Alexander F. Pshenichnikov

Magneto-granulometric analysis of concentrated ferrocolloids

Abstract This study presents two improvements to the reliability of magneto-granulometric analysis. The first considers the use of a two-parametric function to approximate the size distribution of particles. The second consists in taking into account the dipole-dipole interparticle interactions. In the case of magnetite ferrocolloids a Γ-distribution has been found to provide higher accuracy in calculation of the sixth-order moments (in comparison with the commonly used lognormal distribution), i.e. in calculation of the initial susceptibility, analysis of the Rayleigh scattering and aggregation processes. Among the well-known theoretical models developed to describe ferrocolloid magnetization analytically taking into account the interparticle interactions, the mean-spherical model gives the most stable results. A modified variant of the effective field model has been proposed which is nearly the same in accuracy as the mean-spherical model but more compact and suitable for application.

Equilibrium magnetization of concentrated ferrocolloids

Abstract A number of experiments were performed to investigate the temperature and concentration dependences of the initial susceptibility of magnetite ferrocolloids. Measurements were taken at infralow frequency of 0.05 Hz in a wide range of temperatures and concentrations. The experimental results provide verification of different theoretical models dealing with the equilibrium magnetization of ferrocolloids and taking into account magneto-dipole interparticle interactions. The limits of application for the models were qualified. It was shown that mean-spherical and high-temperature approximations provide adequate description of the real ferrocolloids. The Weiss model has been found to give satisfactory results only for diluted and moderately concentrated ferrocolloids, and, as well, in the region of strong magnetic fields.

Equilibrium magnetization and microstructure of the system of superparamagnetic interacting particles: numerical simulation

Abstract The Monte Carlo method is used to study the equilibrium magnetization of a 3D system of superparamagnetic particles taking into account the steric and dipole–dipole interparticle interactions. Two types of systems are considered: magnetic fluids and solidified ferrocolloids containing randomly spatially distributed particles with negligible energy of magnetic anisotropy. The results of numerical simulations confirm the universality of Langevin susceptibility as a main dimensionless parameter determining the influence of interparticle interactions on the magnetization of the system for moderate values of the aggregation parameter. The obtained results are in good agreement with theoretical and experimental data. At large values of the aggregation parameter, the clustering of particles in magnetic fluids is observed resulting in a reduction of their magnetization as compared to solidified systems. It is shown that the magnetization of solidified systems can be well described by the modified effective field approximation within the whole investigated range of parameters.

Magnetophoresis, sedimentation, and diffusion of particles in concentrated magnetic fluids

A dynamic mass transfer equation for describing magnetophoresis, sedimentation, and gradient diffusion of colloidal particles in concentrated magnetic fluids has been derived. This equation takes into account steric, magnetodipole, and hydrodynamic interparticle interactions. Steric interactions have been investigated using the Carnahan-Starling approximation for a hard-sphere system. In order to study the effective interparticle attraction, the free energy of the dipolar hard-sphere system is represented as a virial expansion with accuracy to the terms quadratic in particle concentration. The virial expansion gives an interpolation formula that fits well the results of computer simulation in a wide range of particle concentrations and interparticle interaction energies. The diffusion coefficient of colloidal particles is written with regard to steric, magnetodipole and hydrodynamic interactions. We thereby laid the foundation for the formulation of boundary-value problems and for calculation of concentration and magnetic fields in the devices (for example, magnetic fluid seals and acceleration sensors), which use a concentrated magnetic fluid as a working fluid. The Monte-Carlo methods and the analytical approach are employed to study the magnetic fluid stratification generated by the gravitational field in a cylinder of finite height. The coefficient of concentration stratification of the magnetic fluid is calculated in relation to the average concentration of particles and the dipolar coupling constant. It is shown that the effective particle attraction causes a many-fold increase in the concentration inhomogeneity of the fluid if the average volume fraction of particles does not exceed 30%. At high volume concentrations steric interactions play a crucial role.

Self-organization of magnetic moments in dipolar chains with restricted degrees of freedom.

Equilibrium behavior of a single chain of dipolar spheres is investigated by the method of molecular dynamics in a wide range of the dipolar coupling constant λ. Two cases are considered: rodlike and flexible chains. In the first case, particle centers are immovably fixed on one axis, but their magnetic moments retain absolute orientational freedom. It has been found that at λ≳1.5 particle moments are chiefly aligned parallel to the chain axis, but the total moment of the chain continuously changes its sign with some mean frequency, which exponentially decreases with the growth of λ. Such behavior of the rodlike chain is analogous to the Néel relaxation of a superparamagnetic particle with a finite energy of magnetic anisotropy. In the flexible chain particles are able to move in the three-dimensional space, but the distance between centers of the first-nearest neighbors never exceeds a given limiting value r(max). If r(max)≃d (d is the particle diameter) then the most probable shape of the chain of five or more particles at λ≳6 is that of a ring. The behavior of chains with r(max)≥2d is qualitatively different: At λ≃4 long chains collapse into dense quasispherical globules and at λ≳8 these globules take toroidal configuration with a spontaneous azimuthal ordering of magnetic dipoles. With the increase of r(max) to larger values (r(max)>10d) globules expand and break down to form separate rings.