The effect of inter-particle hydrodynamic and magnetic interactions in a magnetorheological fluid

TL;DR

This paper investigates how inter-particle magnetic and hydrodynamic interactions influence the behavior of magnetorheological fluids, revealing mechanisms that lead to flow cessation and potential instability under magnetic fields.

Contribution

It provides a detailed analysis of particle interactions and their role in dynamical arrest and instability in magnetorheological fluids under shear and magnetic fields.

Findings

Magnetic interactions cause particle orientation changes affecting flow.

Hydrodynamic interactions contribute to anisotropic diffusion and instability.

Concentration fluctuations can lead to flow cessation in magnetic fields.

Abstract

A magnetorheological fluid, which consists of magnetic particles suspended in a viscous fluid, flows freely with well-dispersed particles in a the absence of a magnetic field, but particle aggregation results in flow cessation when a field is applied. The mechanism of dynamical arrest is examined by analysing interactions between magnetic particles in a magnetic field subject to a shear flow. An isolated spherical magnetic particle undergoes a transition between a rotating state at low magnetic field and a static orientation at high magnetic field. The effect of interactions for spherical dipolar and polarisable particles with static orientation is examined for a dilute viscous suspension. There are magnetic interactions due to the magnetic field disturbance at one particle caused by the dipole moment of another, hydrodynamic interactions due to the antisymmetric force moment of a non-rotating particle in a shear flow, and a modification of the magnetic field due to the particle magnetic moment density. When there is a concentration variation, the torque balance condition results in a disturbance to the orientation of the particle magnetic moment. The net force and the drift velocity due to these disturbances is calculated, and the collective motion generated is equivalent to an anisotropic diffusion process. When the magnetic field is in the flow plane, the diffusion coefficients in the two directions perpendicular to the field direction are negative, implying that concentration fluctuations are unstable in these directions. This instability could initiate field-induced dynamical arrest in a magnetorheological fluid.