The Role of Particle Shape in Granular Rheology and Jamming PDF Download

Author: Theodore A. Marschall
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Languages : en
Pages : 243

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Book Description
"Granular materials, non-Brownian suspension, emulsions, and foams are a class of 'soft matter' systems where the dominant interaction between particles arises from direct contact and the particles are sufficiently macroscopic so that thermal fluctuations can be ignored. All of these systems undergo a similar transition from a flowing phase to a disordered solid phase known as the jamming transition, which has been found to share many characteristics with phase transitions in thermalized systems. Most work on granular media has focused on the idealized case of spherical particles. This work focuses on systems with elongated convex grains and several nonconvex grain shapes in two dimensions, using a simple model of frictionless particles in a viscous background medium. These systems are compressed isotropically and sheared continuously over a wide range of densities and strain rates, with a particular focus on the rotational degrees of freedom of anisotropic particles. We find that the jamming transition in systems with anisotropic particles is broadly similar to that of circular disks, though in several ways the limit of circular disks is singular. We identify a nonuniform distribution of contacts on the surfaces of anisotropic particles, both when compressed and sheared, with a peak that diverges in the circular limit. We find that systems of convex particles are hypostatic at jamming, having fewer constraints from contacts than internal degrees of freedom. This leads to modes of motion with zero cost to energy at jamming, but only for small displacements, so that the system remains rigid against macroscopic perturbations. In particular, we identify sliding modes for particles which have flat regions on their surfaces. When sheared, anisotropic particles develop orientational ordering, which usually depends non-monotonically on the packing density. In the limit approaching circular disks, the ordering jumps discontinuously at the jamming transition. We explain the ordering with a mean-field model of particle rotation, finding that it results from the shearing acting as an ordering field combined with local interactions between particles. Finally, we find similar orientational ordering in sheared systems of nonconvex particles, which is modified by the more complex local structures of such particles"--Pages x-xi.