Phase separation of self-propelled disks with ferromagnetic and nematic alignment

Abstract

We present a comprehensive study of a model system of repulsive self-propelled disks in two dimensions with ferromagnetic and nematic velocity alignment interactions. We characterize the phase behavior of the system as a function of the alignment and self-propulsion strength, featuring orientational order for strong alignment and Motility-Induced Phase Separation (MIPS) at moderate alignment but high enough self-propulsion. We derive a microscopic theory for these systems yielding a close set of hydrodynamic equations from which we perform a linear stability analysis of the homogenous disordered state. This analysis predicts MIPS in the presence of aligning torques. The nature of the continuum theory allows for an explicit quantitative comparison with particle-based simulations, which consistently shows that ferromagnetic alignment fosters phase separation, while nematic alignment does not alter either the nature or the location of the instability responsible for it. In the ferromagnetic case, such behavior is due to an increase of the imbalance of the number of particle collisions along different orientations, giving rise to the self-trapping of particles along their self-propulsion direction. On the contrary, the anisotropy of the pair correlation function, which encodes this self-trapping effect, is not significantly affected by nematic torques. Our work shows the predictive power of such microscopic theories to describe complex active matter systems with different interaction symmetries and sheds light on the impact of velocity-alignment interactions in Motility-Induced Phase Separation.

18 pages, 12 figures

References in corpus (17)

• Novel type of phase transition in a system of self-driven particles
• Motility-Induced Phase Separation
• Statistical Mechanics of Interacting Run-and-Tumble Bacteria
• When are active Brownian particles and run-and-tumble particles equivalent? Consequences for motility-induced phase separation
• Non-equilibrium clustering of self-propelled rods
• Pattern formation in self-propelled particles with density-dependent motility
• Minimal model for active nematics: quasi-long-range order and giant fluctuations
• Nonequilibrium equation of state in suspensions of active colloids
• Traffic jams, gliders, and bands in the quest for collective motion
• A mean-field theory for self-propelled particles interacting by velocity alignment mechanisms
• Dynamical mean-field theory and weakly non-linear analysis for the phase separation of active Brownian particles
• New ordered phases in a class of generalized XY models
• Hydrodynamic interactions in dense active suspensions: from polar order to dynamical clusters
• Interparticle torques suppress motility-induced phase separation for rodlike particles
• From scalar to polar active matter: Connecting simulations with mean-field theory
• Motility-Induced Clustering and Meso-Scale Turbulence in Active Polar Fluids
• Long-Range Nematic Order in Two-Dimensional Active Matter