(80c) The Cause of the G" Overshoot in Soft Yielding Materials Explained Via a Novel Rheological Protocol and a Simple Fluid Model

Materials that exhibit yielding behavior are used in many applications, from spreadable foods and cosmetics to direct write 3D printing inks and filled rubbers. Their key design feature is the ability to transition behaviorally from solid-like to fluid-like under sufficient load or deformation. Despite their widespread application, little is known about the dynamics of yielding in real processes, as the non-equilibrium nature of the transition impedes understanding.

We demonstrate a novel, iteratively-punctuated rheological protocol that combines strain-controlled oscillatory shear with stress-controlled recovery tests. This hybrid technique provides an experimental decomposition of recoverable and unrecoverable strains, allowing for solid-like and fluid-like contributions to a yielding material’s behavior to be separated in a time-resolved manner.

Using this protocol, we show that the overshoot in the loss modulus seen in materials that yield is caused by the transition from primarily solid-like, viscoelastic dissipation in the linear regime to primarily fluid-like, plastic flow at larger amplitudes. We compare and contrast this behavior from a wide variety of yielding materials to a viscoelastic liquid with no yielding behavior, where the contribution to energy dissipation from viscous flow dominates over the entire range of amplitudes tested.

Further, we develop a simple viscoelastic model based on the linear Jeffreys model that displays all the relevant rheological responses without the explicit inclusion of a yield stress term. We therefore return and contribute to the decades-old discussion of the yield stress myth.