(8c) Shear thickening and normal stress differences of colloid + polymer mixtures

The rheological or flow properties of materials, such as viscosity, greatly affect the processibility of products. Whereas viscosity is a commonly measured rheological metric, normal stress differences, which are more difficult to measure, inherently impact processibility for non- Newtonian materials. For instance, non-zero first normal stress differences give rise to the Weissenberg (rod-climbing) effect in concentrated polymer solutions. Such behavior would greatly decrease the efficiency of mixing procedures. Many soft material products are non- Newtonian, such as suspensions of colloids with polymers. Addition of non-adsorbing polymers to suspensions of stable colloids gives rise to depletion attractions between the particles, resulting in significant changes in the microstructure of the particles. In depletion mixtures, the size and concentration of the polymer additive control the range and strength, respectively, of the interparticle attractions. Although the viscosity of colloid + polymer suspensions has been widely studied, trends in normal stress differences remain incompletely understood. In this work, we measured the effect of polymer depletant size and concentration on the rheology of colloidal suspensions at a constant, moderate volume fraction. By varying the molecular weight of the polymer depletant over approximately one order of magnitude, we observed that the presence of large polymer depletants enhances shear-thickening and changes the sign of the first normal stress difference. These rheological signatures are reminiscent of the onset of frictional interactions in dense suspensions without polymers. The same model used to describe the viscosity of such suspensions quantitatively fits the viscosity of the attractive suspensions, suggesting that the large polymer depletants enhance frictional interactions between particles at high shear rates. This change in rheology is expected to significantly alter the processibility of suspensions during flow.