Study on the rheological behaviors and micorstructure evolutions in concentrated silica suspension

Abstract

In this thesis, rheological behaviors and microstructure evolutions in concentrated silica suspension were investigated experimentally. First, spherical and monodispersed silica particles were synthesized by sol-gel method. The final morphology of silica was greatly influenced by concentration of ammonium hydroxide, water content and medium solvent. To enhance the colloidal stability, various compounds such as polyvinylalcohol (PVA), vinyltriethoxy silane (VTES) and (γ-methacryloxy propyl triethoxy silane (MPTES) were used. The adsorption of PVA onto silica surface was caused by either the affinity of PVA to the surface siloxane or the hydrogen bonding between the ionized silanol group and -OH group of PVA. Further, the silica surface was successfully modified with the silane coupling agents, and their chemical adsorptions were also proven by elemental analysis of carbon and hydrogen. Secondly, microstructure evolutions of weakly aggregated suspension of monodispersed silica particles were investigated using rheo-optical method. After synthesis of silica using sol-gel method, the transparent dispersion was prepared by dispersing the silica particles in the index-matching solvent, which was composed of equivolume mixture of ethylene glycol and glycerin. Both of the silica dispersions of 30wt% and 50wt% by weight exhibited the non-Newtonian behaviors, although the silica particles were dispersed in Newtonian medium. Furthermore, the deviation from Newtonian behavior was pronounced when the particle concentration was increased. With two different optical alignments designed in the present study, the flow-induced dichroism was measured. The measured dichroisms for 30wt% dispersions showed enhanced microstructure evolution especially at high shear rates. For 50wt% dispersion, the microstructural transition became more complicated and the sign of dichroism was changed with long relaxation behaviors. This was attributed to the formation of micro-domain as particle cl...