Doctor of Philosophy / Department of Physics / Christopher M. Sorensen / We investigated the effect of shear on the structure and aggregation kinetics of unstable
colloids using small angle light scattering. We used an aqueous suspension of 20 nm polystyrene
latex microspheres and MgCl[subscript]2 to induce aggregation. The sample was only sheared once for
approximately 33 sec at different times, typically 1 min, 5 min, or 15 min, after the onset of
aggregation. The average shear rate was in the range of 0.13 - 3.56 sec[superscript]−1 , which was in a laminar
regime. The unsheared sample gelled after ca. 45 min. When the sample was sheared soon after
the onset of aggregation, the aggregation followed the diffusion limited cluster cluster
aggregation (DLCA) kinetics to yield D[subscript f] = 1.80 [plus or minus] 0.04 aggregates unaffected by the shear. The
gel time also remained the same as the unsheared gel. Shearing at later stages of aggregation
shortened the gel time and enhanced the scattered light intensity significantly indicating rapid
growth. Then, depending on the shear rate, there were three different behaviors. At high shear
rates, the aggregate structure was inhomogeneous after the shear was stopped with a crossover in
slope in the scattered light intensity versus q, to imply hybrid superaggregates with two different
fractal dimensions. At intermediate shear rates far from the gel point, there was a similar
crossover after the shear was stopped; however, the fractal dimension regained 1.80 [plus or minus] 0.04 at the
gel point. At low shear rates, the aggregation rate was increased, but the aggregate structure was
uniform, and the fractal dimension remained 1.75 [plus or minus] 0.05.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/522 |
| Date | January 1900 |
| Creators | Mokhtari, Tahereh |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
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