(380c) Effects of Attractive Interparticle Interactions on Normal Stresses and Shear-Induced Migration of Colloids

Particles in flowing suspensions can move across streamlines to form regions with concentrations higher than the bulk through shear-induced migration.^{1, 2} This phenomenon has been observed in pipe or channel flow of suspensions, and during rheological measurements in Couette and cone-and-plate geometries. Controlling the microstructure of particles during flow hence requires understanding the mechanisms that drive particle migration. For suspensions of hard sphere particles, the migration is thought to arise from normal stress differences in the sample.² How attractive interactions between particles affect the normal stresses and/or the shear-induced migration is less studied. Here, we developed a model system in which to systematically measure normal stresses in colloid-polymer depletion mixtures, which are commonly used as simple models of attractive suspensions. We synthesized colloidal particles of a copolymer of trifluoroethyl methacrylate and tert-butyl methacrylate,³ which were nearly refractive-index and density-matched to 80 wt% glycerol in water with Tris buffer added to control the pH. To controllably vary the strength of attraction between the particles, we added poly(acrylic acid) at three concentrations to induce depletion interactions. The first normal stress differences, measured for suspensions formulated at constant particle volume fraction Φ = 0.40 and various concentrations of polymer, approximately represent the contributions of the polymer and colloidal suspensions. We also examined the resulting migration of the particles in a cone-and-plate geometry after rheology measurements at high shear rates to determine the effect of normal stress differences on the migration of attractive particles. These measurements provide insight into the physical processes underlying migration in complex mixtures, which is of practical significance for controlling microstructure during flow-based processing routes in particulate suspensions.

1. D. Leighton and A. Acrivos, Journal of Fluid Mechanics, 1987, 181, 415-439.

2. M. Frank, D. Anderson, E. R. Weeks and J. F. Morris, Journal of Fluid Mechanics, 2003, 493, 363-378.

3. T. E. Kodger, R. E. Guerra and J. Sprakel, Scientific Reports 2015, 5, 14635.