This paper reports the preparation of magnetic fluids consisting of magnetite nanoparticles dispersed in an ionic liquid. Different additives were used in order to stabilize the fluids. Colloidal stability was checked by magnetic sedimentation, centrifugation and direct observation. The results of these tests showed that a true ferrofluid was only obtained when the nanoparticles were coated with a layer of surfactant compatible with the ionic liquid. These experiments also showed that stability could not be reached just by electrostatic repulsion. The conclusions of the stability tests were confirmed by calculations of the interparticle energies of interaction. The rheological behaviour of the magnetic fluids upon magnetic field application was also investigated. The experimental magnetoviscous response was fitted by a microstructural model. The model considered that the fluids consisted of two populations of particles, one with a magnetic core diameter of 9 nm, and another with a larger diameter. Upon field application chain-like structures are supposed to be induced. According to estimations particles of 9 nm are too small to aggregate upon field application. The results of the calculations showed that the intensity of the magnetoviscous response depends on the concentration and size of the large particles, and on the thickness of the surfactant layers.

We present results of an experimental and theoretical study of the magnetic properties of soft magnetic gels consisting of micron-sized magnetizable particles embedded in a polymer matrix. Experiments demonstrate hysteretic dependences of composite magnetization on an applied magnetic field and non-monotonic, with maximum, dependence of the sample susceptibilities on the field. We propose a theoretical approach which describes the main physical features of these experimental results.

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The paper deals with theoretical study of non linear viscoelastic phenomena in ferrofluids placed in magnetic field. Our attention is focused on the study of nonstationary flow and Maxwell-like relaxation of the macroscopical viscous stress after alternation of the shear rate. We propose that these phenomena can be explained by finite rate of evolution of chainlike aggregates, consisting of the ferrofluid particles. Statistical model of the chains growth-disintegration is suggested. In this model the chain-single particle mechanism of the chains evolution is considered, the effects of the chain-chain interaction are ignored. The proposed model allows us to estimate the time-dependent function of distribution over number of particles in the chain. Having determined this function and using methods of hydromechanics of ferrofluids with chainlike aggregates, we have studied evolution of the ferrofluid viscosity after stepwise alternation of the fluid shear rate. The estimated time of relaxation is in a reasonable agreement with experimental results. Thus, our analysis shows that the observed macroscopical viscoelastic phenomena in ferrofluids can be provided by evolution of the chain ensemble.