What is so interesting about the dark ring that a drop of coffee
leaves behind ?
Wong: Coffee rings reveal the dissolved particles present in a
liquid-because these particles gather along the edge of the ring. The way
they do so depends on how quickly the liquid evaporates and water kind of
dissolved particles it contains.
What lessons can be learnt for other liquids from coffee rings ?
Yunker: When coffee evaporates, water and dissolved particles
flow from the centre to the edge. Since the edge of the drop is 'pinned' to the
surface of the table, it cannot hold together at the centre. This increased
concentration of particle along the rim of a drop affects many liquids- and
represents a big problem when a medium is supposed to spread uniformly, for
instance printer ink on paper. But now we have finally figured out how to deal
with this effect.
How did you manage this ?
Yunker: By changing the shape of the particles dissolved in a
liquid. In our tests we established that when the particles were spherical in
shape, the ring effect was evident. But when the particles were elongated, they
deformed the surface of the drop, creating forces of attraction between the
particles. This resulted in the formation of blobs that could resist the flow
to the edge and, in turn, the particles got deposited evenly.
So what is the next step ?
Wong: we can use this insight , for instance, to develop tiny
lab-on-a-chip device to establish the presence of dissolved substances in
blood, a good technique that can improve medical care in poor countries.
Any other applications that are conceivable ?
In micro technology, if we manipulate the evaporation of liquids
with dissolved metal particles, the rings will form tiny structures along their
edges that are very good conductors of electricity.
