Elucidating the G″ overshoot in soft materials with a yield transition via a time-resolved experimental strain decomposition

Abstract

Materials that exhibit yielding behavior are used in many applications, from spreadable foods and cosmetics to direct write three-dimensional printing inks and filled rubbers. Their key design feature is the ability to transition behaviorally from solid to fluid under sufficient load or deformation. Despite its widespread applications, little is known about the dynamics of yielding in real processes, as the nonequilibrium nature of the transition impedes understanding. We demonstrate an iteratively punctuated rheological protocol that combines strain-controlled oscillatory shear with stress-controlled recovery tests. This technique provides an experimental decomposition of recoverable and unrecoverable strains, allowing for solid-like and fluid-like contributions to a yield stress material’s behavior to be separated in a time-resolved manner. Using this protocol, we investigate the overshoot in loss modulus seen in materials that yield. We show that this phenomenon 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 with a viscoelastic liquid with no yielding behavior, where the contribution to energy dissipation from viscous flow dominates over the entire range of amplitudes tested.

Charge‐Induced Structural Changes of Confined Copolymer Hydrogels for Controlled Surface Morphology, Rheological Response, Adhesion, and Friction

“…It is known that the storage modulus depends on the size, type, and number density of microstructural domains, and composition. [ 31 ] This similarity indicates that the presence of the spherical microphase does not impact the rheological response on a macroscale. In contrast, the storage modulus of grafted samples with higher AAc content is ≥71 kPa, while it remains <10 kPa for the free‐swelling hydrogels.…”

“…Such overshoots have recently been shown to be due to the acquisition of unrecoverable plastic strains typical of viscoelastic liquids, while the smaller amplitude behavior is dominated by recoverable strain behaviors typical of viscoelastic solids. While the precise details of how and when materials yield under oscillatory shearing are the subject of ongoing studies, [ 31 ] it is clear that the yield strain of the hydrogel scales with the point at which the storage and loss moduli cross over. We see here that extrapolations of behaviors for compositions other than 40% AAc beyond the amplitudes tested indicate that the stiffer hydrogels are also more brittle.…”

Physical aging in aqueous nematic gels of a swelling nanoclay: sol (phase) to gel (state) transition

“…47 Another possible explanation relates G 00 overshoot to how the strain is acquired, from recoverable (elastic) at small amplitudes to unrecoverable (plastic) at larger amplitudes. 48 This also implies the presence of gradual yielding rather than single yield stress in yield stress fluids. 49 Therefore, the growing strength of G 00 overshoot as a function of concentration (Fig.…”

Gelation and Yielding Behavior of Polymer-Nanoparticle Hydrogels

“…At a constant polymer concentration, increasing the nanoparticle content led to greatly increased the strain-to-yield, which exceeded 500% strain for several formulations. Additionally, at a higher nanoparticle content an increase in the G" is observed during yielding, suggesting a significant deformation energy is lost by de-caging of the nanoparticles during the yielding response 18,19,29 . In contrast, increasing the polymer content at a constant nanoparticle content exhibited the opposite effect.…”