Thibault Roch (University of Amsterdam)

1) The decrease of the static friction coefficient with interface growth from single to multi-asperity contact.

The key parameter for describing frictional strength at the onset of sliding is the static friction coefficient. Yet, how the static friction coefficient emerges at the macroscale from contacting asperities at the microscale is still an open problem. Here, we conducted friction experiments in which the normal load was varied over more than three orders of magnitude, such that a transition from a single asperity contact at low loads to multi-asperity contacts at high loads was achieved. We find a remarkable reduction in the friction drop (the ratio of the static friction force to the dynamic friction force) with increasing normal load. Using a simple stick-slip transition model we identify the presence of pre-sliding and subcritical contact points as the cause of smaller static friction coefficient at increased normal loads. Our measurements and model bridge the gap between friction behavior commonly observed in atomic force microscopy (AFM) experiments at microscopic forces, and industrially relevant multi-asperity contact interfaces loaded with macroscopic forces.

2) Creep regimes in out-of-steady-state yield stress fluids.

The behavior of simple (non-thixotropic) yield stress fluids depends heavily on the stress they are subjected to. Upon loading, they display a transient creep flow that could be followed by either arrest (solid-like behavior) or fluidization (fluid-like behavior), the outcome being determined by the magnitude of the applied stress compared to the material's static yield stress. Here, we propose a general model for the non-steady-state evolution of non-aged and aged athermal systems around jamming, based on a mesoscopic picture of the underlying physics. We verify some experimentally known scaling regimes, such as the fluidization time, and various creep regimes. In particular, we highlight the existence and the transition between different creep regimes (viscosity dominated creep; hard-particle limit; visco-elastoplastic creep) depending on the system characteristics and the loading procedure, shedding light on the mechanisms at play during the creep of simple yield stress fluids.