Increasing ‘ease of sliding’ also increases friction: when is a lubricant effective?

Abstract

We investigate experimentally the effective Coulomb friction exerted by a granular medium on a shearing plate, varying the medium depth. The plate is driven by a spring connected to a motor turning at a constant speed and, depending on the system configuration, performs continuous sliding or stick and slip in different proportions. We introduce an order parameter which discriminates between the different regimes expressing the fraction of time spent in slipping. At low driving speed, starting from zero layers of interstitial granular material, the average friction coefficient decreases when a few layers are added, while the order parameter stays close to zero. By further increasing the granular depth, the friction undergoes a sudden increase but the order parameter does not change notably. At an intermediate driving speed, however, both the friction and the order parameter undergo a sudden increase, which for the order parameter amounts to several orders of magnitude, indicating that the plate is more braked but nevertheless keeps sliding more easily. For medium-high driving speeds, full sliding is obtained for only one layer of interstitial matter, where friction has a minimum, and is maintained for all increasing depths while friction increases. These observations show that the ease of slipping is not determined by friction alone, rather by the highly complex interplay between driving velocity, friction, and the depth of the medium.

Universal Excursion and Bridge shapes in ABBM/CIR/Bessel processes

“…The original ABBM model is recovered in the over-damped limit of small inertia I → 0. An ABBM model with inertia has been studied in [41] and previously introduced [42] to sucessfully reproduce the statistics of stick-slip dynamics in granular friction experiments [43][44][45][46], for which the average avalanche shape has been also measured and discussed [25]. Different inertial terms have also been considered [47,48].…”

Preface to the special section on nano- and mesoscale friction

“…Consequently, understanding friction entails a multi-scale effort. In this special section, the collective dynamics of an ensemble of macroscopic grains under shear are investigated [1]. Possible relations to lubrication of systems at smaller scales are raised.…”

Dynamical modes of sheared confined microscale matter

“…9(a). We attribute this to the fact that the strength of the induced dipole-dipole interaction is proportional to the magnetic torque, see eqn (4). This results in a stronger attraction for larger magnetic torques.…”