Updated: Oct 15, 2018
The rings of dried coffee drops that you see on your table have deep physics in it. Similar ringlike patterns can also be seen on dusted floors when water spills, but contain different set of rules.
Coffee-rings! What stuff is that? This might be the question that came into your mind now. Well, the simplest way that I can explain it is to direct you to your coffee table. This time a little dirty table would be the best. Look at those dried stains of coffee drops on your table. They have a ring-like shape with most of the coffee particles at the rim of the stain. These stains are called coffee rings: thus the phenomenon of formation of such stains is called the coffee- ring effect.
The coffee-ring effect not only occurs with coffee drops but also with any drop that is comprised of a liquid that evaporates mixed with some non-evaporating particles. Drops of ink, paint, and so many other fluids can form such stains.
You may see only see ring-like stains on your coffee table; but physicists go further to ask the question ‘how such stains are formed’. This intriguing phenomenon was first explained in 1997 by Deegan. When the coffee drop evaporates the liquid molecules escape from the surface of the drop. As a result, the drop gets smaller and smaller with time. The edge of the drop, however, is still stuck at the same position as it cannot recede while the drop gets smaller. This is because the liquid molecules at the edge are usually bound with the surface. This means that new liquid molecules have to go to the edge to compensate for the liquid loss there due to evaporation. In short, there is a tiny flow of liquid from the centre of the drop towards the edge. This flow carries the coffee particles to the edge and deposits them there forming a ring-like stain upon drying.
Since drying of liquid drops is interesting to many technological areas, such as printing, biochemical analyses, etc., many scientists are interested in this phenomenon. Scientists have shown that this process can be very complex depending upon the properties of the liquid, particles, and even the surface where the drop is placed. For example, if the coffee particles were of rod-like shape then no ring-like stains would be formed.
‘But not all rings are coffee-rings’! What does that means? Not only drying of a drop but wetting by a drop can also produce ring-like stains. When a drop wets and spreads on a surface that contains dispersed powder, ring-like patterns are formed. The spreading of a drop occurs in a blink of an eye; sometimes much faster than that. When the drop spreads, its edge pushes on the powder particles and forms rings.
As in the case of the coffee-ring effect, several physical concepts are involved here as well. For example, the liquid not only spreads but also penetrates into the pores of the powder layer, influencing the patterns.
You may see such rings of dust when water drops spills over dusted floors. After drying, one may misidentify such stains as those resulted from the coffee-ring effect. In fact, these rings are formed by the wetting and spreading of the drop or even by a combination of wetting and drying.
Next time, after spilling your coffee or water on the floor look down curiously; maybe you will see some ring-like patterns whose formation contains complex physical concepts.