Scientists Fight Friction Down to the Last Atom

MILWAUKEE – In the field of nanotechnology, where devices are one-billionth the size of everyday objects, friction _ the resistant force of two objects rubbing together – is preventing some of these minuscule devices from being reliable enough to enter the commercial market.

Two new studies in the journal Science may get scientists closer to working around the problem of friction. "Nanotechnology is a vast and multidisciplinary area," said Robert Carpick, a physicist at the University of Wisconsin-Madison, "and for certain technologies, friction needs to be overcome."

Researchers in an area called nanotribology study the problem of reducing friction in micro- and nano-scaled electronic devices such as computer chips, microphones and drug-releasing mechanisms. Tribology is the science and technology of friction, lubrication and wear.

The studies outline a new method for reducing friction in micro- and nano-scale devices. Both approaches are unique because they physically alter the existing system – the pair of surfaces involved – rather than adding a chemical to act as a lubricant. In the first study, friction was reduced by applying a voltage to the system. One of the surfaces involved was made of silicon. When an electric charge is applied to silicon, the electrons become either excited or subdued, depending on whether the charge is positive or negative.

Friction creates heat, and electrons are involved in the transfer of heat. So by altering the electrons, scientists can increase or decrease the amount of friction, said Miguel Salmeron, physicist at Lawrence Berkeley National Lab and one of the authors of the paper.

The second study, by Swiss physicist Anisoara Socoliuc, applied mechanical vibrations to the system. "This process essentially reduces the total time the surfaces are in contact," said Michael Dugger, a scientist at Sandia National Labs in New Mexico who was not involved in the research. Carpick described these two results as "clear and convincing science" that friction can be reduced on this scale, although, he said, they may not be able to reduce friction in more complicated nano-devices with more than one point of contact.

Micro- and nano-sized objects are too small for the eye to see. A strand of human hair is approximately 50 micrometers or 50,000 nanometers wide. Objects that measure in the micro- or nanometers are more vulnerable to friction because they have a much higher ratio of surface area to volume than macro-sized objects. That is, the majority of atoms in a micro- or nano-sized object are on the surface, whereas the majority of atoms in a macro-scale object are in the bulk of the material. This high ratio means that surface forces such as friction and adhesion are more significant than other forces, such as gravity, which depend on mass, Carpick said.

Nano-devices are composed of only a handful of atoms, so they wear down quickly or melt from the heat generated by friction. Problems like this have prevented some micro- and nano-machines from becoming commercially practical. Engineers can currently build numerous micro-machines, including gears, motors and accelerometers, but the only machines being commercially deployed are those with no sliding surfaces.

In macro-scale machines, lubricants are used to reduce friction. But lubricants fail on a smaller scale because on a molecular level liquids are viscous, or sticky. This viscosity slows down the nano-scale devices. Researchers hope that nanotribology research will also provide insight into friction on the macro-scale. Under magnification, most surfaces are rough. This means that macro-scale friction is actually nano-scale friction occurring simultaneously at many points.

If researchers can mimic lots of nano-sized contact spots, then they may be able to propagate these results up to the macro-scale, according to Wilfred Tysoe, physical chemist at the University of Wisconsin-Milwaukee and co-editor of the journal Tribology Letters.

Understanding friction on the nano-scale may be important not only for eventual applications, but also for basic science. "Everyone knows about friction," said Carpick, "but we do not have a fundamental theorem of friction, and cannot predict friction between two surfaces."