Steady shear microstructure in dilute colloid-polymer mixtures

The shear-induced microstructure in dilute colloid-polymer mixtures, where the presence of polymer induces a tuneable attractive interaction between the colloids, is investigated using quantitative confocal microscopy, over a wide parameter space including the shear rate, the polymer concentration, and two different polymer molecular weights corresponding to polymer/colloid size ratios of approximately 0.02 and 0.05. Overall, the imposition of low shear rates radically transforms the relatively uniform quiescent structure into one marked by long-range heterogeneities and pronounced segregation of dense clusters and voids. Increasing the rate of deformation effects a consistent decrease in the average cluster size and a gradual transition towards a more homogeneous structure through the redistribution of voids. Interestingly, at high shear rates, the suspension microstructure for the large molecular weight polymer is nearly insensitive to the polymer concentration, and primarily determined by the shear rate alone. The prominent microstructural features of these shear-induced transformations are quantified in detail and discussed in light of the competition between interparticle attraction and microscopic shear forces.