Mystery of force of friction solved

Physicists in the U.S. and China could solve the puzzle of the negative coefficient of friction. The name refers to the effect created during work of an atomic force microscope with a graphite surface with the thickness of the size of a single atom. When the pressure of the scanner on the surface weakens, the friction force does not decrease, but rather, increases.

Clearly, the greater is the load on the surface, the stronger is friction. For example, when we slightly press a finger on the table and trace it on its surface, the finger slides easily. But if we increase the pressure, it becomes more difficult. However, what is common to the macrocosm may appear differently in the microcosm. This applies to the aforementioned friction.

Scientists have long known one paradox - if a cantilever of an atomic force microscope (a mechanical microprobe that scans the surface) is pressed against a graphite surface, the effect will be the opposite to what would have been observed when pressing a finger against the table. Why is this happening?

Everyone who worked with atomic microscopes knows that friction can vary from time to time to the extent that the cantilever moves over the surface of the crystal lattice. The coefficient of friction (i.e., the rate of change of the force depending on the pressure on the surface) may vary non-linearly, however, the decrease of friction with the increase in pressure, or, in more scientific terms, a situation with a negative coefficient of friction, has been never observed before.

For a long time, physicists did not understand why this is the case. Recently, however, researchers from the National Institute of Standards and Technology (USA) and Tsinghua University (China) Cannara Rachel and Dan Zhao were able to solve the puzzle.  

Dan Zhao was measuring the coefficient of friction between the diamond part of the cantilever tip of the microscope and the graphite surface. This is a common procedure commonly conducted by newbies and graduate students. Performing the routine, Mr. Dan suddenly found that when the pressure increases, the friction also increases at first, but with the following decrease of pressure it is not reduced. Conversely, when the pressure decreases, friction continues to increase. Moreover, Dan Zhao recorded that after the first measurement the increase of friction was systematic. It continued to grow as the pressure was decreasing, up until the complete separation of the cantilever tip and graphite.  

Unable to understand what was happening, the researcher turned to his senior colleagues who, on reflection, have put forward an interesting hypothesis. It has long been known that the thinner is the graphite layer, the higher is the friction on the surface. Therefore, it is possible that the researcher had to deal with a layer thickness of only a few atoms.

At lower pressure the tip of the cantilever rises slightly. However, since the atoms have already stuck to it, it turns out that when moving, the cantilever drags graphite behind. It resembles sticking of wet snow to sled runners - the further they go, the more snow sticks to the runners. It is obvious that in such a situation, the friction will grow, because the tip is dragging increasingly more graphite atoms that stuck to it. Only when the tip is removed completely, graphite returns to its original position.

The researchers tested this theory. They built a computer model that shows how this happens. It became clear that this pattern is not unique to graphite. In principle, it is valid for all materials forming layers whose thickness is comparable to the size of an atom. It turns out that if you first apply hard pressure on a surface, and then reduce the pressure, friction will always increase as the atoms that stuck to the surface of the device will slow its movement.

Interestingly, this discovery seems to be purely theoretical, but it can actually be used in the design of MEMS devices where minimization of friction is especially important. With sizes that small, friction magnitude compared with the forces in the mechanism increases and eventually becomes very significant. However, unlike in macro-mechanisms, it cannot be reduced by conventional lubricants. Therefore, in order to reduce friction, completely different measures are required.          

Before suggesting a way to combat malicious friction, one needs to understand how this power works at such a small scale. The discovery of Mr. Deng and his colleagues is a great breakthrough in this field of research. Now at least it is clear that nothing depends on the level of pressure.

Anton Evseev

Pravda.Ru 

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