(440f) Shear-Induced Structure and Migration of Colloidal Particles in Concentrated Polymer Solutions

Suspensions with a viscoelastic continuous phase can exhibit interesting and unexpected microstructural features. One of these is the formation of small, vorticity-aligned particle clusters in simple shear, which has occasionally been observed in the past [e.g., DeGroot et al. (J. Colloid Interface Sci., 1994) and Pasquino et al. (Rheol. Acta, 2010)], without fully uncovering the underlying mechanism. We used rheo-SALS (in situ small-angle light scattering on a rheometer) to probe the dynamics of this phenomenon for a suspension of colloidal silica particles in shear-thinning entangled polymer solutions. It was found that shear rates in the shear-thinning regime resulted in particle aggregation and orientation of aggregates along the vorticity axis. In contrast, application of strain in the Newtonian, low-shear regime did not yield clusters, and even led to restoration of a uniform particle distribution in an already ordered, pre-sheared suspension. Comparison of the SALS patterns with simultaneously measured viscosity data provided interesting insight into the microstructural evolution in these suspensions under shear, revealing the decoupled dynamics of aggregation and flow orientation. Additional experiments with a Boger fluid that exhibits less shear-thinning, but enhanced normal stresses showed that these effects were predominantly caused by shear-thinning.

In addition to the simple shear flow experiments, these dilute colloidal suspensions were also subjected to channel flow through rectangular glass capillaries. Concentration profiles were determined via confocal microscopy. Depending on shear rate, cross-channel migration of particles was observed in different directions: toward the center of the channel at low shear rates and toward the walls at high shear rates.

Controlling and optimizing the migration, aggregation and alignment of colloidal particles in concentrated polymer solutions is highly relevant for the processing of “mixed matrix membranes”, which combine a continuous polymer phase with embedded inorganic particulates that enhance the membrane properties.