FRICTION: TEAM TRACKS FUNDAMENTAL FORCE By Emil Venere Friction plays a key role in everyday life. Without it, people wouldn't be able to walk, cars wouldn't stay parked on hilly grades, and buildings would fall down. At the same time, friction is a serious handicap. It consumes as much as 20 percent of the energy used by an internal combustion engine and causes engine parts to wear out. If this fundamental force can be mastered, scientists might one day create substances that reduce or enhance the effects of friction, resulting in superior materials and more efficient machines. But researchers have a lot to learn about the workings of friction before that day is here. They don't know, for example, how all the energy consumed by cars gets converted into heat by friction. "Our understanding of the molecular origins of this important force is relatively primitive," Mark O. Robbins, a Hopkins professor of physics, wrote in the Aug. 26 issue of Science. Dr. Robbins worked with Hopkins graduate student Elizabeth D. Smith and visiting associate professor Marek Cieplak, from the Institute of Physics in the Polish Academy of Science, to create a computer model that sheds some light on the mechanics of friction at the atomic level. Their simulation reproduced the surprising results of recent experiments conducted at Northeastern University. The Northeastern scientists found that friction behaves much differently at the microscopic level than it does in the familiar, macroscopic world. They injected krypton, which is a gas at room temperature, into a vacuum chamber. The researchers then cooled the chamber until a single layer of krypton only one atom thick stuck to the surface of a gold wafer inside the vacuum chamber. They were able to change the krypton from a liquid to a solid by regulating the number of atoms in the chamber, and they used a quartz crystal like the one in a wristwatch to measure the friction produced when the krypton layer slid over the gold. Their experiments yielded a startling discovery: a solid layer of krypton slides more easily than a liquid layer over the gold surface. This is just the opposite of what happens in the macroscopic realm of everyday existence, where fluids clearly slide with more ease and with less friction than solids. But a closeup view, via the computer simulation, enabled the Hopkins scientists to actually follow the motion of krypton atoms. To explain why solids slide more readily than fluids over the gold surface, Dr. Robbins likens the gold to a layer of pingpong balls on a table and the krypton atoms to tennis balls. The tennis balls would not sit right on top of a pingpong ball, but slide down into the spaces between several pingpong balls. When the krypton is fluid, its atoms (tennis balls) have more freedom to move into the spaces. But when it is solid, the krypton atoms are pressed together so tightly that they can't slide sideways enough to find the nearest hole to fall into. Therefore, the gold doesn't grab onto the solid layer as well as the fluid layer, so the solid layer slides with more ease and requires less frictional force than the fluid layer, Dr. Robbins said. The simulation helped scientists develop a simple equation describing the amount of friction involved in the experiment. One factor is how readily krypton atoms fall into "holes" in the gold surface. When sliding krypton atoms fall into the holes, vibrations or sound waves are excited in the layer of krypton. The more the atoms fall into the holes, the greater the vibrations. The second factor is how rapidly these molecular vibrations decay into heat. A more rapid decay leads to higher friction. The equation reproduces the results of the Hopkins simulation and the Northeastern experiments and could be used to predict how other materials might behave in the same experiment, Dr. Robbins said.
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