Most people are annoyed by nagging coffee stains, but to physicist Sidney Nagel they were inspiration. If you’re a coffee lover (or you live with one), I guarantee that you’ve seen the characteristic light brown stain with dark edges. I assume it was after he’d already had his morning coffee that this color difference piqued Nagel’s interest. The resulting 1997 journal article kicked off a flurry of research activity, and not just among java fans. It turns out that the physics behind these stains is both complex and potentially really useful.
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| A drop of wet coffee (left) and the resulting dried stain (right). Image Credit: Baiou Shi. |
Coffee is, of course, mostly water. Its brownish color comes from the tiny particles of coffee beans that make it through your filter and spread out in the water. When you splash coffee onto the kitchen counter and don’t clean it up, the water eventually evaporates and leaves the particles behind. Exactly where the particles end up—primarily around the edges of the stain—is controlled by the forces they experience. New work by Lehigh University researchers tackles this “coffee ring effect” from a new angle, using massive processing power to better understand the forces involved.
Just in case you’re wondering if it is a worthwhile use of processing power, time, and money to study a stain, consider this. Coffee isn’t the only substance that’s made of tiny particles suspended in a liquid. Blood, paint, ink…understanding the way these kinds of liquids behave could have huge implications in areas from medicine to high-tech manufacturing. For example, in the future we may be able to create tiny structures with unique properties by carefully dropping a liquid filled with nanoparticles onto a surface and evaporating the liquid. In order to do this, though, scientists need to be able to accurately predict a mixture’s behavior. This requires an understanding of the forces involved.
In an article in Physical Review E, graduate student Baiou Shi and her advisor Edmund Webb share the results of their recent research on these forces. With a powerful computer program, they ran simulations of a drop spreading out on a surface and calculated the forces experienced by suspended particles. In this case it’s not a drop of coffee spreading out on a kitchen counter, but a drop of lead (filled with tiny copper particles) spreading out onto a copper surface. While not as appetizing as coffee, this combination let them get at the physics behind what’s going on while simplifying things a bit. It also has relevance to situations that involve joining materials, as in welding, among other benefits.
Picture a drop of coffee hitting a table and spreading out. The boundary that separates the coffee from the air and runs along the surface of the table, all the way around the drop, is called the contact line. Previous research has shown that the edges of a drop evaporate more quickly than the middle. As the edges evaporate, more liquid flows in to take its place. As liquid continues to evaporate from the edges and more flows in from the middle to take its place, coffee particles are being constantly carried to the contact line and left behind. The particles pile up at the edges and stick to the surface, essentially pinning the contact line in place so that the size of the stain doesn’t shrink or grow. This creates the thick, dark outline around a coffee stain.
Shi and Webb are particularly interested in how a drop spreads out on a surface and which factors affect whether the contact line is pinned. In this work, they simulated how drops spread out on copper surfaces that have different crystal structures. This let them vary the way that a drop spreads out in a very specific way and study the impact on contact line pinning.
Through a very fundamental approach, their results provide insight on the forces that control where the particles suspended in a liquid end up after the liquid evaporates. In addition, their work indicates that a very thin film forms on the surface ahead of a spreading droplet, and this continues moving forward even when a contact line is pinned. What exactly does this mean? It means that there is much more to learn about the coffee ring effect before we can accurately predict how different mixtures will behave under different conditions, but scientists are on their way.
Although this work was primarily done by computer simulation, there was a bit of an experiment involved. During a meeting with her advisor one day, Shi accidentally spilled some coffee. When Webb saw her about to wipe it up, he exclaimed, “Don’t do it!” Although the computational research took a couple of years, it took just a couple of minutes for Shi to verify experimentally that a dark ring does indeed form around the edges of a coffee stain. Check it out for yourself tomorrow morning as you fuel up for your day.
—Kendra Redmond