then allow the fluid to flow around them. This process would leave them in place. However, close experimental examination of the actual process reveals a different mechanism.

The modified flow between the ellipsoidal particles actually draws the particles together to produce loosely packed structures at the interface between the fluid and air. These structures prevent the suspended particles from independently flowing to the edge to form a ring. The jammed structures deposit where they are formed, and the result is a uniform deposit in the droplet area, and not a ring at the edge. Quite interestingly, adding just a small faction of ellipsoidal particles to a suspension of spheres produces a uniform deposition, also.

Coffee rings with spherical and ellipsoidal particles.

"Coffee stains" when particles are spherical (Left), and ellipsoidal (Right). (Images: Peter J. Yunker and Arjun G. Yodh, University of Pennsylvania, via NSF Web Site). A video demonstration is available on the Internet.[8]

Peter J. Yunker, one of the authors of the Nature paper, has a simple kitchen demonstration of the effect.[6]

"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."

Although clumped ellipsoidal particles do reach the edge of the droplet, their packing isn't dense, so there is an even coating of particles when the fluid evaporates.[6] Surfactants spoil the effect, so adding soap to a colloid of ellipsoidal particles results in the same behavior as one of spherical particles; that is, a ring is formed.[6]

Of course, one good experiment leads to another. The Penn team plans to investigate different particle sizes and shapes, the interplay of mixtures of different particles, and colloids of different types of fluids.[6] When formulating paints and inks , control of particle shape allows modification of the deposition process without modifying the chemistry of the particles or the fluid.[4]

Coffee Rings (Part II)

References: