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Coffee Physics

Here's something to ponder over your next cup of coffee. The physics of a humble bag of ground coffee still holds surprises for scientists...

by Patrick L. Barry and Dr. Tony Phillips

Perhaps you wonder about it - just for a moment - each time you open a fresh brick of vacuum-packed coffee to make that coveted morning mug.

Remarkable, you might think to yourself, how hard and strong the brick is before the "ssss" sound of cutting it open, and how quickly it becomes soft and pliable afterward. It's as if the coffee grounds themselves have transformed: A solid one moment, a powder the next.

Why does this happen?

Coffee grains have jagged irregular shapes. (Look at some through a magnifying glass and you'll see.) In a vacuum-packed bag, the pressure exerted inward by the atmosphere squeezes the coffee grounds from all sides; their odd shapes interlock to help hold them in place. Because each particle fiercely resists motion, the brick of coffee as a whole will be rigid. When the bag is opened and the pressure relaxes, the coffee grounds can tumble and flow like a powder.

Simple. Yet physicists cannot predict from theory exactly how hard a vacuum-packed bag of coffee should be € or when it will change from a solid to a powder.

There's no mystery to an individual coffee ground. We can readily determine its chemical composition, its jagged shape, its density, its crystal structure, and so on. Individual grains are not the problem. It's the millions of individual grains rubbing together that are so hard to predict.

Coffee is an example of a granular material - substances that are as common as the sand beneath your feet, but which have no complete physical theory to describe their behaviour. NASA is interested in granular materials for several reasons.

Credit Nasa

The towering Dhaulagiri Range of the Himalayas in central Nepal as shown here in a photo from the International Space Station. Like all mountain ranges; at times they will experience avalanches, which show a more devestating example of the granular effect.

"It's likely that large amounts of granular materials will have to be processed in order to provide oxygen and fuel for humans on Mars and the Moon," explains physics professor James Jenkins, a researcher at Cornell University. "Also granular flows are important in the formation of geological features such as dunes and avalanche deposits seen on distant planets and moons. A better understanding of grain flow could provide an indication of the conditions under which those features were formed." Planetary rings are granular, too, and astronomers would like to understand them better as well.

"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.

NASA-supported researchers are working to develop such a theory through a combination of experimentation and mathematics. Jenkins, for example, is studying differential equations that describe molecular gases. It might be possible, he says, to adapt them for granular flows. He plans to test some of his ideas using a rotating chamber filled with beads; the device is slated for launch to the International Space Station (ISS) in 2007.

A sand column is compressed during an MGM experiment onboard shuttle flight STS-79. The speed of the movie is misleading; the complete sequence takes about an hour.

"We do this on the ISS," he explains, "because granular flows are affected by both gravity and internal collisions. We need to get Earth's gravity out of the picture to create a simpler system."

For the same reason, engineering professor Stein Sture of the University of Colorado is leading a series of experiments called "Mechanics of Granular Materials" (MGM) onboard the space shuttle. His device repeatedly squeezes a column of damp sand and records what happens. The goal, explains Sture, is to understand the liquid-like behaviour. of soil during some earthquakes. MGM has flown before on shuttle missions STS-79 and STS-89, and it's slated for another flight in 2003 onboard the shuttle Columbia (STS-107).

Earthquakes, avalanches, planetary rings, coal mines ... even bags of coffee. From the alien to the ordinary, we'll understand them all a little better when this research is done.


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First Science 2014