Magnetorheological (MR) fluids of interest in current applications are prepared by dispersion of magnetisable particles in non-magnetic liquid carriers. They exhibit a remarkable rheological change (so-called MR effect) upon the application of a magnetic field. The reason for this is the magnetic field-guided colloidal assembly of the dispersed magnetisable particles. The largest MR effect is achieved under saturating magnetic fields [1].

Measuring the rheological properties of MR fluids under saturation can be problematic as strong inhomogeneities emerge along the sample volume. However, this can be circumvented by using a strategically positioned ferromagnetic plate in a double-gap configuration [2]. Nonetheless, the use of plate-plate geometries does not permit the measurement of steady shear rheological curves because the sample history changes within the volume and MR fluids are generally thixotropic [3].

In this communication we design and construct a measuring double-gap MR cell that is capable to generate homogeneous saturating magnetic fields with a biconical shape to generate a constant shear rate within the whole sample. The device is validated using Finite Element Method Magnetostatics simulations, Computational Fluid Dynamics calculations and experiments with Newtonian liquids and model MR fluids.

This new device allows us to construct the first “complete” MR flow curves (i.e. including the pre- and post-yield regimes) at saturating magnetic fields. The generated data are fitted to a viscoplastic Casson model and collapsed to a previously reported master curve for lower magnetic fields [4]. Moreover, apparent yield stress values follow a linear dependence on particle volume fraction at low particle loading, for the first time validating already existing theoretical models.



Figure 1: (a) Scheme of the experimental double-gap bicone geometry. (b) Flow curves at two different particle volume fractions in the absence of magnetic fields (open symbols) and in the presence of saturating magnetic fields (solid symbols).

[1] J. R. Morillas and J. de Vicente, Soft Matter 16, 9614-9642 (2020).
[2] J. R. Morillas, et al., J. Rheol. 62, 1485 (2018).
[3] J. de Vicente and C. L. A. Berli, Rheol. Acta 52, 467–483 (2013).
[4] C. L. A. Berli and J. de Vicente, Appl. Phys. Lett. 101, 021903 (2012).
 

Steady shear flow, master curve, strong MR fluids