It is twenty years since Sidney Nagel and colleagues at the University of Chicago started to work on the “Coffee Ring” problem. When spilled coffee dries, it forms rings rather than blobs of dried coffee. Why does it do that? Why doesn’t it just form into a homogeneous mass of brown dried coffee? Surely someone knew the answer to these questions?
Well, it turns out that until 1997 no one had asked these questions. Did we all assume that someone somewhere knew? A bit like those ubiquitous white mists that form on hot drinks, surely someone knew what they were? (They didn’t, the paper looking at those only came out two years ago and is here). Unlike the white mists though, coffee rings are of enormous technological importance. Many of our electronic devices are now printed with electrically conducting ink. As anyone who still writes with a fountain pen may be aware, it is not just coffee that forms ‘coffee rings’. Ink too can form rings as it dries. This is true whether the ink is from a pen or a specially made electrically conducting ink. We need to know how coffee rings form so that we can know how to stop them forming when we print our latest gadgets. This probably helps to explain why Nagel’s paper suggesting a mechanism for coffee ring formation has been cited thousands (>2000) of times since it was published.
More information on the formation of coffee rings (and some experiments that you can do with them on your work top) can be found here. Instead, for today’s Daily Grind, I’d like to focus on how to avoid the coffee ring effect and the fact that bacteria beat us to it. By many years.
There is a bacteria called Pseudomonas aeruginosa (P. aeruginosa for short) that has been subverting the coffee ring effect in order to survive. Although P. aeruginosa is fairly harmless for healthy individuals, it can affect people with compromised immune systems (such as some patients in hospitals). Often water borne, if P. aeruginosa had not found a way around the coffee ring effect, as the water hosting it dried, it would, like the coffee, be forced into a ring on the edge of the drop. Instead, drying water droplets that contain P. aeruginosa deposit the bacteria uniformly across the drop’s footprint, maximising the bacteria’s survival and, unfortunately for us, infection potential.
The bacteria can do this because they produce a surfactant that they inject into the water surrounding them. A surfactant is any substance that reduces the surface tension of a liquid. Soap is a surfactant and can be used to illustrate what the bacteria are doing (but with coffee). At the core of the bacteria’s survival mechanism is something called the Marangoni effect. This is the liquid flow that is caused by a gradient in surface tension; there is a flow of water from a region of lower surface tension to a region of higher surface tension. If we float a coffee bean on a dish of water and then drop some soap behind it, the bean accelerates away from the dripped drop (see video). The soap lowers the surface tension in the area around it causing a flow of water (that carries the bean) away from the soap drop.
If now you can imagine thousands of bacteria in a liquid drop ejecting tiny amounts of surfactant into the drop, you can hopefully see in your mind’s eye that the water flow in the drying droplet is going to get quite turbulent. Lots of little eddies will form as the water flows from areas of high surface tension to areas of low surface tension. These eddies will carry the bacteria with them counteracting the more linear flow from the top of the droplet to the edges (caused by the evaporation of the droplet) that drives the normal coffee ring formation. Consequently, rather than get carried to the edge of the drop, the bacteria are constantly moved around it and so when the drop finally dries, they will be more uniformly spread over the circle of the drop’s footprint.
Incidentally, the addition of a surfactant is one way that electronics can now be printed so as to avoid coffee ring staining effects. However, it is amusing and somewhat thought provoking to consider that the experimentalist bacteria had discovered this long before us.