Thixotropy, non-monotonic stress relaxation, and the second law of thermodynamics

TL;DR

This paper presents a structural kinetic model that explains thixotropic behavior and non-monotonic stress relaxation in viscoelastic materials without violating the second law of thermodynamics, aligning well with experimental data.

Contribution

It introduces a simple formalism that captures aging and rejuvenation in viscoelastic materials, explaining complex behaviors while respecting thermodynamic principles.

Findings

Model predicts non-monotonic stress relaxation accurately.

Model demonstrates viscoelastic aging and rejuvenation.

No violation of the second law of thermodynamics occurs.

Abstract

Many thixo-viscoelastic 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 non-monotonic 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 non-monotonic stress relaxation very well.