Paper Number
IN12 

Session
Flow Induced Instabilities and Non-Newtonian Fluids

Title
An experimental explanation of the G’’ overshoot in yield stress soft materials

Presentation Date and Time
October 21, 2019 (Monday) 4:35

Track / Room
Track 4 / Room 305B

Authors (Click on name to view author profile)

1. Donley, Gavin J. (University of Illinois at Urbana-Champaign, Department of Chemical and Biomolecular Engineering)
2. Rogers, Simon A. (University of Illinois at Urbana-Champaign, Department of Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Gavin J. Donley and Simon A. Rogers
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801

Speaker / Presenter 
Rogers, Simon A.

Text of Abstract
In an effort to gain a deeper understanding of the rheology behind the yielding transition, we perform experimental decompositions of the rheological response of two model yield stress fluids, a Carbopol microgel and a concentrated colloidal silica suspension, to transient shear strains. These decompositions are achieved at different amplitudes of applied shear by means of oscillatory stress jump tests (in the case of stress) and oscillatory shear/recovery tests (in the case of strain). The flow cessation and recovery steps are performed at multiple points over the course of a period of oscillation, in order to map out the decomposition in a thorough manner. The stress jump tests separate stress acquired through elastic and inelastic processes and show that nearly all the stress in the system is elastic under oscillatory shear. The shear/recovery tests, on the other hand, allow for recoverable and unrecoverable strains to be resolved, and show that when above the yield point, both of these components contribute significantly to the overall material response . We utilize the interplay between these two strain components to develop a more comprehensive rheological picture of the yielding behavior in soft materials. We demonstrate that the overshoot in G’’ seen in many amplitude sweeps is the result of the transition between retardation dominated dissipation at small amplitudes and flow dominated dissipation at large amplitudes. These results suggest that any numerical decompositions for yield stress materials need to consider the existence of both unrecoverable viscoplastic flow above the yield point and a recoverable strain which persists once yielding has taken place.