Activity enhances heterogenous dynamics in 2D granular glasses

Abstract

We study the effect of self-propulsion in the approach to the glassy regime of 2D bidisperse mixtures of frictional discs, confined to a horizontal plane, and fluidized by vertical vibrations. The discs are designed either to be passive, that is, exhibiting isotropic motion in the plane, or active, with a preferred polar mobility. When dynamics in mixtures of large active and small passive discs are compared to mixtures of large and small passive discs, the active system shows faster dynamics at equal area fractions. To explore whether activity merely speeds up motion, we select active and passive mixtures with the same relaxation time and compare their dynamics. Many indicators show that dynamical heterogeneity is much greater in the active glass with longer-ranged spatial velocity correlations, larger temporal variations in displacement, and higher amplitude of the four-point correlator. In this very crowded regime, despite frequent collisions, the activity directions of the discs remain persistent. However, the activity direction of the discs remains spatially uncorrelated, and furthermore, the particle displacements are not aligned with the activity direction, thus presenting a puzzle as to how activity leads to such strong heterogeneity.