Paper Number
SC37 

Session
Suspensions & Colloids

Title
First normal stress difference of model attractive colloid polymer depletion mixtures

Presentation Date and Time
October 17, 2018 (Wednesday) 4:10

Track / Room
Track 1 / Galleria I

Authors (Click on name to view author profile)

1. Park, Nayoung (University of Houston, Chemical and Biomolecular Engineering)
2. Conrad, Jacinta C. (University of Houston, Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Nayoung Park and Jacinta C. Conrad
Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204

Speaker / Presenter 
Park, Nayoung

Text of Abstract
Non-adsorbing polymers added to colloidal suspensions induce controlled depletion attractions between the particles that modify the particle microstructure. While the effect of interparticle attractions on viscosity is well understood, it is not yet clear how normal stress differences change with attractive interactions. We studied the effect of adding polymers to colloidal suspensions at constant particle volume fraction of φ=0.40 on the shear rheology, including the first normal stress difference N₁. We used a new depletion system comprised of poly(2,2,2-trifluoroethyl methacrylate – co – tert-butyl methacrylate) particles [Kodger et al., Sci. Rep. 5, 14635 (2015)] that were refractive index- and density-matched in 80 (w/w)% glycerol in water. Gels induced by adding one of three polyacrylamide depletants (PAM) of differing molecular weight and dispersity (PDI) at similar normalized PAM concentrations in the free volume c/c* (where c* is the overlap concentration of the polymer) exhibited indistinguishable quiescent microstructure and dynamics. When the PAM M_w was small, the shear thickening exponent β of the suspension did not change with the addition of PAM up to c/c* = 1.2. Correspondingly, N₁ remained negative and only increased slightly with the addition of PAM. In the presence of large polymers, however, β increased slightly, irrespective of the polymer PDI, and N₁became positive at high shear rates. Analysis using a friction-based model [Wyart and Cates, Phys. Rev. Lett. 112, 098302 (2014); Guy et al., Phys. Rev. Lett. 115, 088304 (2015); Royer et al., Phys. Rev. Lett. 116, 188301 (2016)] suggested that the presence of large polymers drives the particles to form enough frictional contacts to increase N₁ to positive values. These results suggest that the sign of N₁ can be tuned in colloid + polymer mixtures through the M_w of the polymer additive.