Thixotropy, nonmonotonic stress relaxation, and the second law of thermodynamics

Many thixoviscoelastic materials have been reported to undergo enhancement in elastic modulus with time and decrease in the same under application of deformation field. Incorporation of this feature in a viscoelastic structural kinetic model has an apparent possibility of violating the second law of thermodynamics. Furthermore, in a related experimental observation, stress has been reported to undergo a nonmonotonic change as a function of time under the application of constant strain. We analyze both these scenarios through a structural kinetic model that shows viscoelastic aging but undergoes rejuvenation only due to the viscous rate of strain. We observe that such formalism does not violate the second law. Interestingly, the proposed simple formalism predicts the experimental observation of the nonmonotonic stress relaxation very well.(c) 2021 The Society of Rheology.

Automated summary

This study investigates the characteristics of the elastic modulus and stress changes over time in thixoviscoelastic materials. By analyzing a structural kinetic model, the phenomenon of viscoelastic aging and rejuvenation is observed, and it is shown that this form does not violate the second law of thermodynamics. Moreover, the proposed simple formalism successfully predicts the nonmonotonic stress relaxation observed experimentally.