Paper Number
TM24
Session
Rheometry: Advanced Techniques and Methods
Title
How to produce unbiased experimental results for time-dependent materials: Pre-shear with strain recovery
Presentation Date and Time
October 22, 2019 (Tuesday) 3:45
Track / Room
Track 1 / Room 305A
Authors
- Choi, Jiho (University of Illinois at Urbana-Champaign, Department of Chemical and Biomolecular Engineering)
- Rogers, Simon A. (University of Illinois at Urbana-Champaign, Department of Chemical and Biomolecular Engineering)
Author and Affiliation Lines
Jiho Choi and Simon A. Rogers
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801
Speaker / Presenter
Choi, Jiho
Text of Abstract
Most traditional pre-shear protocols can be classified into one of two classes based on the manner by which mechanical energy is added: one-directional shear and oscillatory shear. Residual stress after pre-shearing can occur as a result of both steady and oscillatory shearing. This residual stress, which can be small, can be a manifestation of unrecovered elastic strain. This residual stress and unrecovered elastic strain are caused by unwanted structural anisotropies. If structural anisotropy exists in the system, the interpretation of any subsequent experimental results must be modified to reflect the complex material state. Here, the criteria for an optimal pre-shear protocol for time-dependent materials are suggested. An optimal pre-shear should: (1) lead to a state of zero residual stress and zero recoverable strain, (2) lead to a material state that has rheological responses that are independent of the choice of shearing direction, and (3) not induce an irreversible (chemical or physical) change in the material. It is experimentally shown that addition of strain recovery step to traditional one-directional pre-shear guarantees not only repeatability, but also unbiased experimental results by comparing positive and negative directions of fixed shear rate with thixotropic fumed silica suspension. This result has implications for determining model parameters for thixotropic and aging materials when it is assumed the experiments are carried out on unbiased isotropic structures. Further, we apply pre-shear with strain recovery to traditional experiments such as frequency sweeps and the generation of flow curves. Traditionally, data points in those tests have been measured consecutively, but we apply our new pre-shear before measuring every single point of trajectory in separate manner. This method enables us to separate the effects of different shear histories.