Spatial and Temporal Control of Avalanches in Dense Active-Passive Mixtures

Abstract

Dense active--passive mixtures are often described within an effective-temperature picture, in which active dopants promote cage breaking and fluidize an arrested amorphous host. However, at sufficiently large persistence, the same dopants drive stress accumulation and localized void formation. Here we investigate the avalanche dynamics across this progression, focusing on the intermediate regime between dynamic arrest and void formation, where the system remains structurally homogeneous on average but relaxes through intermittent collective rearrangements rather than as a simple activated liquid. Using overdamped simulations of dilute active dopants embedded in a dense amorphous solid, we show that the avalanche dynamics separates into distinct spatial and temporal controls. For a given system size, the active fraction sets the size of the largest stress-release events, with a cutoff that grows as the square root of the fraction of dopants, $S_c\sim ϕ_a^{1/2}$, i.e., with the number of mean dopant spacings across the system, whereas persistence controls how avalanches unfold in time at fixed released stress. A decomposition of the stress into active and passive contributions, together with measurements of avalanche participation and morphology, shows that the active minority provides a localized and persistent driving channel while the surrounding amorphous packing carries the collective stress release. While the active minority controls the cutoff of the avalanche size statistics and the timing of the stress releases, the long-range elastic response and the building correlations in the passive host sets the size exponent, $τ_S\simeq1.1$, that is shared with externally sheared and uniformly active amorphous solids.