Abstract
In granular physics experiments, it is a persistent challenge to obtain the boundary stress measurements necessary to provide full a rheological characterization of the dynamics. Here, we describe a new technique by which the outer boundary of a 2D Couette cell both confines the granular material and provides spatially- and temporally- resolved stress measurements. This key advance is enabled by desktop laser-cutting technology, which allows us to design and cut linearly-deformable walls with a specified spring constant. By tracking the position of each segment of the wall, we measure both the normal and tangential stress throughout the experiment. This permits us to calculate the amount of shear stress provided by basal friction, and thereby determine accurate values of $\mu(I)$.
Highlights
It is an open question what constitutive equations best describe flows of dense cohesionless granular materials [1,2,3]
We develop our technique using a standard annular Couette geometry, which has the advantage of allowing continuous shear of a granular material to arbitrary total strain
The apparatus consists of a rotating inner disk (Ri = 15 cm) and a fixed outer wall (Ro = 28 cm) made up of 52 leaf springs which can dilate slightly and thereby provide a stress measurements at the outer wall
Summary
It is an open question what constitutive equations best describe flows of dense cohesionless granular materials [1,2,3]. There has come to be a consensus that two dimensionless parameters play a key role: interial number I and the friction μ. Each of these can be defined at the particle scale. The inertial number is given by I ≡ |γ|d (1). P/ρ where ρ is density of the solid granular material, d is their diameter, γis the local shear rate, and P is the local pressure. Higher values of I correspond to rapid flows, and lower values to slower flows.
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