Nonlinear Susceptibility of a Viscoelastic Ferrocolloid: Effect of Displacement Field

A model non-Newtonian ferrocolloid has been considered in the work. The ferrocolloid is a nanodispersion of ferrite in a viscoelastic fluid, the properties of which are described by the Jeffry’s rheological scheme. This scheme is advantageous in the fact that it enables one to adequately describe the Brownian motion of particles under the conditions of retarded viscous stresses. Nonlinear responses of the ferrocolloid to a probe alternating magnetic field have been studied in the simultaneous presence of a constant magnetic field of an arbitrary strength (displacement field). In the aforementioned situation, the spectrum of the system upon longitudinal probing (both imposed films are collinear) contains additional harmonics, i.e., the dynamic magnetization acquires a contribution quadratic with respect to the amplitude of the probe field, while the optical anisotropy arises already in the first order. It has been shown that the spectra of these modes substantially differ from the fundamental components of the response and strongly vary with a rise in the viscoelasticity of the carrier fluid of the colloid. The measurement of the aforementioned characteristics may widen the set of microrheological tests and increase their sensitivity.