Mode-Coupling Theory for Active Brownian Particles

TL;DR

This paper develops a mode-coupling theory for dense active Brownian particles, capturing how self-propulsion and persistence influence glassy dynamics and predicting a complex transition diagram.

Contribution

It introduces a novel MCT framework for active particles that explicitly includes both translational and rotational degrees of freedom, extending understanding of non-equilibrium glass transitions.

Findings

Self-propulsion speeds up structural relaxation at fixed density.

Decreasing persistence length slows down the dynamics.

Predicted a non-trivial glass transition diagram in parameter space.

Abstract

We present a mode-coupling theory (MCT) for the high-density dynamics of two-dimensional spherical active Brownian particles (ABP). The theory is based on the integration-through-transients (ITT) formalism and hence provides a starting point for the calculation of non-equilibrium averages in active-Brownian particle systems. The ABP are characterized by a self-propulsion velocity $v_0$, and by their translational and rotational diffusion coefficients, $D_t$ and $D_r$. The theory treats both the translational and the orientational degrees of freedom of ABP explicitly. This allows to study the effect of self-propulsion of both weak and strong persistence of the swimming direction, also at high densities where the persistence length $\ell_p=v_0/D_r$ is large compared to the typical interaction length scale. While the low-density dynamics of ABP is characterized by a single P\'eclet number, $Pe=v_0^2/D_rD_t$, close to the glass transition the dynamics is found to depend on $Pe$ and $\ell_p$ separately. At fixed density, increasing the self-propulsion velocity causes structural relaxatino to speed up, while decreasing the persistence length slows down the relaxation. The theory predicts a non-trivial idealized-glass-transition diagram in the three-dimensional parameter space of density, self-propulsion velocity and rotational diffusivity. The active-MCT glass is a nonergodic state where correlations of initial density fluctuations never fully decay, but also an infinite memory of initial orientational fluctuations is retained in the positions.