Local rotational dynamics reveals strain dependence in amorphous particle packings

Glassy dynamics in amorphous packings is commonly probed by quantifying various nonaffine displacement metrics such as D²_min, χ4, Okubo-Weiss etc. While the rigidity-inducing proliferation of steric hindrance makes such metrics interesting, it also makes these metrics difficult to interpret. Particle rotations are much less sensitive to steric hindrance and other confinement effects. We study the dynamics of rotations in sheared amorphous quasi-two-dimensional disk packings at various friction coefficients and packing densities. We demonstrate that even for round particles, particle rotations display the hallmarks of glassy dynamics. Studying rotations offers a few advantages: the diffusive nature of rotational dynamics evidences imminent shear jamming already after a few percent transient strain, in the absence of any measurable pressure. Additionally, the spatially varying rotation dynamics is correlated and can be quantified with a well-studied spatial autocorrelation measure called Moran’s I. Such analysis show the emergence of clusters of anticorrelated motion. Our work suggests that particle rotation dynamics is an essential field to consider in the understanding of amorphous material mechanics.