Thixotropy of magnetorheological gel composites: Experimental testing and modelling

Abstract Magnetorheological gel (MRG) is known as an aspiring controllable composite which exhibit rapid and reversible structural changes subjected to magnetic fields. Its rheological manifestation of the structural changes induced by both the shearing flow and the magnetic field is the varying in viscosity or shear stress. This effect is called thixotropy if the structural changes are reversible and time dependent. In this study, time dependent rheological properties were characterised by a proposed stepwise experiment which revealed the effects of shear rate, magnetic field, and resting time on the structural break-down and rebuilding in MRG. The results indicated up to 15% decrease in the shear stress of MRG under the constant shear rate of 6000 1/s within 5 s. 4 s and 30 s of resting allows approx. 50% and 98% recoveries of the reversible structural change, respectively. The application and intensifying of magnetic fields tend to increase thixotropic yield stress but decrease the fluctuation state. A simple model was formulated to capture the unique time dependent structural changes of MRG under constant shear flow and magnetic field; and, showed accurate predictions of the thixotropic responses of MRG under the magnetic fields, shear rates and resting time considered.