Optical Analysis and Theoretical Modelling of Colloidal Maghemite Sedimentation in Magnetic Field

Abstract

Superparamagnetic fluids are colloidal suspensions of ferri- or ferromagnetic nanoparticles in a liquid solvent. For applications of these magnetic fluids, it is essential that sedimentation of the suspended particles in an external magnetic field is slow. This Master’s Thesis examines the sedimentation of suspended nanoparticles of a superparamagnetic fluid in a magnetic field, both in theory and experiment. Two simple models to predict the behavior of superparamagnetic nanoparticles in a magnetic field are developed, a sedimentation-diffusion equilibrium model (I) and a dynamic model (II) respectively. Both calculate the forces acting on the nanoparticles and evaluate concentration profiles from this. Model I evaluates the concentration profile at sedimentation-diffusion equilibrium. Model II calculates dynamically the concentration profiles in time between the start of the sedimentation and as the magnetic fluid approaches the sedimentation-diffusion equilibrium. Additionaly, an optical detection technique which measures the absorption of visual light of the superparamagnetic fluid to capable of measuring the nanoparticle concentration profiles in time was developed. Experiments were performed on nitrate coated maghemite superparamagnetic fluids in a magnetic field. The results obtained from the two models were compared with one another and with the concentration profiles obtained from the experimental setup. The optical setup to monitor sedimentation in a magnetic field yielded promising results. The experimental equilibrium and time-dependent concentration profiles were qualitatively similar but not quantitatively the same as predicted by our theoretical calculations.