Effective magnetic susceptibility of suspensions

TL;DR

This study measures how suspensions of magnetic particles respond to magnetic fields, revealing the influence of particle alignment and packing on effective susceptibility, and challenging existing models with new experimental insights.

Contribution

It introduces experimental data on magnetic susceptibility of suspensions, highlighting the effects of particle alignment and packing, and identifies limitations of current predictive models.

Findings

Measured $ ext{χ}_ ext{eff}$ with 17% accuracy for aligned particles.

Random particle orientations significantly reduce $ ext{χ}_ ext{eff}$, not accounted for in models.

Maximum $ ext{χ}_ ext{eff}$ observed at lower particle aspect ratios than predicted.

Abstract

We characterize how suspensions of magnetic particles in a liquid respond to a magnetic field in terms of the effective magnetic susceptibility $\chi_{eff}$ using inductance measurements. We test a model that predicts how $\chi_{eff}$ varies due to demagnetization, as a function of sample aspect ratio, particle packing fraction, and particle aspect ratio. For spherical particles or cylindrical particles aligned with external magnetic field, the model can be fitted to the measured data with agreement within 17\%. However, we find that the random alignment of particles relative to the magnetic field plays a role, reducing $\chi_{eff}$ by a factor of 3 in some cases, which is not accounted for in models yet. While suspensions are predicted to have $\chi_{eff}$ that approach the particle material susceptibility in the limit of large particle aspect ratio, instead we find a much smaller particle aspect ratio where $\chi_{eff}$ is maximized. A prediction that $\chi_{eff}$ approaches the bulk material susceptibility in the limit of the packing fraction of the liquid-solid transition also fails. We find $\chi_{eff}$ no larger than about 4 for suspensions of iron particles.