Pair correlations in magnetic nanodispersed fluids
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AL, NONLINEAR, AND SOFT MATTER PHYSICS
Pair Correlations in Magnetic Nanodispersed Fluids E. A. Elfimova* and A. O. Ivanov** Ural State University, Yekaterinburg, 620083 Russia *email: [email protected] **email: [email protected] Received December 9, 2009
Abstract—The pair distribution function of a monodisperse magnetic fluid simulated by a liquid made of dipolar hard spheres with constant magnetic moments is calculated. The anisotropy of the pair distribution function and the related structure factor of scattering in a dc uniform magnetic field are studied. The calcu lation is performed by diagrammatic expansion in the volume concentration of particles and the interparticle magneticdipole interaction intensity using a thermodynamic perturbation theory. Limitation by threepar ticle diagrams makes it possible to apply the results obtained to magnetic fluids with a moderate concentra tion. Even for lowconcentration and weakly nonideal magnetic fluids, the anisotropic interparticle mag neticdipole correlations in a magnetic field lead to the repulsion of particles in the direction normal to the field and to the formation of particle dimers along the field. DOI: 10.1134/S1063776110070149
1. INTRODUCTION Magnetic nanodispersed fluids (ferrofluids, ferro colloids) represent stable suspensions of nanoparticles of ferro and ferrimagnetic materials (iron and cobalt oxides) in liquid carriers [1]. The characteristic diam eter of the magnetic particle core is about 10 nm, and the particle shape is close to a spherical shape. Since the ferroparticle size is smaller than the singledomain threshold, such particles are traditionally considered as uniformly magnetized spheres [2], whose magnetic moment m is proportional to the product of the satu ration magnetization of the crystalline magnetic mate rial into the ferroparticle magnetic core volume. Such small particles are involved in intense Brownian motion, which leads to sedimentation stability of fer rofluids. To ensure aggregative stability and to prevent irreversible coalescence, particles are coated with spherical shells made of surfactant molecules, which serve as elastic shock absorbers to neutralize the irre versible action of the van der Waals attraction. Thus, with allowance for a stabilized surfactant shell, outside particle diameter d is 4–6 nm larger than the magnetic core diameter of the particle. An applied magnetic field orients the magnetic moments of the particles; as a result, a magnetic fluid can substantially interact with the magnetic field and retain the physical proper ties of the liquid state. On the one hand, this unique combination of properties leads to many unusual physicochemical, hydrodynamic, and thermophysical effects. On the other hand, this combination makes it possible to attribute magnetic fluids to the class of nanostructured substances with controllable proper ties, which allows their wide application in modern technologies and medicine.
From the fundamental standpoint of the physics of liquids, the main sp
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