Hopkins Scientists Probe Atomic Workings of Friction
It consumes as much as 20 percent of the energy used by a car's engine. Yet, scientists have much to learn about this force. They don't know, for example, how all that energy consumed by cars gets converted into heat by friction.
In fact, "our understanding of the molecular origins of this important force is relatively primitive," Mark O. Robbins, a Johns Hopkins professor of physics, said in a scientific paper on the research findings. The paper will be published Aug. 26 in the journal Science. Dr. Robbins led a team of researchers who created a computer simulation that 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 everyday, macroscopic world. They injected krypton, which is a gas at room temperature, into a vacuum chamber. They then cooled the chamber until a single layer of krypton only one atom thick stuck onto the surface of a gold wafer inside the vacuum chamber. The scientists changed the krypton from a liquid to a solid by regulating the number of atoms in the chamber. They used a quartz crystal like the one in a quartz watch to measure the friction when the krypton layer slid over the gold.
Their experiments yielded some startling discoveries: the friction was a thousand times weaker for the one-atom-thick layers of krypton sliding over the gold surface than it was for atoms inside a container of bulk krypton fluid. Scientists also were surprised to find that a solid layer of krypton slides more easily than a liquid layer over the gold surface.
This is just the opposite of what happens with familiar macroscopic objects, where fluids clearly slide with more ease and with less friction than solids.
The computer simulation allowed 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 prefer not to sit right on top of a pingpong ball, but would rather 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.
The simulation's results helped the Hopkins scientists to develop a simple equation to describe 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 agrees with the Hopkins simulations and the Northeastern experiments and could be used to predict how other materials would behave in the same experiment. By learning more about the atomic roots of friction, scientists might one day design superior materials that either reduce or enhance the effects of friction, resulting in more efficient machines.
The other researchers involved in the Hopkins work were visiting associate professor Marek Cieplak, from the Institute of Physics in the Polish Academy of Science, and Hopkins graduate student Elizabeth D. Smith.
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