Steady state rheological behaviour of multi-component magnetic suspensions

TL;DR

This study investigates the rheological properties of multi-component magnetic suspensions, revealing enhanced viscosity and yield stress due to particle interactions and structures formed under magnetic fields, supported by microscopy and theoretical modeling.

Contribution

It introduces a new understanding of how multi-component suspensions behave rheologically, especially the effects of particle adsorption and structure formation under magnetic fields.

Findings

Multi-component suspensions have higher viscosity and yield stress than single-component ones.

Adsorption of iron particles on PMMA particles creates core-shell structures observable via microscopy.

Theoretical model predicts yield stress based on magnetic permeability changes during shear.

Abstract

In this paper we study the rheological behaviour (in the absence of magnetic field and upon its application) of multi-component magnetic suspensions that consist of a mixture of magnetic (iron) and non-magnetic (PMMA) particles dispersed in a liquid carrier. These suspensions exhibit considerably higher viscosity and yield stress in the absence of magnetic field than single-component suspensions of the same solid fraction, as a consequence of the adsorption of the iron particles on the PMMA ones. The adsorbed layer of iron particles on the PMMA ones is observed through optical microscopy of dilute samples and confirmed by attenuated total reflectance. Microscopic observations also show that the resulting non-magnetic-core-magnetic-shell composites move upon magnetic field application and aggregate into particle structures aligned with the applied field. These structures, which consist of both types of particles, give rise to high values of the static and dynamic yield stresses upon field application. Actually, both quantities are much higher than those of a suspension with the same volume fraction of magnetic particles, and increase when the amount of non-magnetic ones increases. These trends are adequately predicted by a theoretical model that considers that the main contribution to the yield stress is the change of the suspension magnetic permeability when particle chains are deformed by the shear.