Paper Number
GG2 

Session
Rheology of Gels, Glasses and Jammed Systems

Title
Strain shift in a model yield stress fluid – Evidence for a continuous yielding transition

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

Track / Room
Track 3 / Sheraton 5

Authors (Click on name to view author profile)

1. Griebler, James (University of Illinois at Urbana-Champaign, Department of Chemical and Biomolecular Engineering)
2. Donley, Gavin J. (Georgetown University, Department of Physics)
3. Wisniewski, Victoria (University of Illinois, Chemical and Biomolecular Engineering)
4. Rogers, Simon A. (University of Illinois at Urbana-Champaign, Department of Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
James Griebler¹, Gavin J. Donley², Victoria Wisniewski¹ and Simon A. Rogers^1
1Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801; ²Department of Physics, Georgetown University, Washington, DC 20057

Speaker / Presenter 
Griebler, James

Keywords
experimental methods; gels; polymer solutions; rheometry techniques

Text of Abstract
Yield stress fluids are often thought about in a piecewise manner as behaving like elastic solids below the yield stress and like generalized Newtonian liquids above it. This simplistic approach has defined their study for much of the last 50 years. In this work we study the behavior of Carbopol 980, a model yield stress fluid, using a new experimental protocol that shows that yielding is a gradual and continuous transition across a wide range of stresses. We make use of recent observations that the strain response of viscoelastic materials subjected to stress-controlled oscillations can oscillate predictably and understandably about a non-zero value. This strain shift only occurs when unrecoverable strain is acquired, or when a material has flowed. We show measurable strain shifts in Carbopol well below the yield stress determined by a Herschel-Bulkley fit to steady-shear flow measurements. We also show that the measured strain shift changes continuously as a function of stress amplitude, angular frequency, and applied stress phase angle. These experimental results are compared to a popular piecewise model, which predicts abrupt yielding, as well as a recently published continuous model that predicts flow below the yield stress as a coupling between recoverable and unrecoverable processes. We also make use of iterative recovery rheology to show how the strain shift is acquired in the first period through acquisition of unrecoverable strain, which oscillates about a non-zero value, while the recoverable strain oscillates about zero. This experimental study adds nuance to our understanding of yield stress fluids by showing that piecewise descriptions are oversimplifications and emphasizes the importance of recovery rheology in determining conditions under which materials flow.