Paper Number
GG43 

Session
Rheology of Gels, Glasses and Jammed Systems

Title
Non-monotonic stress relaxation in a “simple” yield stress fluid

Presentation Date and Time
October 12, 2022 (Wednesday) 2:10

Track / Room
Track 3 / Sheraton 5

Authors (Click on name to view author profile)

  1. Owens, Crystal E. (MIT)
  2. McKinley, Gareth H. (Massachusetts Institute of Technology, Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Crystal E. Owens1 and Gareth H. McKinley2
1MIT, Cambridge, MA 02139; 2Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02142

Speaker / Presenter 
Owens, Crystal E.

Keywords
computational methods; glasses

Text of Abstract
Yield stress fluids show a menagerie of rheological responses, with a range of time-, rate-, and history-sensitive behaviors. Here we report a new time-dependent effect: a non-monotonic stress relaxation/growth sequence in response to a small (γ <5%) imposed step strain in a carbopol gel showing no thixotropy or aging. This “simple” yield stress fluid behavior was evidenced by a lack of stress overshoot in start-up of steady shear rate tests, repeatability of all types of experiments including creep tests without preconditioning, and a time-invariant (over 12 months) rheological flow curve. Our rheometric protocol consists of three steps in series:

  1. Preconditioning at a high constant shear rate (0.1-100 s-1) followed by
  2. A waiting period at fixed zero stress (for 10 = t_w = 3,600 s) and
  3. A step in strain to probe the material’s elastoplastic response at low strains corresponding to 0.01-10% of the material yield strain.
In response to the imposed step strain, material shear stress relaxes slowly, as we would observe in viscoelastic polymeric liquids. However, instead of a continued monotonic decay, the shear stress turns and increases logarithmically in time up to levels of 20% of the material yield stress (100 Pa), far exceeding the initial elastoplastic response to the probe strain. Intriguingly, this stress increase exhibits memory of the direction of the initial pre-shear (step 1), but not the direction of the probe strain (step 3), indicating long-term effects of residual strain from preconditioning, even though the carbopol sample is nominally isotropic and non-thixotropic. We characterize the features of this anomalous non-monotonic response via simulations of time-varying rheological models to elucidate underlying microstructural anisotropy and to give continuum level insights to this time-dependent but non-thixotropic effect. Finally, we explore the possible use of this phenomenon for data storage.