The benefits of a formalism built on recovery: Theory, experiments, and modeling

A new rheological formalism based on the ideas of recovery is presented. Our new formalism contains recoverable and unrecoverable contributions to arbitrary deformations. The introduction of the two displacement gradients leads to two distinct measures of strain and strain rates, which highlights the importance of performing recovery experiments. Having established the new formalism, we show the benefits of this way of thinking by performing transient step strain and startup shear recovery measurements in a wide range of shear strains and shear rates on a model viscoelastic solution. With recovery, we show clear similarities in the material behavior between the two test protocols. The resultant recovery material functions – recoverable modulus and flow viscosity – allow the development of a new constitutive model, which consists of nonlinear elastic and viscous functions, along with a retarded viscous term. The predictions of the model are compared favorably with the experimental data, including responses to extremely large step strains. These observations allow us to revisit the transient entanglement length, relaxation time, and damping function based on the idea of recovery rheology. The present findings suggest a clear correlation exists between microstructural evolution and recoverable and unrecoverable components and provide a new direction for the exploration of the relation between recovery material functions and material responses under different dynamic flows.