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Low-dimensional amplitude-intrinsic material functions for nonlinear viscoelasticity

N. Ashwin Bharadwaj (UIUC), Randy Ewoldt (UIUC)

Complex Fluids: Suspensions, Emulsions, and Gels

Wed 9:00 - 10:30

Barus-Holley 160

Rheological material functions are used to form our conceptual understanding of a material response. For a nonlinear rheological response, the possible deformation protocols and material measures span a high-dimensional space. Here we use asymptotic expansions to outline low-dimensional measures for describing leading-order nonlinear responses in large-amplitude oscillatory shear (LAOS). This amplitude-intrinsic regime is sometimes called medium amplitude oscillatory shear (MAOS). These intrinsic nonlinear material functions are only a function of oscillatory frequency, and not amplitude. Such measures have been suggested in the past, but here we clarify what measures exist and give physically meaningful interpretations. Both shear strain-control (LAOStrain) and shear stress-control (LAOStress) protocols are considered, and nomenclature is introduced to encode the physical interpretations. We measure experimentally the four intrinsic shear nonlinearities of LAOStrain for a polymeric hydrogel (PVA-Borax) and observe typical integer-power function asymptotics. The magnitudes and signs of the intrinsic nonlinear fingerprints are used to conceptually model the mechanical response and to infer molecular- and micro-scale features of the material.