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[Display omitted] The elastic contribution to the fluid dynamics of wormlike micellar solutions makes these fluids unique due to the distinctive self-assembled micellar network formed by tubular micelles. Measured mesoscopic scales of the micellar network related to the degree of entanglement can give guidelines for understanding the origin of elastic forces and their effect on rheological response. Different experiments were made as flow curves, rotating the internal or external cylinder in a Couette geometry, small and large oscillatory shear tests, and linear shear banding observations, all of them to determine how elastic forces modify the rheological behavior in systems made of different ratios of hexadecyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal) and different ratios of CTAB/NaNO 3. Diffusive wave spectroscopy micro-rheology was also performed to measure the mean square displacements of microspheres embedded in the micellar fluids to obtain their high-frequency viscoelastic spectra. With this information, the entanglement index κ , the ratio of the total contour of the micelles to the entanglement length, was estimated and correlated with the rheological behavior. The entanglement index is a valuable piece of information to understand the origin of the contribution of the elastic forces from a molecular point of view on the fluid dynamics of wormlike micelle solutions. [ABSTRACT FROM AUTHOR]

Oscillatory shear testing, used to characterize the viscoelastic response of soft materials, is often divided into small, medium, and large amplitude oscillatory shear (SAOS, MAOS, and LAOS) regions. SAOS is a common test that gives us a unified analysis and interpretation of linear viscoelastic behavior, whereas understanding MAOS and LAOS is still an active area of research. While numerous mathematical techniques have been proposed, a consensus interpretation is still missing. Recently, our understanding of nonlinear behavior in the LAOS regime has been developed using iterative recovery tests. Recovery rheology decomposes the strain into two components, allowing an unambiguous interpretation of the nonlinear behavior in terms of sequences of recoverable and unrecoverable processes. In this work, we revisit the MAOS material functions for polyvinyl alcohol-borax hydrogel and worm-like micelles using recovery rheology. We show that two mathematical formalisms, the Chebyshev and sequence of physical processes analyses, provide competing physical interpretations when they are derived from the total strain, but provide unified interpretations when describing the decomposed strains. We, therefore, show that what has often been treated as a mathematical problem can instead be solved experimentally by acknowledging the extra information provided by recovery rheology. [ABSTRACT FROM AUTHOR]

Understanding how microscopic rearrangements manifest in macroscopic flow responses is one of the central goals of nonlinear rheological studies. Using the sequence-of-physical-processes framework, we present a natural 3D structure–rheology space that temporally correlates the structural and nonlinear viscoelastic parameters. Exploiting the rheo-small-angle neutron scattering (rheo-SANS) techniques, we demonstrate the use of the framework with a model system of polymer-like micelles (PLMs), where we unveil a sequence of microscopic events that micelles experience under dynamic shearing across a range of frequencies. The least-aligned state of the PLMs is observed to migrate from the total strain extreme toward zero strain with increasing frequency. Our proposed 3D space is generic, and can be equally applied to other soft materials under any sort of deformation, such as startup shear or uniaxial extension. This work therefore provides a natural approach for researchers to study complex out-of-equilibrium structure–rheology relationships of soft materials. [ABSTRACT FROM AUTHOR]