The increase of the yield stress vs. the magnetic field is the most important quantity characterizing the efficiency of a magnetorheological suspension. The theory based on the formation of columnar aggregates predicts a linear variation with the volume fraction of magnetic particles. In this paper we review previous models used to calculate forces and yield stress and will introduce a new model based on rupture at zero strain. Predictions of these models are compared with the experimental data obtained for carbonyl iron particles, by different authors. Whereas, previous analytical prediction strongly overestimates experimental yield stress, those calculated using the Finite Element Method (FEM), together with affine trajectories, reproduce the experiments well and show a linear dependence with the volume fraction and a H 3/2 behavior between 50 and 200 kA/m. Nevertheless, at very high-volume fractions (>55%), where the suspension can only flow in the presence of specific additives, the dependence of the yield stress vs. the volume fraction and the magnetic field is dramatically changed. We observed a jamming transition, which is triggered by the application of a low magnetic field and which depends strongly on the volume of the fraction. Here, we will discuss new perspectives arising from the use of these very high-volume fractions.
Very concentrated suspensions of iron particles in water or ethylene glycol can be obtained thanks to the use of superplasticizer molecules used in cement industry. At high volume fractions, these suspensions show a discontinuous shear thickening which was thoroughly characterized in rotational geometries. We will show that the jamming transition is also present in a capillary flow, and that it manifests through the formation of a non-consolidated porous medium at the constriction between the barrel and the capillary. In suspension of iron particles, the dynamics of formation of this porous medium, and so the pressure, can be controlled by a low magnetic field and is reversible for a constant volume flow rate, opening potential new applications in the domain of dampers and force control devices.
This work is devoted to the detailed study of jet instability occurring in concentrated aqueous mixtures of calcium carbonate (CC) isotropic-shaped particles and rigid polyamide (PA) fibers. These mixtures exhibit very sharp discontinuous shear thickening (DST). The jets were subjected to a free fall under gravitational stretching at a constant flow rate. In the absence of PA fibers, we observed relatively strong lateral oscillations occurring for jet lengths [Formula: see text] and accompanied by small periodic undulations of the jet diameter. Two-dimensional Direct Fourier Transform analysis reveals approximately linear dispersion relations for propagation of lateral oscillations and diameter undulations with similar wave speeds [Formula: see text]. This instability is ascribed to complex rheological behavior in an extensional flow above the DST transition. Theoretical modeling reveals abrupt jumps of the tensile stress along the jet likely leading to fluctuation of longitudinal and transverse velocity fields within the jet perceived through jet diameter and centerline undulations. The addition of PA fibers to CC suspension damps lateral oscillations but favors ruptures along the jet. This is tentatively explained by the interplay between growing lower and decreasing upper DST threshold stresses with increasing fiber volume fraction [Formula: see text] along with the thinning of the jet diameter down to the size of fiber flocs. Quantitatively, the stabilizing effect of PA fibers is manifested through an abrupt decrease in the lateral drift amplitude at [Formula: see text].
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Discontinuous shear thickening (DST) is usually found in very concentrated suspensions of solid particles and is characterized by a sudden jump in stress during a ramp of shear rate. We have investigated this transition in suspension of magnetic particles with the idea of monitoring the critical shear rate with the help of a magnetic field. Furthermore, the use of conductive particles allows one to relate the conductivity of the suspension to the setup of a network of contacts between particles during this transition. We shall compare our experimental rheograms to the prediction of the standard model, and we shall show how the instability observed above a critical stress can be related to the inertia of the rotating tool. Last, we shall present results related to applications in the domain of the control of forces with a magnetic field and emphasize the fact that the control of the DST allows to improve the efficiency of the devices by more than order of magnitude.
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