Revisiting the basis of transient rheological material functions: Insights from recoverable strain measurements

Abstract

Recent studies have shown that rheological material functions that can be linked to structural measures are defined in terms of the recoverable and unrecoverable strains [for example, Lee et al., Phys. Rev. Lett. 122, 248003 (2019)]. In this study, we explore the consequences of applying these ideas to transient nonlinear rheological tests, using new material functions including an elastic modulus and a flow viscosity defined in terms of the recoverable strain and the rate of acquisition of unrecoverable strain, respectively. These material functions, based on recoverable and unrecoverable strains (rather than total strain), are defined in the same way independent of the test protocol and provide the viscoelastic properties of a material in a clear and succinct way without requiring a priori knowledge of the constitutive model. At short times, we observe that for a self-assembled wormlike micellar solution, the new material functions exhibit a constant (plateau) modulus and a constant (zero-shear) viscosity independent of the applied shear rate, even under conditions that eventually lead to nonlinear responses. This observation quantitatively corroborates the intuitive picture of material deformation that when the material is close to equilibrium, it responds according to its linear viscoelastic material properties for both linear and nonlinear deformations. The fundamental, universal, and unifying nature of these new transient measures are showing promise in explaining a range of phenomena, including the Payne effect for filled polymers and yield stress materials, as well as suggesting new improved dimensionless groups for more accurate flow diagnosis. This work lays the foundation for measuring material functions that are directly related to the material structure and are agnostic of the testing protocol.