Phases and excitations of active rod-bead mixtures: simulations and experiments

TL;DR

This study combines simulations and experiments to explore phase transitions, band formations, and collective behaviors in active mixtures of polar rods and beads under vertical vibration.

Contribution

It provides new insights into phase behavior, band formation, and collective excitations in active rod-bead mixtures through combined experimental and numerical analysis.

Findings

Observation of a direct phase transition from isotropic to flock with increasing bead concentration.

Formation of a dense, ordered band of rods and beads at higher concentrations.

Detection of sound modes and giant number fluctuations consistent with flocking theories.

Abstract

We present a large-scale numerical study, supplemented by experimental observations, of a quasi-two-dimensional active system of polar rods and spherical beads confined between two horizontal plates and energised by vertical vibration. For low rod concentrations $\Phi_r$ we observe a direct phase transition, as bead concentration $\Phi_b$ is increased, from the isotropic phase to a homogeneous flock. For $\Phi_r$ above a threshold value, an ordered band dense in both rods and beads occurs between the disordered phase and the homogeneous flock, in both experiments and simulations. Within the size ranges accessible we observe only a single band, whose width increases with $\Phi_r$. Deep in the ordered state, we observe broken-symmetry "sound" modes and giant number fluctuations. The direction-dependent sound speeds and the scaling of fluctuations are consistent with the predictions of field theories of flocking, but sound damping rates show departures from such theories. At very high densities we see phase separation into rod-rich and bead-rich regions, both of which move coherently.