Force fluctuations at the transition from quasi-static to inertial granular flow

• Published in: Soft matter 2019-10, Vol.15 (42), p.8532-8542
• Main Authors: Thomas, A. L, Tang, Zhu, Daniels, Karen E, Vriend, N. M
• Format: Article
• Language: English
• Subjects: Avalanches; Fluctuations; Fluidity; Force measurement; Mathematical analysis; Particle tracking; Rheological properties; Rheology; Shear rate; Stress; Stress ratio; Tensors; Two dimensional flow
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c9sm01111k

We analyse the rheology of gravity-driven, dry granular flows in experiments where individual forces within the flow bulk are measured. We release photoelastic discs at the top of an incline to create a quasi-static erodible bed over which flows a steady 2D avalanche. The flowing layers we produce are dense ( 0.8), thin ( h 10 d ), and in the slow to intermediate flow regime ( I = 0.1 to 1). Using particle tracking and photoelastic force measurements we report coarse-grained profiles for packing fraction, velocity, shear rate, inertial number, and stress tensor components. In addition, we define a quantitative measure for the rate of generation of new force chain networks and we observe that fluctuations extend below the boundary between dense flow and quasi-static layers. Finally, we evaluate several existing definitions for granular fluidity, and make comparisons among them and the behaviour of our experimentally-measured stress tensor components. Our measurements of the non-dimensional stress ratio μ show that our experiments lie within the local rheological regime, yet we observe rearrangements of the force network extending into the quasi-static layer where shear rates vanish. This elucidates why non-local rheological models rely on the notion of stress diffusion, and we thus propose non-local effects may in fact be dependent on the local force network fluctuation rate. We analyse the rheology of gravity-driven, dry granular flows in experiments where individual forces within the flow bulk are measured.