Large Amplitude Oscillatory Shear Data Research Articles

Small and large deformation rheology on pizza cheese as an example of application to study anisotropic properties of food soft materials

Various rheological techniques were applied to study pizza cheese, as a model anisotropic material. Small Amplitude Oscillatory Shear (SAOS) was combined with Large Amplitude Oscillatory Shear (LAOS) rheology, and the data compared to large deformation tests and confocal microscopy. Pizza cheese was analyzed at various stages of processing: before/during mixing, freshly stretched and after 1 and 2 weeks of storage. Directional sampling and testing were of critical importance for data interpretation. LAOS data were evaluated using a recently developed software that, with visual interactive analysis, including layover plots and similarity maps, allowed to provide a rheological fingerprint of the cheese's process and storage-induced structural changes. This work demonstrated that by using complementary rheological and microstructural techniques it is possible to improve our understanding of the anisotropy at different length scales, and to distinguish the differences in the networks among samples after various processing stages.

Measurements of human blood viscoelasticity and thixotropy under steady and transient shear and constitutive modeling thereof

New in vitro measurements from two human donors for both steady and transient hemorheology are presented that are particularly suited for testing thixotropic and viscoelastic models for human blood and are made available to the community along with the relevant blood physiology. This more complete data set is analyzed within the context of recent thixo-elasto-visco-plastic models, and we improve upon the Horner-Armstrong-Wagner-Beris (HAWB) model to better model transient hemorheology. The improved model features an additional viscoelastic equation for the red blood cell rouleaux within blood, along with simplifications to the structure kinetics equation. The resulting model is fit to additional experimental data on human whole blood for a step shear change and steady shear experiments, and the corresponding parameter values are used to predict oscillatory shear moduli, transient large amplitude oscillatory shear, and unidirectional large amplitude oscillatory shear data. The modified HAWB model shows significant improvement in fitting and predicting the experimental data. Specifically, a significant overshoot is captured in step shear changes from a low shear rate to a moderate shear rate, and a plateau for the storage modulus is observed at low frequencies. Highly nonlinear behavior is observed for the third harmonic in unidirectional large amplitude oscillatory shear in the form of multiple extremum separated widely in frequency, which can be identified with the two dominant mechanisms for generating viscoelasticity in human blood, namely, rouleaux structuring and red blood cell deformation. We also discuss the potential applications and difficulties associated with the constitutive modeling of transient blood rheology and provide insight into microstructural sources of the stress contributions to the bulk flow behavior.New in vitro measurements from two human donors for both steady and transient hemorheology are presented that are particularly suited for testing thixotropic and viscoelastic models for human blood and are made available to the community along with the relevant blood physiology. This more complete data set is analyzed within the context of recent thixo-elasto-visco-plastic models, and we improve upon the Horner-Armstrong-Wagner-Beris (HAWB) model to better model transient hemorheology. The improved model features an additional viscoelastic equation for the red blood cell rouleaux within blood, along with simplifications to the structure kinetics equation. The resulting model is fit to additional experimental data on human whole blood for a step shear change and steady shear experiments, and the corresponding parameter values are used to predict oscillatory shear moduli, transient large amplitude oscillatory shear, and unidirectional large amplitude oscillatory shear data. The modified HAWB model shows sig...

Rheological study of different mashed potato preparations using large amplitude oscillatory shear and confocal microscopy

Texture is a major driver for mashed potato products. Measuring food texture via descriptive sensory analysis can be expensive and timeconsuming, so rheometry is often used as a method of estimating food texture. The objective of this study was to determine small- and largestrain rheological behavior of different mashed potato formulations. Rheological data, including large amplitude oscillatory shear data, and confocal microscopy images were collected. Comparison of rheological data and confocal images showed relationships between sample rheological behavior and sample structural damage were found. With the exception of freeze-dried samples, samples exhibiting greater starch damage exhibited more linear, fluid-like rheological behavior. Freeze-dried samples showed microfractures on the surface of the starch granules. It was hypothesized that these microfractures allowed amylose strands to leach from the granules without destroying structural integrity. Based on these results, mashed potato rheological behavior, and therefore texture, can be controlled by controlling extent of starch damage.

Nonlinear elasticity and cavitation of a triblock copolymer gel.

Polymer gels are subjected to large-strain deformation during their applications. The gel deformation at large-strain is non-linear and can often lead to failure of the material. Here, we report the large-strain deformation behavior of a physically cross-linked, swollen triblock copolymer gel, which displays unique strain-stiffening response at large-strain. Investigations were performed using large amplitude oscillatory shear (LAOS) and custom developed cavitation rheology techniques. The Gent constitutive equation, which considers finite extensibility of midblock, was fitted with the LAOS data, thereby, linking the estimated parameters from LAOS analysis to the structure of the gel. The pressure responses obtained from the cavitation experiments were modeled using neo-Hookean and Gent constitutive equations. Our results capture the failure behavior of a gel with finite chain extensibility, initiated from a defect within the gel.

Modeling of human blood rheology in transient shear flows

We offer here an extension of our previous work [Apostolidis and Beris, J. Rheol. 58, 607–633 (2014)] of modeling blood flow rheology in simple shear steady state flows to time-dependent conditions. The basis of our model is a scalar, structural, parameter thixotropic model. More specifically, we show that a modified version of the “Delaware model” [A. Mujumdar et al., J. Non-Newtonian Fluid Mech. 102, 157–178 (2002)] is capable of predicting the time-dependent shear flow rheology of blood at low and moderate values of shear rate. At steady state, the model reduces to the Casson constitutive model for low and moderate shear rate values consistent to the findings of our previously mentioned work. At high shear rates it reduces to a Newtonian model, correcting our previous model and consistent to data from the literature [Merrill and Pelletier, J. Appl. Physiol. 23, 178–182 (1967)]. Exploiting the existing parameterization developed before for the steady state Casson model and the Merrill and Pelletier steady state data, the transient thixotropic model introduces only three additional parameters. Each one of these parameters has a specific physical meaning: A zero-shear rate maximum strain, and two kinematic parameters governing the relaxation of the structural parameter and the elastic modulus, respectively. The model is able to naturally account for the additional yield strengthening effect attributable to the red blood cell rouleaux structures developed within blood. The proposed model can fit excellently the experimental data of Bureau et al. [Biorheology 17, 191–203 (1980)], on simple triangular steps in shear flow at low shear rates. The predictions of the present model are then validated comparing them against additional experimental data, collected on the same samples, either on triangular steps in shear flow but at higher shear rates or on rectangular step-up and step-down experiments. The model is further validated by comparing its predictions against recent large amplitude oscillatory shear data [P. C. Sousa et al., Biorheology 50, 269–282 (2013)]. In all these comparisons there is good, at least semiquantitative, agreement, with the observed discrepancies only appearing at the higher shear rates, where the isotropic description resulting from the use of a single scalar internal parameter to describe the blood microstructure naturally breaks down.

Flow instabilities in large amplitude oscillatory shear: a cautionary tale

In recent years, large amplitude oscillatory shear (LAOS) has become a prime tool to investigate the nonlinear rheology of complex fluids. More specifically, most studies use LAOS data as a macroscopic probe for hypothetical microscopic changes, sometimes successfully. Nevertheless, we would like to raise awareness on the potential impact of secondary flows triggered by instabilities in LAOS performed in curved shear flows exemplified by the Taylor–Couette geometry. First, we show that even for Newtonian fluids, where no micro-structural changes are expected, complex flow patterns can emerge in the LAOS data. Second, we stress the potential impact of similar effects in complex fluids by studying LAOS flows of a well-known shear-banding surfactant solution. Our hope is that our thorough study of the Newtonian case together with preliminary experiments on a complex fluid will reinforce the already successful analogy existing between inertial and elastic instabilities, suggest caution for future LAOS studies focused solely on a structural perspective, and open new research avenues combining flow instabilities and unsteady flows in complex fluids.