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

Abstract

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.