Coffee Physics

"Granular flows are ubiquitous on Earth," adds Jenkins. "Avalanches of rock and granular snow are two examples. Flows of granular materials that resemble avalanches are important in coal-fired power plants, in the manufacture of pharmaceuticals, in the processing of aluminium, and in the production of plastics from pellets. It's hard to think of an industry that does not employ a granular flow during some processing operation."

Unfortunately, the physics of granular materials doesn't boil down to simple equations as easily as some other phenomena. The helium in a balloon, for example, is also made of many millions - in fact, billions of trillions - of particles. Yet one simple equation governs all of its important traits: pressure, volume and temperature. (Remember "PV=nRT" from high school physics?)

The difference is that the helium atoms are widely separated (on a molecular scale). One helium atom is mostly identical to any other. There are no irregular edges or complicated atom-to-atom interactions. It really is simple.

Image courtesy NASA. Electron micrographs of irregularly-shaped sand grains.

In a bag of coffee, however, the grounds bump, rub, and press against each other. Each grain is unique and it interacts strongly with its neighbours. Because these interactions can't reasonably be ignored, the coffee must be considered as more than just the sum of its parts. Instead, it is the sum of its parts plus their interactions!

Computers are ideal for solving such problems, but there's a snag: There are enough interactions in a single bag of coffee to overwhelm a supercomputer.

When scientists and engineers need to deal with granular materials like soils and powders, they usually approach the problem empirically - that is, they measure how the material behaves in real life and make predictions accordingly. But the empirical approach is limited to things easily measured. Some things aren't. For example, what triggers avalanches on the Moon? How much soil can flow down a chute on Mars? Or, right here on Earth, what happens to damp sand underneath a building during an earthquake? To answer such questions we need a theory, a "PV=nRT" for granular flows, that can make predictions under a wide range of circumstances.