Aggregation Effects on the Rheological, Sedimentation, and Drying Behavior of Colloidal Silica Suspensions
Guo, Joe Jiyou
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https://hdl.handle.net/2142/82900
Description
Title
Aggregation Effects on the Rheological, Sedimentation, and Drying Behavior of Colloidal Silica Suspensions
Author(s)
Guo, Joe Jiyou
Issue Date
1998
Doctoral Committee Chair(s)
Lewis, Jennifer A.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Language
eng
Abstract
Aggregation effects on the properties of aqueous and nonaqueous colloidal silica (SiO$\sb2)$ suspensions were investigated via a combination of theoretical modeling of interparticle interactions and experimental studies of their rheological (shear and compressive), sedimentation, and drying behavior. In aqueous solutions at low salt concentration, stable suspensions were predicted to form. At higher salt concentrations or in toluene, a weakly flocculated system was predicted to form when short-range effects were accounted for; whereas a strongly flocculated system was predicted to form in their absence. Rheological studies revealed that SiO$\sb2$ suspensions at low salt concentrations exhibited Newtonian behavior indicative of a dispersed system. Their osmotic pressure was well described by the modified Carnahan-Starling equation. Further salt additions produced weakly flocculated suspensions. SiO$\sb2$ suspensions in toluene displayed the strongest shear thinning behavior, highest apparent viscosity and compressive yield stress at a given volume fraction indicative of a strongly flocculated system. Sedimentation studies showed that dispersed suspensions exhibited a constant settling velocity. Aggregated systems exhibited a more complicated settling behavior. Cluster growth in highly-salted, aqueous systems followed RLCA mode $(D\sb{f}=2.35),$ revealing the underlying importance of short-range repulsive interactions in such systems. In contrast, cluster growth in the nonaqueous system followed DLCA mode $(D\sb{f}=1.83),$ highlighting the significance of short-range effects solvation effects in this system. Remarkably different drying stress histories were observed for films produced from suspensions of varying stability. In each case, $\rm\sigma\sb{max}$ was found to coincide with fully saturated state where the supersaturated region disappeared. In the region of stress rise, the measured stress was shown to correspond to the osmotic pressure or compressive yield stress of the dispersed and flocculated systems, respectively, and $\rm\sigma\sb{max}$ reflected these values at $\rm\Phi\sb{sat}.$ The stress decay beyond $\rm\sigma\sb{max}$ coincided with an emptying of liquid in the pores. For systems containing salt additions, residual stresses were observed due to salt bridging effects. Structural rearrangement occurred in the drying films due to capillary-induced effects. These findings demonstrates the importance of extending classical models (i.e., DLVO theory) for predicting interparticle interactions to account for short-range structural contributions.
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