Orbital Magnetism of Active Viscoelastic Suspension

TL;DR

This paper investigates how active particles in a viscoelastic fluid exhibit finite orbital magnetism influenced by their inertia and the interplay of active fluctuations and visco-elastic dissipation, revealing a transition between diamagnetic and paramagnetic states.

Contribution

It provides an exact calculation of orbital magnetic moment in active viscoelastic suspensions and uncovers a tunable transition between diamagnetic and paramagnetic behavior driven by physical time scales.

Findings

Finite inertia is necessary for orbital magnetism in active suspensions.

System transitions from diamagnetic to paramagnetic by tuning visco-elastic and active fluctuation time scales.

Magnetic moment exhibits non-monotonic behavior with increasing activity.

Abstract

We consider a dilute suspension of active (self-propelling) particles in a visco-elastic fluid. Particles are charged and constrained to move in a two dimensional harmonic trap. Their dynamics is coupled to a constant magnetic field applied perpendicular to their motion via Lorentz force. Due to the finite activity, the generalised fluctuation-dissipation relation (GFDR) breaks down, driving the system away from equilibrium. While breaking GFDR, we have shown that the system can have finite classical orbital magnetism only when the dynamics of the system contains finite inertia. The orbital magnetic moment has been calculated exactly. Remarkably, we find that when the elastic dissipation time scale of the medium is larger (smaller) than the persistence time scale of the self-propelling particles, the system is diamagnetic (paramagnetic). Therefore, for a given strength of the magnetic field, the system undergoes a novel transition from diamagnetic to paramagnetic state (and vice-versa) simply by tuning the time scales of underlying physical processes, such as, active fluctuations and visco-elastic dissipation. Interestingly, we also find that the magnetic moment, which vanishes at equilibrium, behaves non-monotonically with respect to increasing persistence of self-propulsion, that drives the system out of equilibrium