OrthoChirp: A fast spectro-mechanical probe for monitoring transient microstructural evolution of complex fluids during shear

Abstract

Shear-induced microstructural changes are observed ubiquitously in materials as diverse as hydrogels, or drilling fluids, as well as many consumer products. Superposition rheometry, consisting of superimposing Small Amplitude Oscillatory Shear (SAOS) on top of a constant rate unidirectional shear has often been used to gain insight into these shear-induced changes. Orthogonal superposition (OSP), in which the two modes of deformation are perpendicular, has been preferred over parallel superposition to avoid non-linear cross-coupling of the steady shear and small amplitude oscillatory deformation fields. This cross-coupling can lead to unphysical sign changes in the measured material properties, and makes it difficult to unambiguously interpret the flow-induced mechanical properties. Recently, orthogonal superposition has been used to investigate the shear-induced anisotropy produced in colloidal gels by comparing the transient evolution of the orthogonal moduli with the parallel moduli immediately after cessation of shear. However, probing transient evolution using the OSP technique can be challenging for rapidly mutating complex materials which evolve on time scales comparable to the time scale of the experiment. Using a weakly associated alginate gel, we demonstrate the potential to substantially reduce the measurement time by superimposing fast Optimally Windowed Chirp (OWCh) deformations orthogonally to the steady shearing deformation. We evaluate the changes in the rate-dependent relaxation spectrum in the direction of the applied unidirectional shear rate, as well as in the orthogonal direction, by measuring the damping function and shear-induced changes in the orthogonal moduli respectively. We observe systematic differences between the two sets of spectra that we measure in the two orthogonal directions, enabling us to quantify the flow-induced generation of microstructural anisotropy in the alginate gel. • Reduction of measurement time for orthogonal superposition (OSP) rheometry. • Possible to superpose optimally windowed chirps (OWCh) orthogonally to steady shear. • Shear flow induces microstructural anisotropy in weakly associating alginate gels. • Observation of transient evolutions in rapidly mutating complex materials • Different rate-dependent relaxation spectra in orthogonal directions.