From scalar to polar active matter: Connecting simulations with mean-field theory

Abstract

We study numerically the phase behavior of self-propelled elliptical particles interacting through the "hard" repulsive Gay-Berne potential at infinite Péclet number. Changing a single parameter, the aspect ratio, allows to continuously go from discoid active Brownian particles to elongated polar rods. Discoids show phase separation, which changes to a cluster state of polar domains, which then form polar bands as the aspect ratio is increased. From the simulations, we identify and extract the two effective parameters entering the mean-field description: the force imbalance coefficient and the effective coupling to the local polarization. These two coefficients are sufficient to obtain a complete and consistent picture, unifying the paradigms of scalar and polar active matter. arXiv:1910.06547v1 [cond-mat.stat-mech]

A particle-field approach bridges phase separation and collective motion in active matter

“…Similar observations were reported from numerical investigations of hard rods, implemented via a Gay–Berne potential, with isotropic mobility tensor in the limit of infinite Péclet number 62 . The breakdown of MIPS aggregates was attributed in ref.…”

“…The breakdown of MIPS aggregates was attributed in ref. 62 to a different mechanism from the one reported here, namely to a reduction of force imbalance as a consequence of local alignment. In contrast, our analysis (for theoretical considerations, see below) suggests an anisotropic coupling of density gradients to the polar order parameter field to stabilize the homogeneous state for self-propelled rods.…”

“…These findings provide an understanding of the role played by particle anisotropy regarding the robustness of active phase separation described in terms of scalar field theories for the particle density. Our smooth-particle approach underpins the restabilization of the disordered, homogeneous phase—respectively, the breakdown of MIPS—to specific microscopic interaction mechanisms, which, as we argue, are also involved in the emergence of order, moving beyond previous studies that reported the destabilization of MIPS based on numerical measurements of effective transport coefficients 61 or identified a different destabilization mechanism 62 .…”

“…That is in stark contrast to the arguments given in ref. 62 , where this type of coupling is absent.…”

“…Within the same framework, it consistently reproduces MIPS for self-propelled discs 12 and predicts the emergence of orientational order as well as polar clustering for self-propelled hard rods 27 , 32 . Thereby, it contributes to the connection of the realms of scalar and vectorial active matter 37 , 61 , 62 . Combining numerical simulations and analytically derived coarse-grained order parameter equations, we show how MIPS breaks down for anisotropic objects due to the combined action of self-propulsion and anisotropic repulsion.…”