Impact of dipole-dipole interactions on motility-induced phase separation

Abstract

We present a hydrodynamic theory for systems of dipolar active Brownian particles which, in the regime of weak dipolar coupling, predicts the onset of motility-induced phase separation (MIPS), consistent with Brownian dynamics (BD) simulations. The hydrodynamic equations are derived by explicitly coarse-graining the microscopic Langevin dynamics, thus allowing for a quantitative comparison of parameters entering the coarse-grained model and particle-resolved simulations. Performing BD simulations at fixed density, we find that dipolar interactions tend to hinder MIPS, as first reported in [Liao et al., Soft Matter, 2020, 16, 2208. Here we demonstrate that the theoretical approach indeed captures the suppression of MIPS. Moreover, the analysis of the numerically obtained, angledependent correlation functions sheds light into the underlying microscopic mechanisms leading to the destabilization of the homogeneous phase.

Microscopic field theory for structure formation in systems of self-propelled particles with generic torques

“…Vicsek-like aligning rules, which are decoupled from the excluded-volume forces, are considered in 74 . On the contrary, dipolar interactions between permanent point dipoles, which couple spatial and angular degrees of freedom, are analysed in 76 .…”

“…In this case, originally considered in 45 , the phase behavior is exclusively controlled by the competition between activity and interparticle collisions. Alternatively, effective torques can also lead to ε 0 in some particular microscopic achiral models, such as systems with Vicsek-like alignment 74 or dipoledipole interactions 76 , where the symmetry of the alignment interaction together with the symmetry of the angular correlation leads to a rotational friction coefficient that is identically zero upon integration (see discussion on angular symmetries in Section II).…”

“…The interplay between both excluded-volume and aligning interparticle interactions, or torques, has been addressed in a series of works 11,25,27,[65][66][67][68][69][70][71][72][73][74][75] , focused on activity-induced aggregation. Following the approach first introduced for pure ABP systems in 45 , a hydrodynamic description has been derived for specific types of polar and nematic (Vicsek-like) alignment rules 74 , for chiral ABP 64 and for dipolar ABP 76 . However, a general theoretical framework encompassing both chiral and achiral self-propelled particles interacting via generic central forces and aligning torques, is still missing.…”