Magnetorheological effect in the magnetic field oriented along the vorticity

Abstract

International audienceIn this work, we have studied the magnetorheological (MR) fluid rheology in the magnetic field parallel to the fluid vorticity. Experimentally, the MR fluid flow was realized in the Couette coaxial cylinder geometry with the magnetic field parallel to the symmetry axis. The rheological measurements were compared to those obtained in the cone-plate geometry with the magnetic field perpendicular to the lower rheometer plate. Experiments revealed a quasi-Bingham behavior in both geometries with the stress level being just a few dozens of percent smaller in the Couette cylindrical geometry at the same internal magnetic field. The unexpectedly high MR response in the magnetic field parallel to the fluid vorticity is explained by stochastic fluctuations of positions and orientations of the particle aggregates. These fluctuations are induced by magnetic interactions between them. Once misaligned from the vorticity direction, the aggregates generate a high stress independent of the shear rate, and thus assimilated to the suspension apparent (dynamic) yield stress. Quantitatively, the fluctuations of the aggregate orientation are modeled as a rotary diffusion process with a diffusion constant proportional to the mean square interaction torque. The model gives a satisfactory agreement with the experimental field dependency of the apparent yield stress and confirms the nearly quadratic concentration dependency Sigma_Y proportional to Phi^2.2, revealed in experiments. The practical interest of this study lies in the development of MR smart devices with the magnetic field non-perpendicular to the channel walls

How nonmagnetic particles intensify rotational diffusion in magnetorheological fluids

“…As a result, the improvement of the MR effect in bi-component suspensions is more noticeable for cylindrical Couette geometry, in agreement with experiments. 4) and ( 8)) are calculated as a function of the magnetic field H using finite element simulations [18,29] The simulation results for f m and χ a , have been fitted by the following expressions, valid in the range of the magnetic field intensities, 0 30 H ≤ ≤ kA/m: ( )…”

“…In addition to it, magnetic interactions between aggregates may induce stochastic fluctuations of their orientation and result in a supplementary contribution to the yield stress, coming from stochastic interaction torques. These fluctuations may be regarded as an effective rotary diffusion process, which is likely responsible for the unexpectedly high yield stress when the magnetic field is parallel to either the suspension velocity or the vorticity [19,29].…”

“…The orientation state of the aggregates is described by the equation of evolution of the second statistical moments, i k e e , which is conventionally derived by multiplying the Fokker-Planck equation for the orientation distribution function by i k e e , and averaging over all possible orientations. In the case of long aggregates possessing an induced magnetic moment, this equation reads [19,29,34]:…”

Magnetorheology: a review

“…133,134 In this particular geometry, the formation of Taylor vortex flows was reported by Güth and Maas. 134 Furthermore, the magnetorheological behavior of MRFs when the field is oriented in the vorticity direction was investigated by Kuzhir et al 135 They reported an unexpected high MR response that was explained by stochastic fluctuations of positions and orientations of the particle aggregates. The fluctuations were modeled as a rotary diffusion process with a diffusion constant proportional to the mean square interaction torque.…”