Thixotropy is a phenomenon related to time dependent change in viscosity in the presence or absence of flow. The yield stress, on the other hand, represents the minimum value of stress above which steady flow can be sustained. In addition, the yield stress of a material may also change as a function of time. Both these characteristic features in a material strongly influence the steady state flow curve of the same. This study aims to understand the interrelation between thixotropy, yield stress, and their relation with the flow curve. In this regard, we study five thixotropic materials that show yield stress. The relaxation time of all the five systems shows power-law dependence on aging time with behaviors ranging from weaker than linear, linear to stronger than linear. Furthermore, the elastic modulus and yield stress have been observed to be constant for some systems while time dependent for the others. We also analyze the experimental behavior through a viscoelastic thixotropic structural kinetic model that predicts the observed experimental behavior of constant as well as time-dependent yield stress quite well. These findings indicate that a nonmonotonic steady-state flow curve in a structural kinetic formalism necessarily leads to time-dependent yield stress, while constant yield stress is predicted by a monotonic steady-state flow curve with stress plateau in the limit of low shear rates. The present work, therefore, shows that thixotropic materials may exhibit either monotonic or nonmonotonic flow curves. Consequently, thixotropic materials may show no yield stress, constant yield stress, or time-dependent yield stress.
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On the nature of flow curve and categorization of thixotropic yield stress materials
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March 2023
Research Article|
March 01 2023
On the nature of flow curve and categorization of thixotropic yield stress materials
Tulika Bhattacharyya
;
Tulika Bhattacharyya
1
Department of Chemical Engineering, Indian Institute of Technology Kanpur
, Kanpur, Uttar Pradesh 208016, India
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Alan R. Jacob
;
Alan R. Jacob
2
Department of Chemical Engineering, Indian Institute of Technology Hyderabad
, Hyderabad, Telangana 502284, India
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George Petekidis
;
George Petekidis
a)
3
IESL-FORTH and Materials Science and Technology Department, University of Crete
, Heraklion 71110, Greece
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Yogesh M. Joshi
Yogesh M. Joshi
a)
1
Department of Chemical Engineering, Indian Institute of Technology Kanpur
, Kanpur, Uttar Pradesh 208016, India
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J. Rheol. 67, 461–477 (2023)
Article history
Received:
September 02 2022
Accepted:
January 03 2023
Citation
Tulika Bhattacharyya, Alan R. Jacob, George Petekidis, Yogesh M. Joshi; On the nature of flow curve and categorization of thixotropic yield stress materials. J. Rheol. 1 March 2023; 67 (2): 461–477. https://doi.org/10.1122/8.0000558
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