“The understanding and control of friction has been a great challenge for a long time,” Anisoara Socoliuc of the University of Basel told nanotechweb.org. “As the scale of devices is continuously reduced, the frictional forces become more and more important. Our work shows that friction experienced on a nano-asperity is drastically reduced, and even suppressed, if the applied load is below a certain threshold.”
To study the effect, Socoliuc and colleagues used a home-built atomic-force microscope with high mechanical stability and an optimized signal-to-noise ratio. They claim this enabled them to measure friction with an unprecedented resolution, especially at small loads. The scientists dragged the tip of the microscope’s silicon cantilever back and forth across the surface of single crystals of sodium chloride.
In the stick-slip regime, a plot of lateral force against position showed two sawtooth profiles, one for the forward motion and one for backwards motion. The sawtooth modulation had the periodicity of the crystal lattice. The forwards and backwards sawtooth profiles were mismatched, enclosing a hysteresis loop that showed the energy dissipated in one cycle. Lowering the externally applied load on the tip decreased the amount of energy dissipated. What’s more, further reduction of the applied load caused the hysteresis loop and dissipation to disappear completely, within the limits of the researchers’ measurements. The sawtooth modulation also became continuous.
“These optimized conditions lead to the observation of the transition from a dissipative regime characterized by stick-slip motion to an ultralow friction regime, when the tip slides continuously over the underlying surface,” said Socoliuc. “Indeed, the stick-slip effect can be suppressed if the normal load applied on the tip is reduced below a certain limit. In that moment the tip follows or overcomes the support driving it.”
The researchers will report their work in Physical Review Letters.