Abstract
We use x-ray tomography to investigate the translational and rotational dynamical heterogeneities of athree dimensional hard ellipsoid granular packing driven by oscillatory shear. We find that particles which translate quickly form clusters with a size distribution given by a power law with an exponent that is independent of the strain amplitude. Identical behavior is found for particles that are translating slowly, rotating quickly, or rotating slowly. The geometrical properties of these four different types of clusters are the same as those of random clusters. Different cluster types are considerably correlated or anticorrelated, indicating a significant coupling between translational and rotational degrees of freedom. Surprisingly, these clusters are formed already at time scales that are much shorter than the α-relaxation time, in stark contrast to the behavior found in glass-forming systems.
Highlights
Our system consists of 4100 hard prolate ellipsoids made of polyvinyl chloride with an aspect ratio of 1.5, i.e. a shape that makes the crystallization of the system difficult [28] and allows for a rather strong T-R coupling
We find that particles which translate quickly form clusters with a size distribution given by a power-law with an exponent that is independent of the strain amplitude
Scans were made after each complete cycle, giving a stroboscopic view of the dynamics with the time unit of one shear cycle, and in the SI we show the mean squared displacement for the translational degrees of freedom (TDOF) and the rotational degrees of freedom (RDOF) which allows to get an idea on the relavant time and length scales in the system
Summary
(Since the definition of the population depends on tf , it is clear that the overlap will depend on tf as well.) We see that for small and intermediate tf the overlap for FT-FR is around 0.27, i.e. by a factor of 2.7 above the trivial value of 0.1 expected for random clusters, and that this enhancement is independent of γ This implies that there is a significant probability that a particle which is translating quickly is rotating quickly. The cluster size distribution of all four populations of particles can be described well by a power-law, a distribution that is very different from the one of random clusters These distributions are independent of the strain amplitude, indicating that the dynamics is system universal, in qualitative agreement with earlier findings about the van Hove function [29]. The mechanisms leading to these DH are, strongly dependent on the system considered
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