Studies of the effects of shear on colloidal aggregation and gelation using small angle light scattering

by Mokhtari, Tahereh

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.