Why do coffee stains dry to leave a ring?
Scientists from the University of Pennsylvania have discovered how coffee spills evaporate to leave hollow circular stains, and how to leave a uniform deposition instead.
As a drop of coffee dries, all of the tiny, spherical, particles suspended in the drop move to the edge, leaving a 'ring' of coffee once dry. This phenomenon, known as the 'coffee-ring' effect, is true of many suspensions, and can be a major problem in industrial applications where a uniform deposition of particles in required.
But why do these coffee-rings form? In a spilled drop without particles, such as a solution or pure water, evaporation causes a gradual reduction in the size of the droplet, whereby the edges move inwards, the height reduces, and the droplet shape remains similar throughout.
For a drop with suspended spherical particles, however, the edges get 'pinned down' by the particles (a surface tension effect) and so the edges can't move inwards. The height still reduces with water loss and so there is a gradual change in drop shape - towards a flatter profile. Water flows towards the drop edge to replenish the water lost during evaporation, and carries particles with it. Once at the drop edge, the particles get deposited. This process continues throughout drying, until there are no particles to transport and no water left to do it.
Non-spherical, or ellipsoidal, particles are also drawn to the edge of the drop during evaporation, but as soon as they get to the edge and touch the drop surface, it is energetically more favourable for them to stay on the surface (another surface tension effect). Ellipsoidal particles therefore aggregate on the drop surface, moving towards the centre of the drop to make room for more. An open network of spheroids then results on the drop surface, which is able to resist the flow to the droplet edge. This network remains on the drop surface until no fluid exists, and is then deposited on the surface, evenly.
Peter Yunker, a physicist at the University of Pennsylvania who worked on the study, explained that "when you change from round [suspended] particles or spheres to elongated particles the physics changes significantly." He and his team have realised that simply by changing the shape of these particles, from spheres to spheroids, the coffee-ring effect is suppressed.
"We also found that you could add a small number of ellipsoids to a suspension of spheres, and you can force the spheres to be deposited uniformly", explains Peter Yunker, providing that the spheres are larger than the ellipsoids. "They'll get entangled in the network of ellipsoids that exists on the surface of the drop, and they'll be deposited uniformly".
Suppression of the coffee-ring effect would simplify many industrial applications which require a uniform distribution of particles, such as ink-jet printing, paint technology and complex assembly. Previous work has concentrated on altering the flow properties of the fluid, and other complex formulation changes. This latest research, however, provides an alternative technique, either by changing the shape of the particles or adding inert particles of the required shape.
To listen to Peter Yunker discuss the 'coffee ring' effect, click here