The influence of a sinusoidal magnetic field on the effective viscosity of a magnetorheological dispersion in the low particle concentration regime is studied experimentally. When a sinusoidal magnetic field of low amplitude, conceived as perturbation, is applied transversally to the static field, a significant enhancement occurs in the measured effective viscosity. The magnitude of changes depends on a number of factors such as the amplitude and frequency of the perturbation, the particle concentration, the application time of the fields, and the shear rate. It has also been found that the behavior of the effective viscosity as a function of frequency presents a critical behavior. Therefore, an average Mason number is proposed, whose behavior as a function of frequency is similar to that shown by the effective viscosity.
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