Shear thickening prevents slip in magnetorheological fluids

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Magnetorheological (MR) fluids are smart fluids that solidify upon the application of a magnetic field by virtue of suspended magnetic particles forming chains. Here, the effect of shear thickening additives on the performance of MR fluids is explored and found to increase yield stress by 60% in fluids that have the same iron content. In order to explore the origin of this improvement, a series of flow- and oscillation-based rheology experiments were performed at various plate separations and the results were compared with a discrete dipole model of chain formation and breakage. Ultimately, conventional MR fluids, which are typically shear thinning, are found to yield at small strains that are consistent with slip failure where chains shear apart at one particle-particle joint. In contrast, shear thickening fluids reliably yield at high strain in a manner consistent with affine failure of the chain where each joint is pulled apart equally. The prevention of slip failure is attributed to the shear thickening additive locally increasing the effective viscosity as slip begins, thus preventing it from leading to yield. In addition to motivating the use of shear thickening agents broadly in MR fluids, these findings highlight the importance of additive choice in tuning rheological properties of MR fluids and the benefit of considering the microstructural and bulk behavior of the fluid simultaneously when synthesizing smart fluids.