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What causes coffee grounds to dry into the pattern shown at the bottom of this coffee cup (see photo)?
鈥� The coffee dregs essentially consist of two components 鈥� very large coffee grains which remain settled in one place and very fine colloidal material that moves with the liquid. Because of surface-tension effects, the large grains retain a pool of liquid around them. As this liquid droplet dries, evaporation happens preferentially at the edge of the droplet because it is more exposed. This creates a flow of liquid towards the edge which carries with it the fine colloidal particles. As a result, the majority of them end up being deposited at the droplet periphery, which remains anchored in place some distance from the particle until nearly all of the moisture has evaporated.
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This explains the resultant pattern of large particles, with a clear space around them, then a very concentrated ring of fine material. A New 杏吧原创 article many years ago called 鈥�The thrill of the spill鈥� (25 October 1997) explains this effect in detail.
Simon Iveson, School of Engineering, Faculty of Engineering and Built Environment, University of Newcastle, Callaghan, New South Wales, Australia