Rheological modelling of elongated particles

Flows of elongated particulate granular systems are encountered in many industrial and natural settings. However, in comparison to spheres, the rheological properties of such systems are much less understood. Notably, behaviours not encountered with spherical systems have previously been reported, including non-monotonic jamming and macroscopic friction responses with respect to particle aspect ratio. Moreover, important microscopic effects including the tendency of elongated particles to align preferentially in the flow direction are expected to be crucial for capturing the rheology of such systems. In this work, using the multi-sphere discrete element method, the steady-state rheology of monodisperse elongated particle systems is investigated by performing volume controlled simple shear simulations using the Lees-Edwards boundary condition. Moderately dense to jammed systems of elongated particles with four different aspect ratios and different inter-particle friction coefficients are studied while fixing other primary particle properties. The stress shows different functional dependencies on the shear rate, depending on the solid volume fraction and the shear rate, similar to what is observed for spherical particles. A rheological framework developed for spherical particles, which employs the volume fraction distance to jamming as a key scaling factor, has been extended to accurately predict elongated particle flow behaviour. The key extensions include an aspect-ratio-dependent jamming volume fraction and a lengthscale accounting for particle alignment in the flow direction. This lengthscale can be expressed analytically as a function of simple geometric properties of the particles. Crucially, the new model allows the use of identical parameter values as those used for spherical systems with the same interparticle friction, providing a powerful and convenient tool to predict the flow behaviour of elongated particles based on the calibration to spherical particles.