While cleaning dishes, I noticed a symmetric pattern in the bottom of a coffee cup. Cream had separated from the coffee making a six-fold pattern. Since the coffee and cream were well mixed earlier, what caused them to un-mix? What gave rise to the six-fold pattern? Would this happen again. Subsequently, several patterns have been photographed and subjected to time lapse photography.
Patters in Coffee
Water in the cup evaporates cooling the surface and leaving the cream fat globules behind. This process unmixes the coffee and cream. It is a non-equilibrium process produced by a flow of energy out of the coffee. The evaporative cooling at the surface produces an unstable situation in the coffee. Warmer, less dense, coffee below rises and can produce convective rolls, where coffee moves in a circle, up on one side and down on the other. This can be seen on the surface of tea in a cup, where lines of bubbles appear on the downward convectiing regions of the tea (see below). The large scale patterns are likely due to Maragoni convection, because cream on a surface has a lower surface tension than pure water? To equalize this tension, the surface will flow towards the pure water regions.