The effect of chain-shaped aggregates on the dynamic properties of magnetic liquids is studied under the assumption that the chains can be modeled as straight and rigid. On the basis of microscopical analysis the macroscopical expression for average stress tensor is obtained. The effective viscosity and viscoelastic characteristics are estimated.

We analyze the effect of nanoparticle concentration on the physical properties of magnetic hydrogels consisting of polymer networks of the human fibrin biopolymer with embedded magnetic particles, swollen by a water-based solution. We prepared these magnetic hydrogels by polymerization of mixtures consisting mainly of human plasma and magnetic nanoparticles with OH⁻ functionalization. Microscopic observations revealed that magnetic hydrogels presented some cluster-like knots that were connected by several fibrin threads. By contrast, nonmagnetic hydrogels presented a homogeneous net-like structure with only individual connections between pairs of fibers. The rheological analysis demonstrated that the rigidity modulus, as well as the viscoelastic moduli, increased quadratically with nanoparticle content following a square-like function. Furthermore, we found that time for gel point was shorter in the presence of magnetic nanoparticles. Thus, we can conclude that nanoparticles favor the cross-linking process, serving as nucleation sites for the attachment of the fibrin polymer. Attraction between the positive groups of the fibrinogen, from which the fibrin is polymerized, and the negative OH⁻ groups of the magnetic particle surface qualitatively justifies the positive role of the nanoparticles in the enhancement of the mechanical properties of the magnetic hydrogels. Indeed, we developed a theoretical model that semiquantitatively explains the experimental results by assuming the indirect attraction of the fibrinogen through the attached nanoparticles. Due to this attraction the monomers condense into nuclei of the dense phase and by the end of the polymerization process the nuclei (knots) of the dense phase cross-link the fibrin threads, which enhances their mechanical properties.

We consider a ferrofluid consisting of identical spherical particles with a permanent magnetic moment. Under the assumption that linear flexible chains can appear in the ferrofluid, we estimate the distribution function of the number of particles inside the chain. The analysis is done and simple expressions for the size distribution function are obtained in asymptotics of a strong magnetic interaction between the particles inside one chain. We studied the influence of the linear chains on conditions and scenarios of bulk “gas-liquid” phase transition in the ensemble of the particles under an infinitely strong magnetic field. In order to study the influence of the chains on bulk “gas-liquid” phase transition in the ensemble of the particles, their chemical potential $\mu$ is calculated in the model of separate interacting particles as well as in the model with chains, taking into account the interaction between them. When the temperature is low enough, van der Waals loops appear on the plots of $\mu$ versus volume concentration $\phi$ of the particles in the first model; function $\mu \left(\phi \right)$ increases monotonically in the second model for all examined temperatures. This means that the condensation “gas-liquid” phase transition can take place in the model of individual particles; however, formation of the chains in real ferrofluids prevents the appearance of this transition.