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Geologic Column

Coffee, Tea or Phi?
Lisa Rossbacher

Geologists run on coffee. It’s fuel — better then any fossil fuel — for getting out of bed in the morning. Just ask the Beloit College Geology Department. Its Web site proudly announces: "Coffee: The life blood of the geology department. Coffee maker: The life regulator of the Geology Department."

When students declare that they are going to major in geology at Beloit, they receive a personalized coffee mug. When a geology major graduates, the year is painted on the bottom, and the mug itself is archived for future return visits.

Many geologists lug a thermos of coffee with them in the field. Others make it themselves on the spot. Small, single-serving French presses make this easier — and tastier — than it used to be, but the rituals of having a "coffee break" to review data, check the maps and consider the next steps can be an important element of field work.

When we were doing field work in the Swedish Lapland, Dallas and I learned that our Scandinavian colleagues required official coffee breaks in the morning and afternoon: Everything stopped while we boiled water, fixed the coffee from scratch and drank it. Only then could the field work continue.

Coffee as an earth material

Coffee not only fuels geologists, it also fuels the educational process of teaching major concepts in geology. Coffee can be treated as an earth material. It’s a favorite substance, for example, to illustrate angle of repose, including the relationship between size of the grains (the phi of the title) and slope angle. Generations of students have learned about the differences between angles of repose for finely ground coffee, coarsely ground coffee and whole beans. A master’s thesis at the University of Georgia focused on determining "the effect of moisture content on angle of internal friction and angle of repose of coffee beans." It’s a great material for modeling.

Preparing coffee is all about leaching, which is a critical topic in hydrologic discussions. Whether the process is filtering, percolating or pressing, the leaching process applies. Water percolating through coffee grounds can model the origin of many features in caves and karst regions.

Back in the old days, making coffee in the field meant using a container of cheap instant coffee. When geologists carried this coffee during work at high elevations, the unwary learned what pressure differential really means. A jar that was sealed at low elevation and opened at 10,000 feet offered a graphic lesson to those who failed to heed the warning to stick an ice pick in the lid before opening the jar. The positive pressure in the jar blew out the instant coffee as soon as the seal was broken, usually in the face of the person opening it. The phrase "raccoon eyes" comes to mind as a description of the result.

What we still don’t know

Recent research into the behavior of materials has helped us understand how much we still don’t know about a material as familiar as coffee. Just think about the transition that a "brick" of coffee undergoes when the seal is broken. In a vacuum-packed bag, the irregular coffee grains are pressed together, closely interlocking their jagged edges and creating a rigid block. When the pressure is released after opening the bag, the coffee grains flow easily.

Yet physicists don’t understand the theory enough to tell Starbucks, Peet’s or Caribou Coffee exactly how rigid a bag of vacuum-packed coffee should be. Nor can they predict the exact moment at which the ground coffee will turn from a solid to powder.

The complexities of coffee-grain behavior seem to be a function of the interaction of the individual beans or grounds, which creates a system that is extremely difficult to model. A single bag of coffee produces enough granular interactions to swamp a supercomputer.

Examples of granular flows on Earth include dune formation and migration, as well as avalanches of rock and granular snow. Images from the surfaces of other planetary bodies — such as the moon and Mars — indicate landforms that were created by granular flow.

One way to simplify granular flow processes, as a way to understand the system and predict its behavior, is to eliminate some of the variables. Gravity, for example, can be removed from the equation through experiments in space. A series of experiments on "mechanics of granular materials" (MGM) were included on space shuttle missions STS-79 and STS-89.

An MGM experiment was also part of the science package on STS-107, the final flight of Columbia. The experiment studied the behavior of saturated sand when exposed to confining pressure; some of the goals were to learn more about how to strengthen building foundations against earthquakes and to better understand how vibrations affect grains of soil and sand. The MGM project scientists have estimated that, with careful analysis of data returned during the Columbia mission, they have achieved 50 to 60 percent of the scientific goals of this experiment. Similar experiments are planned for the International Space Station in 2007.

James Jenkins, a physics professor at Cornell University, studies granular materials for NASA. In an interview with NASA Science News (Dec. 4, 2002), he noted some of the economic value of understanding such a process: "Flows of granular materials that resemble avalanches are important in coal-fired power plants, in the manufacture of pharmaceuticals, in the processing of aluminum, 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."

And as the coffee flows, so too do the geologic ideas. Learning more about the behavior of coffee can help us make sense of a wide range of behaviors of other earth materials. In addition, this knowledge will help us understand more about other material behavior as well, including everything from breakfast cereal being poured into a bowl to the balls that generate winning lottery numbers. Right now, I’m staring into my coffee cup looking for a little inspiration.


Rossbacher, a geologist, is president of the Southern Polytechnic State University in Marietta, Ga.

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