The primary objective of this study is to estimate the parameters of models characterizing the rheological properties of nanoparticle-based magnetorheological fluids. Experiments were carried out for suspensions of nanometer sized iron and cobalt particles. Shear stresses and the strain rates have been calculated for different magnetizing currents. The MR fluid was characterized using Bingham-Plastic and Herschel-Bulkley constitutive models. Both these models have two regimes: a rigid pre-yield behavior for shear stress less than a field-dependent yield stress, and viscous behavior for higher shear rates. Determination of model parameters is a complex problem due to the non-linearity of the model of the large amount of scatter in the experimentally observed data. To estimate the rheological parameters, we used a genetic algorithm and carried out a global optimization. The genetic scheme produces results with trends comparable to the gradient-based techniques and is more accurate. The Herschel-Bulkley fluid model is seen to fit the data with lower errors over a larger range as compared to the Bingham-Plastic model.

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