Paper Number
GG6 

Session
Rheology of Gels, Glasses and Jammed Systems

Title
Activated dynamics theory of the transient and steady state shear rheology of ultra-dense glass-forming colloidal suspensions

Presentation Date and Time
October 10, 2022 (Monday) 11:30

Track / Room
Track 3 / Sheraton 5

Authors (Click on name to view author profile)

1. Schweizer, Kenneth S. (University of Illinois at Urbana-Champaign)
2. Ghosh, Ashesh (Stanford University, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Kenneth S. Schweizer¹ and Ashesh Ghosh^2
1University of Illinois at Urbana-Champaign, Urbana, IL 61801; ²Department of Chemical Engineering, Stanford University, Palo Alto, CA

Speaker / Presenter 
Schweizer, Kenneth S.

Keywords
theoretical methods; colloids; glasses; suspensions

Text of Abstract
We formulate a microscopic force-based theory for the continuous startup shear rheology of ultra-dense Brownian hard sphere fluids and colloidal suspensions. The approach is built on the Elastically Collective Nonlinear Langevin Equation theory of coupled local cage-nonlocal collective elasticity activated structural relaxation in equilibrium, and its microrheology-inspired generalization to treat deformation. The central quantity is a dynamic free energy that is a function of instantaneous particle displacement, which depends on thermodynamic state, the static structure factor, and deformation variables. Important physical elements include how hopping driven relaxation is accelerated due to stress-induced barrier reduction, and strain-induced disordering of the cage structure. Deformation-induced change of local structure is predicted to be not crucial for understanding the steady state response. The theory is in good accord with experiments and simulations for: (a) the onset of nonlinearity at remarkably low values of renormalized Peclet number, (b) power law shear thinning of the alpha relaxation time and viscosity, (c) a flow curve of the Herschel-Buckley form, (d) an apparent static yield stress that grows nearly exponentially with packing fraction, and (e) post-yield reduction of the dynamic heterogeneity of structural relaxation. In contrast, understanding the rich behavior of the stress overshoot requires an explicit treatment of the softening of local pair correlations under deformation and its self-consistently coupled rheological consequences. With this new ingredient, the theory nearly quantitatively captures the most striking features of the stress overshoot: a roughly logarithmic growth of the overshoot strain with rate, a non-monotonic variation of its amplitude with shear rate, and a strong reduction of its amplitude with increasing packing fraction. In all rheological regimes deformation-modified activated hopping is important.