Time Resolved Linear And Non Linear Rheology Of Thixotropic And Aging Complex Fluids

Temporal changes in microstructure and relaxation dynamics are ubiquitously observed in materials such as hydrogels, food products and drilling fluids. These materials are in general known as mutating materials and the build-up or breakdown of microstructure is commonly both time- and shear-rate (or shear-stress)-dependent resulting in a range of complex phenomena collected under the term thixotropy. It is becoming increasingly im- portant to develop time-resolved rheometric techniques to quantify the behavior of mutating materials accurately. In the present study we first discuss the introduction of better time-resolved techniques in superposition rheometry. Conventional superposition rheometry consists of combining Small Amplitude Oscillatory Shear (SAOS) with a steady unidirectional shear rate to gain insight into the shear-induced changes to the viscoelastic properties of a complex fluid. 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 oscillatory deformation fields. This cross coupling can lead to unphysi- cal sign changes in the measured material properties, and makes it difficult to interpret the flow-induced mechanical properties. Recently, orthogonal superposition has been used to investigate the shear-induced anisotropy taking place in colloidal gels by comparing the transient evolution of orthogonal moduli with the parallel moduli immediately after cessa- tion of shear. However, probing transient evolution using the OSP technique can be chal- lenging 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 of superimposing fast optimally windowed chirp (OWCh) deformations or- thogonally to the shear deformation which substantially reduces the measurement time. We evaluate the changes in the rate-dependent relaxation spectrum in the direction of applied unidirectional shear rate and in the orthogonal direction deduced from the damping function and orthogonal moduli data respectively. We measure systematic changes between the two spectra measured in orthogonal directions thus revealing and quantifying flow-induced anisotropy in the alginate gel. Secondly, we develop a signal processing technique to monitor accurate temporal evolution of the complex modulus for a specified deformation frequency. Oscillatory rheometric techniques such as Small Amplitude Oscillatory Shear (SAOS) and, more recently, Large Amplitude Oscillatory Shear (LAOS) are now quite widely used for rheological characterization of the viscoelastic properties of complex fluids. However, the conventional application of Fourier transforms for analyzing oscillatory data assume the signals are time- translation invariant, which constrains the rate of mutation of the material to be extremely small. This constraint makes it difficult to accurately study shear-induced microstructural changes in thixotropic and gelling materials. We explore applications of the Gabor transform (a Short Time Fourier Transform (STFT) combined with a Gaussian window), for providing optimal joint time-frequency resolution of a mutating material's viscoelastic properties. First, we show using simple analytic models that application of the STFT enables extraction of useful data from the initial transient response following the inception of oscillatory flow. Secondly, using measurements on a Bentonite clay we show that using a Gabor transform enables us to more accurately measure rapid changes in both the storage and loss modulus with time, and also extract a characteristic thixotropic/aging time scale for the material. Finally, we consider extension of the Gabor transform to non-linear oscillatory deformations using an amplitude-modulated input strain signal, in order to track the evolution of the Fourier-Chebyshev coefficients characterizing thixotropic fluids at a specified deformation frequency. We show that there is a trade-off between frequency and time resolution (effectively a rheological uncertainty principle). We refer to the resulting test proto col as Gaborheometry and construct an operability diagram in terms of the imposed ramp rate and the mutation time of the material. This unconventional, but easily implemented, rheometric approach facilitates both SAOS and LAOS studies of time-evolving materials, reducing the number of required experiments and the data post-processing time significantly. Finally, we use the time-resolved techniques developed in this thesis to understand the thixotropic aging behavior of bentonite dispersions. In soft glassy materials such as ben- tonite clays, the relaxation dynamics and the microstructure slowly but continuously evolve with time to progressively form more stable structures. We investigate and quantify this complex aging behavior of bentonite dispersions by measuring the evolution in the linear viscoelastic behavior at different age times and temperatures. We model the linear viscoelastic properties using a material time domain transformation and a fractional Maxwell gel model which allows us to develop a rheological master curve to quantify and predict the aging behavior of this soft glass over a range of temperatures and time scales. The time-resolved rheometric techniques and procedures for quantifying the rheology of rapidly mutating complex fluids can be extended to a wide range of soft materials and allows us to obtain insight into how microstructural changes drive the evolution in the bulk rheological behavior for thixotropic and aging materials.