Thermodynamic and transport properties of non-magnetic particles in magnetic fluids

Abstract

Magnetic composites, obtained on associating magnetic fluid with non-magnetic particles, offer interesting opportunities in separations, assemblies and other applications, where the microstructure of the composite can be altered reversibly by an external field without altering the composition. The goal of our work in this area is to develop computational and simulation tools to assist in the in-depth understanding of the thermodynamic and transport properties of such non-magnetic nanoparticles immersed in magnetic fluids under varying magnetic field conditions. Also, in this work we have studied the relaxation and magnetization characteristics of magnetic nanoparticle clusters in presence of low external magnetic fields. Theoretical analysis of such a complex system is difficult using conventional theories, and hence we have used Monte Carlo Simulations to explore these effects. We simulated the interactions between non-magnetic particles (1000 nm) and magnetic nanoparticles (10 nm and 20 nm diameter) dispersed in organic phase. We observed that the presence of the non-magnetic particle in the system induces magnetic non-homogeneity. The magnetic nanoparticles present in the equatorial place of the non-magnetic particle with reference to the applied magnetic field have a higher magnetization as compared to the particles in the polar region. This effect was much more dominant for 20 nm particles than 10nm particles, because the magnetic inter-particle interactions are much stronger for the larger particles. We have also studied the effect of radial distance from the nonmagnetic particle on the magnetization and radial distribution function characteristics of the magnetic nanoparticles.

(cont.) We have evaluated the magnetophoretic forces the non-magnetic particles experience when subjected to magnetic field gradient. We have identified such forces arising from the inter-particle interactions between the magnetic nanoparticles. These forces were found to be significant for larger magnetic particles, smaller non-magnetic particles and lower magnetic fields. Diffusion coefficients were evaluated for non-magnetic nanoparticles in magnetic fluids using Brownian Dynamics Simulation. The chain-like structures formed by magnetic nanoparticles introduce anisotropy in the system with the diffusion coefficients higher along the direction of applied external magnetic field and lower in the perpendicular direction. It was observed that the anisotropy increases with higher magnetic particle concentration and larger non-magnetic particles. Anisotropy is negligible for small sized magnetic particles for which the inter-particle interaction is smaller, increases with increasing magnetic particle size and becomes constant thereafter. Results were compared with theoretical predictions. Néel Relaxation was studied for magnetic nanoparticle clusters. Chain-like, spherical and planar clusters were evaluated for the relaxation times. For chain-like structures the relaxation times increase significantly on increasing the chain length and particle size. For spherical clusters the relaxation times were fairly similar to that of individual magnetic nanoparticles. Hence, such a fast relaxation makes them ideal candidates for HGMS separations, since they will be released quickly from the magnetic wires during the elution step.

(cont.) Also, we studied the magnetization characteristics of rectangular and hexagonal packing arrangements of magnetic clusters in presence of remnant fields. The hexagonal arrangement revealed a novel oscillatory behavior. A theoretical model was developed to predict the magnetic particle size beyond which the oscillations are observed.