The Influence of Particle Softness on Active Glassy Dynamics

TL;DR

This study investigates how particle softness influences the relaxation dynamics of active glassy systems, revealing that the non-monotonic enhancement by activity persists across different softness levels, emphasizing the role of cage length and active persistence.

Contribution

It demonstrates that the non-monotonic relaxation behavior in active glassy systems is robust to particle softness and clarifies how softness affects cage length and active motion.

Findings

Non-monotonic relaxation dynamics are preserved for softer particles.

Longer-range interactions in softer particles decrease cage length.

Qualitative changes occur when persistence length exceeds cage length.

Abstract

Active matter studies are increasingly geared towards the high-density or glassy limit. This is mainly inspired by the remarkable resemblance between active glassy materials and conventional passive glassy matter. Interestingly, within this limit it has recently been shown that the relaxation dynamics of active quasi-hard spheres is non-monotonic and most enhanced by activity when the intrinsic active length scale (e.g., the persistence length) is equal to the cage length, i.e. the length scale of local particle caging. This optimal enhancement effect is claimed to result from the most efficient scanning of local particle cages. Here we demonstrate that this effect and its physical explanation are fully retained for softer active spheres. We perform extensive simulations of athermal active Brownian particles (ABPs) and show that the non-monotonic change of the relaxation dynamics remains qualitatively similar for varying softness. We explain quantitative differences by relating them to the longer range of the softer interaction potential, which decreases the cage length and obscures the intrinsic active motion. Moreover, we observe that only when the persistence length surpasses the cage length, distinct qualitative changes with respect to an equivalent passive Brownian particle system start to manifest themselves. Overall, our results further strengthen the importance of the cage length and its relation to the relevant active length scale in the context of active glassy materials.