Nanoparticle complex fluids

Abstract

The work presented in this thesis concerns the study of complex nanofluids. The interaction of particles in dispersions under the influence of electric and magnetic fields has been studied. The main focus has been the investigation of the behavior of carbon particle dispersions. A novel type of carbon material, namely carbon cone (CC) material, has been characterized using atomic force microscope, scanning tunnelling microscope and scanning electron microscope. The CC material is a mixed powder consisting of carbon particles with the shape of disks and cones and a small amount of amorphous carbon particles. The length or diameter of the particles vary between 0.5-5 um with thickness varying between 10-50 nm. The results confirm the 5 cone angles as predicted by theory. The dispersion of CC particles in silicon oil was studied under the influence of an electric field. The particles were found to align in an ac electric field and structure formation was observed at very low fields. The growth rate was found to vary exponentially with the electric field. The structure formations were permanent (at zero shear rate), not dissolving when the electric field was turned of. This was attributed to the strong Van der Waals forces associated with carbon particles. Electrorheological measurements were carried out for dispersions with varying CC particle concentrations. All samples showed a Bingham fluid behavior with a finite yield stress. The yield stress was found to depend only weakly on the electric field. The results showed that the ER efficiency as measured by the relative increase in viscosity compared to the zero field viscosity, increases with decreasing concentration with a maximum factor of approximately 10 for the dispersion with lowest particle concentration. This is relatively low compared to commercial ER fluids and was attributed to the high conductivity of the particles and to the low relaxation frequency as determined by impedance measurements. The structure formations could be used to produce one dimensional conductive paths in i.e composites. Carbon cones were also dispersed in ferrofluid to observe their behaviour in a magnetic field. A small increase in the viscosity was obtained for CC in a ferrofluid and this was attributed to purely hydrodynamic forces. No evidence of CC particle alignment was found. The interaction of non-magnetic spherical particles (magnetic holes) in a ferrofluid was studied and used to develop a microrheological method for measuring the local viscosity of the ferrofuid itself. By using two magnetic holes which oscillate in a rotating magnetic field, the apparent viscosity of the ferrofluid was determined. The advantage of this method is the small sample volume needed.