Fine Points of Friction
by David Pescovitz
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Miquel Salmeron does much of his research at the new Molecular Foundry at Berkeley Lab.
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When we scratch an itch, it's the friction of our nails rubbing against our skin that satisfies us. Of course, friction also enables sandpaper to scrape paint off an old piece of furniture. But what does friction really mean at the atomic scale? UC Berkeley materials science professor Miquel Salmeron is answering that question using the tools of nanoscience. His research could someday help inform the design of tiny micromachines with parts tinier than the period at the end of this sentence or even much smaller.
"It's not until very recently that we have had tools that allow us to see in great detail what happens when bodies are in contact ," says Salmeron, who is also a staff scientist at the Lawrence Berkeley National Laboratory. "Now we can study that contact, almost atom by atom."
Salmeron and his colleagues are masters of the Scanning Tunneling Microscope (STM). Unlike the lens of an optical microscope, an STM has an extremely sharp tip as its probe. The tip is just a single atom across. As the stylus scans across a sample, the amount of electrical current flowing between the tip and surface is measured. This enables a profile of the sample to be generated and visualized with atomic-scale resolution. STMs can also be used to push atoms around into desired structures.
Salmeron uses a scanning tunneling microscope to create aan atomic-scale contour map of the surface of a sample and measure frictional forces. (courtesy Berkeley Lab) [larger image]
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Salmeron wields the STM to study various alloys, such as a mixture of aluminum, nickel and cobalt. As he drags the tip of the STM across the sample material, he measures the force necessary to move the stylus while keeping it in contact with the metal.
"We can then ask questions of the data, like where does the energy that's dissipated in friction actually go?" Salmeron explains. "Did we move atoms around or did we excite them so they vibrated and that's how energy was lost?"
At the macroscale, friction looks very different. If you run your hand across your desk, it looks like the whole surface of your hand is touching the tabletop. At the atomic scale though, a relatively small number of points are actually touching. The STM allows the researchers to examine a single point of contact at a time.
Atoms are spaced periodically in one direction on a surface perpendicular to a quasicrystal's 10-fold rotational axis. But at right angles they are spaced in a Fibonacci sequence, in which the ratio of short to long spacings is an irrational number like that of the Golden Mean. Friction is eight times greater in the periodic direction than in the aperiodic direction. (courtesy Berkeley Lab) [larger image]
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Recently, Salmeron's team published two groundbreaking papers, both in the journal Science, about friction on the nanoscale. The first was a study of friction across a particular kind of crystal. Scratching the surface of the crystal in one direction with the STM tip exhibited a huge difference in frictional force compared to moving across it in the opposite direction. The experiment provides insight into how atomic structures of material affect the dissipation of frictional energy. The second paper showed how friction in seminconductors, the stuff computer chips are made from, can be controlled by applying voltages.
A better understanding of friction is essential to fully realize the promise of MEMS, tiny devices like sensors and actuators that are fabricated using processes similar to the way integrated circuits are manufactured. MEMS are considered by some to be "training wheels" for nanoscale machines. (A nanometer is a billionth of a meter).
"When you have all of these tiny moving parts, friction becomes a major problem," Salmeron says. "Friction, adhesion and other related phenomena are much more important in the micro- or nano-world than in the macro-world."
Salmeron looks forward to his colleagues building micro- and nano-machines that will put his concepts to the test. Meanwhile, he's exploring how materials can be altered to react in different ways to friction. For example, can a specific material be made to produce light at the friction point of contact?
"I have a deep curiosity about this phenomena that we usually only see at the macroscopic scale," he says. "I want to really know what happens when atoms come together."
Miquel Salmeron's home page at Berkeley Lab
The Salmeron Group
"Of Friction and 'The Da Vinci Code'" by Lynn Yarris (Berkeley Lab, August 25, 2005)
"Did You Ever Wonder? Scientist profile: Miquel Salmeron" (Berkeley Lab)
Molecular Foundry
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