Abstract
Powders or cohesive granular materials are widely handled in industries. However, our understanding of the rheology of these materials is limited. Here, we provide a comprehensive analysis of the rheology of a cohesive granular medium, sheared in a normal-stress-imposed plane shear cell over a wide range of shear rate, employing numerical simulations. At high imposed shear rates, the flow is homogeneous, and the rheology is well described by the existing scaling laws, involving the inertial number and the “effective” cohesion number [S. Mandal et al., Insights into the Rheology of Cohesive Granular Media, Proc. Natl. Acad. Sci. U.S.A. 117, 8366 (2020)]. However, at low imposed shear rates, the flow is inhomogeneous, exhibiting the coexistence of flowing and nonflowing regions in the material, popularly known as shear banding. We thoroughly analyze the crucial features of this shear-banded flow regime and discuss striking similarities between the shear banding for granular media and other complex fluids. We reveal that the occurrence of shear banding is related to the existence of a nonmonotonic intrinsic rheological curve and that increasing adhesion increases the nonmonotonicity and the tendency toward shear localization. A simple theoretical model based on a nonlocal rheological model coupled with a nonmonotonic flow curve is proposed and is shown to successfully reproduce all the key features of the shear banding observed in the numerical simulations. The results have important implications for the handling of powders in industries.
- Received 3 November 2020
- Revised 25 February 2021
- Accepted 15 March 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021017
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
From our daily experience, we realize that flour flows less easily than sugar because of adhesion between the grains. Characterization of the “flowability” of powders such as flour is of paramount importance in many industries that handle powders or cohesive granular materials. However, it remains challenging because we lack a detailed physical understanding of the flow behavior of powders. Toward that end, we provide a detailed numerical investigation into how a model cohesive granular material flows when sheared between two rough plates.
We show that when the material is slowly sheared, it exhibits shear banding—the coexistence of flowing and nonflowing regions. Although shear banding is widely studied in a broad class of complex fluids, it has remained relatively unexplored in granular media. We present an in-depth analysis of this shear banding, showing rich behavior such as hysteresis and a dependence on system size. We find that the source of shear banding is the existence of a regime where the stresses decrease when increasing the shear velocity (contrary to what is observed for simple fluids), leading to flow instability and heterogeneity in the flow.
These results may help researchers understand the jamming and clogging of powders in industrial applications and may also suggest a better way to characterize the flowability of powders.
