The physics behind coffee rings

WASHINGTON - Usually the formation of coffee rings on a table doesn’t prompt a response further than ‘Use a coaster’, but a team of researchers have gone beyond and explored the physics behind the stain patterns left behind by coffee droplets on a surface.

The potential applications of their study include printing, making industrial coatings, fabricating electronics, and designing new medicines.

Shreyas Mandre of Brown University, Ning Wu from Colorado School of Mines and L. Mahadevan and Joanna Aizenberg from Harvard University have devised a predictive model that combines laboratory studies of microscopic glass particles in solution with mathematical theories to predict the existence, thickness and length of the banded ring patterns that formed.

Their results suggest the patterned deposition of particles can be controlled by altering physical parameters such as evaporation and surface tension - and perhaps one day manipulated to create small-particle tools.

“Controlling the ring deposition process would be useful for creating such things as new microphysics tools operating at a scale where pliers or other traditional tools for moving particles cannot operate,” noted Mandre.

The team found that during ring deposition, a particle layer of uniform thickness is deposited if the concentration is above a certain threshold. Below that threshold the deposits form non-uniform bands.

The threshold is formed because evaporation at the solid-liquid interface of the rim occurs faster than a replenishing flow of water from the center of the droplet can replace the evaporating rim fluid. This leaves the particles on the rim high, dry — and deposited.

Exploiting this competition between evaporation and replenishment is the key to controlling the process as a microtool, said Mandre. (ANI)