September 24, 2013 | 11
Friction is both the boon and the bane of our everyday lives. It’s the force that drags against your car’s tires, making you use more gas to keep going. It’s also the force that allows your car to stop at all: Without friction, brakes would be dead weight. Although most of us take friction for granted when we hit the stop pedal, many of its details are still a mystery.
A recent study helped broaden our understanding by probing the dynamics of friction an atom at a time. Using a technique called atomic force microscopy, scientists measured how the orientation of single atoms plays a crucial role in determining the strength of frictional forces between two materials. “We know that friction has a directional dependence—it’s easier to pet a cat in one direction than another,” says Jay Weymouth at the University of Regensburg in Germany. “But we were able to measure clearly the directional dependence at the atomic level, and fully describe it.”
Friction is the force working against the sliding of one object against another, and generally increases with the force pushing the surfaces together. Even seemingly smooth surfaces, such as a wooden table, are really jagged and rough on a microscopic scale, and all these tiny outcroppings on one surface drag and catch the grooves on any other being slid across it. Atomic bonds break and form and break again. And this movement sets the atoms oscillating, which, in turn, generates heat. All of this costs the system energy, which results in the motion between the surfaces slowing. When you walk along a sidewalk, molecules in the rubber soles catch on the molecules in the cement, resisting your motion. Without this resistance, you wouldn’t be able to walk at all—your shoes would slide backward with every step you took, as if you were imitating Michael Jackson’s Moon Walk.
While this basic description generally holds true, even for Michael Jackson, many subtleties of friction remain mysterious. “We’ve got these nice little high school formulas that work in a lot of basic cases,” Weymouth says. “But we’re very bad at taking two arbitrary surfaces and saying what the friction will be.” That’s why Weymouth and his colleagues aimed to get a better understanding of what happens to single atoms when one brushes up against another. “When you rub your hands against each other and feel this frictional force, you’re feeling the interaction between millions of atoms,” Weymouth says. “But we’re trying to simplify it down to the smallest physical system we can really reasonably measure—that’s one atom against another.”
The researchers studied the case of a tiny tungsten tip coated with a thin layer of silicon sliding against a surface of pure silicon crystal coated with hydrogen. The crystalline silicon took the form of pairs of atoms, and as the tip slid over them, the pairs rocked back and forth. The energy expended by this rocking motion can be thought of as the amount of friction between the materials. The researchers found that friction depended strongly on whether the tip was dragged along the direction in which the pairs were oriented, or whether it was swept in the perpendicular direction.
“Experimentally this has not been done before,” Weymouth says. “People have probed the directional dependence of friction, but never on a single atom level.” The findings are detailed in a paper published September 18 in Physical Review Letters.
Beyond adding to the overall picture of how friction works on a fundamental level, the research could help improve the efficiency of all sorts of machines that lose energy to friction. “Such microscopic insights may help eventually move friction beyond previous empirical approaches to a point where, through bottom-up understanding of how friction originates, the properties of materials in sliding contact can eventually be predicted and controlled,” Philip Egberts and Robert W. Carpick at the University of Pennsylvania, who weren’t involved in the study, wrote in an essay accompanying the paper published by the American Physical Society. “A better understanding of friction will benefit applications ranging from the study of geological faults to the prediction of wear in automobile components or microelectromechanical systems.”