Contact Networks Enhance Shear Thickening in Attractive Colloid-Polymer Mixtures

Increased shear thinning arising due to strong attractive interactions between colloidal particles is thought to obscure shear thickening. Here, we demonstrate how moderate attractions, induced by adding a nonadsorbing polymer, can instead enhance shear thickening. We measure the rheology of colloidal suspensions at a constant particle volume fraction of ϕ=0.40 with dilute to weakly semidilute concentrations of three polyacrylamide depletants of different molecular weights. Suspensions containing large polymer exhibit increased shear thickening and positive first normal stress differences at high shear stress, and increased heterogeneous fluctuations in the boundary stress. These results are consistent with a friction-based model for shear thickening, suggesting that the presence of large, extended polymers induces the formation of near-spanning networks of interparticle contacts.Received 31 December 2018Revised 24 March 2019DOI:https://doi.org/10.1103/PhysRevLett.122.228003© 2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNon-Newtonian fluidsRheological propertiesPhysical SystemsColloidsPolymer solutionsPropertiesShear thickeningTechniquesConfocal imagingRheologyPolymers & Soft MatterInterdisciplinary PhysicsFluid Dynamics