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Frictional unjamming probed with a transverse shear ultrasonic oscillator
Friction between solids depends essentially on the response of the
interfacial amorphous layer to shear and compressive stresses. Hence,
the transition from static to dynamic friction corresponds to the
unjamming transition of confined amorphous materials [1]. With a shear
ultrasonic oscillator, we study the boundary lubrication due to
molecular films confined between a plane and a sphere [2]. We observe
a linear viscoelastic behaviour at low oscillation amplitude and a
nonlinear frictional microslip regime at high amplitude. In an
additionnal set of experiments, the system is brought near the
unjamming transition by applying a static force on which we superpose
the oscillatory displacement. The interfacial layer softens before
unjamming, as indicated by the linear response of the oscillator. We
suggest an interpretation based on a stress-induced increase of the
free volume, and propose a corresponding heuristic model. Last, we
show how non linear in-plane shear oscillations of 10 nm amplitude
can trigger unjamming, and we discuss the possible related mechanisms.
[1] P. Thomson and G. Grest and M.O. Robbins, Phys. Rev. Lett 68, 3448 (1992)
[2] J. Léopoldès and X. Jia, Phys. Rev. Lett 105, 266101 (2010)
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Contact : michael.schindler@espci.fr