A. Yu. Zubarev

Rheological properties of dense ferrofluids. Effect of chain-like aggregates

Rheological properties of dense ferrofluid are studied both experimentally and theoretically. Experimental dependence of the fluid effective viscosity on magnetic field is much more than predicted by known theories. New theoretical model is suggested to explain and describe these results. This model is based on assumption that linear chain-like aggregates appear in the ferrofluid; these chains induce strong magnetoviscous effect. The results of the theoretical calculations are in good agreement with the experiments.

Rheological properties of polydisperse magnetic fluids. Effect of chain aggregates

A theoretical model of medium-density polydisperse magnetic fluids is proposed. The model takes into account that the major fraction of particles in typical ferrofluids is characterized by a magnetic core diameter of about 10 nm. In addition, there is a certain proportion of large particles with a core diameter of about 16 nm. As a result of the magnetic dipole interaction, the large particles form chain aggregates. Small particles, for which the magnetic dipole interaction energy (both with each other and with large particles) is smaller than the thermal energy, remain in the individual nonaggregated state. The distribution of chains with respect to the number of (large) particles and some rheological characteristics of the ferrofluids are determined. The proposed model is capable of explaining, in principle, the giant magnetoviscosity effect and a strong dependence of the rheological properties of ferrofluids on the shear rate observed in some recent experiments.

Dynamic properties of moderately concentrated magnetic liquids

On the basis of statistical analysis, we derive expressions for the dynamic susceptibility, magnetization relaxation times, and the effective rheological characteristics of a moderately concentrated homogeneous ferrocolloid consisting of identical spherical ferroparticles suspended in a Newtonian liquid. The magnetic moment of a particle is assumed constant and rigidly “frozen” into the body of the particle. We also estimate how the magnetodipole and hydrodynamic interactions of the particles influence the effective dynamic properties of the ferrocolloid.

On the theory of the magnetoviscous effect in ferrofluids

The microscopic origin of viscoelastic effects in ferrofluids is studied theoretically. The growth kinetics of chain aggregates formed by magnetic ferroparticles under the action of the dipole-dipole interaction between them is analyzed. It is shown that the evolution rate for an ensemble of chains determines the rate of variation in the macroscopic stress of the medium upon a change in the applied external field and/or in the shear flow velocity. Consequently, the viscoelastic properties of magnetic fluids can be explained by the chain formation-destruction processes. The proposed microscopic model of a ferrofluid makes it possible (apparently, for the first time) to estimate the characteristic time of viscoelasticity corresponding to experimental results.

Rheological properties of ferrofluids with microstructures

This paper presents results of a theoretical study of the effects of linear chain-like as well as bulk drop-like heterogeneous aggregates on the rheological properties and behaviour of ferrofluids. The results demonstrate that the appearance of both these internal structures leads to a strong (one–two orders of magnitude) increase of the ferrofluid effective viscosity under the action of the magnetic field applied parallel to the gradient of the ferrofluid flow. When the ferrofluid fills a thin channel (gap) placed into a normal magnetic field, the drop-like structures can overlap with the channel. In the case of a rigid connection between the drop-like domains and the channel walls, the appearance of elastic and yield stress effects on the ferrofluid is expected.

On rheophysics of high-concentrated suspensions

Experiments with high-concentrated suspensions of solid particles show that that their viscosity increases by several times or even orders of magnitude, when the shear rate in the suspensions exceeds some critical value, which depends on the concentration and shape of particles and the physicochemical composition of carrier liquids. It is experimentally found that, within a certain range of shear rates, which is also predetermined by the composition of the suspensions, a steady flow becomes unstable, and regular or random oscillations arise in the flow with a period of several seconds. A theory is proposed for these phenomena, which is based on the fact that the increase in the viscosity with the shear rate is explained by the contact friction between particles, while the flow oscillations arise when the differential viscosity of a suspension acquires negative values.

On the theory of birefringence in magnetic fluids

The influence of homogeneous correlations between the positions and orientations of ferroparticles on the effect of light birefringence in magnetic fluids has been studied. It has been demonstrated that, for typical magnetic fluids, the optical effects associated with the homogeneous correlations can be stronger than the effects caused by elongated primary aggregates formed at the stage of ferrofluid synthesis, as well as heterogeneous chains resulting from magnetic attraction between the largest particles of a magnetic fluid.

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.

Statistical thermodynamics of ferrocolloids

Abstract A thermodynamic model of concentrated ferrocolloids is proposed with an account of the steric, molecular and ionic as well as dipole interparticle interaction. The model permits both to evaluate equilibrium properties inherent to a homogeneous state of a ferrocolloid and to find the conditions of its instability leading to the formation of aggregates whose evolution is studied with the help of the corresponding kinetic equation.

Structurization of ferrofluids in the absence of an external magnetic field

Structural transformations in a model ferrofluid in the absence of an external magnetic field have been theoretically studied. The results agree with well-known laboratory experiments and computer simulations in showing that, if the concentration of particles and their magnetic interaction energy are below certain critical values, most particles form separate linear chains. If these parameters exceed the critical values, most particles concentrate so as to form branched network structures. The passage from chains to network has a continuous character rather than represents a discontinuous first-order phase transition.