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.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.In a bag of coffee, howee 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!