Probing the role of mobility in the collective motion of non-equilibrium systems

TL;DR

This study investigates how varying particle mobility affects collective motion in a non-equilibrium 2D system, revealing similarities to glassy systems and identifying the origin of the boson peak through mode analysis.

Contribution

It introduces a mode analysis approach to non-equilibrium systems, linking particle mobility to collective motion and boson peak formation.

Findings

Large-movement modes are spatially correlated and quasilocalized.

Small-movement modes are highly localized.

A boson peak-like feature emerges with increased mobility.

Abstract

By systematically varying the mobility of self-propelled particles in a two-dimensional (2D) lattice, we experimentally study the influence of particle mobility on system's collective motion. Our system is intrinsically non-equilibrium due to the lack of energy equipartition. By constructing the covariance matrix of spatial fluctuations and solving for its eigenmodes, we obtain the collective motions of the system with various magnitudes. Interestingly, our structurally ordered non-equilibrium system exhibits almost identical properties as disordered glassy systems under thermal equilibrium: the modes with large overall motions are spatially correlated and quasilocalized while the modes with small collective motions are highly localized, resembling the low- and high-frequency modes in glass. More surprisingly, a peak similar to the boson peak forms in our non-equilibrium system as the number of mobile particles increases, revealing the possible origin of the boson peak from a dynamic aspect. We further illustrate that the spatially-correlated large-movement modes can be produced by the cooperation of highly-active particles above a threshold fraction, while the localized small-movement modes can be created by adding individual inactive particles. Our study clarifies the role of mobility in collective motions, and further suggests a promising possibility of extending the powerful mode analysis approach to non-equilibrium systems.