Oblique instability of a stratified oscillatory boundary layer

The instability and dynamics of a vertical oscillatory boundary layer in a container filled with a stratified fluid are addressed. Past experiments have shown that when the boundary oscillation frequency is of the same order as the buoyancy frequency, the system is unstable to a herringbone pattern of oblique waves. Prior studies assuming the basic state to be a unidirectional oscillatory shear flow were unable to account for the oblique waves. By accounting for confinement effects present in the experiments, and the ensuing three-dimensional structure of the basic state, we are able to numerically reproduce the experimental observations, opening the door to fully analysing the impacts of stratification on such boundary layers.

Novel tensorial Thixo-Visco-Plastic framework for rheological characterization of human blood

Characterizing human blood, a complex material with a spectrum of thixo-elasto-visco-plastic properties, through the development of more effective and efficient models has achieved special interest of late. This effort details the development a new approach, the tensorial-enhanced-Thixo-Visco-Plastic model (t-e-TVP), which integrates elements from the proven Bingham and generalized Maxwell systems to create a more robust framework and subsequently cast into a tensorial format. Here, the elastic and viscoelastic stress contributions from the microstructure are superimposed upon the viscoelastic backbone solution for stress offered by the modified TVP frame. The utility of this novel model is tested against the contemporary tensorial-ethixo-mHAWB (t-ethixo-mHAWB) framework, a similar model with a greater number of parameters, using rheological data of human blood collected on an ARESG2 strain-controlled rheometer. The blood samples are parametrically and statistically analyzed, entailing the comparison of the t-e-TVP and t-ethixo-mHAWB models with their capacity to accurately predict small and large amplitude oscillatory shear as well as unidirectional large amplitude oscillatory shear flow in blood.

Visualizing and exploring nonlinear behavior, timescales, and mechanical signatures of human blood

Human blood is an excellent example of a thixo-elasto-visco-plastic (TEVP), shear-thinning fluid, with an apparent yield stress. We demonstrate a series of unique strain-controlled experiments to elucidate the evolving elastic and viscous properties of human blood and show the associated unique viscoelastic and thixotropic signatures. The experimental techniques and procedures outlined here, and the robust mechanical analysis framework offers a window into the complex nonlinearity of the relationship between the microstructure and mechanical properties of human blood. Rather than using the traditional discrete Fourier transform to analyze the data, two contemporary methods, the MITlaos, Chebychev analysis technique, and the other sequence of physical processes (SPP), are applied to these oscillatory tests. These methods are advantageous because they both allow for the analysis of large amplitude oscillatory shear flow. These frameworks will highlight how the blood undergoes thickening-thinning and hardening-softening cycles that are directly related to microstructures during large amplitude oscillatory shear flow. We show these results with a series of Cole-Cole and Lissajous-Bowditch plots, then compare to an aqueous solution of xanthan gum and glycerol. The analysis presented here is independent of a specific TEVP rheological model. (We acknowledge that the protein film on surface of blood air interface issue may exist; based on literature we estimate this to contribute no more than +0.375 mPa s to viscosity at, or 0.0375 Pa to the stress measurement at shear rates below 100 s−1.) BACKGROUND: Human blood is a thixo-elasto-visco-plastic (TEVP) material that exhibits unique fluctuations in mechanical properties based on physiology, and shear rate. We demonstrate new visual tools to help visualize and characterize these varied mechanical properties. OBJECTIVE: Our objective is to demonstrate contemporary visual and numerical tools to help visualize and characterize the varied mechanical properties of human blood. METHODS: Using the ARESG2 strain-controlled rheometer with double wall couette geometry and eight human blood donors, with lab test results, elastic and viscous properties are investigated using Series of Physical Processes (SPP) and MITLaos to both analyze and visualize the mechanical signatures of the blood. RESULTS: Variations of mechanical properties are shown via SPP generated Cole-Cole plots and MITLaos analysis. These variations are a function of physiological properties of blood on the day of the blood draw based on hematocrit, fibrinogen, cholesterol, triglycerides, and a host of other proteins and constituents. Each rheological experiment with blood is replicated with an analogous experiments with 0.04 wt% xanthan in glycerol, and water to demonstrate that the mechanical properties of the human blood, and its rheological signatures are unique to human blood. CONCLUSIONS: Human blood is proven to be a TEVP material, as shown on a series of Cole-Cole plots for eight different donors, at two different frequency and strain amplitude combinations. Variations in Cole-Cole plots for each donor are shown. MITLaos average mechanical properties are calculated and shown. Aggregated elastic and viscous projections and a Cole-Cole plot is shown for Donors 1–8, along with 95% confidence interval.

Steady and Oscillatory Shear Flow Behavior of Different Polysaccharides with Laponite®

The rheological behavior, in terms of steady and oscillatory shear flow, of Laponite® with different polysaccharides (alginate, chitosan, xanthan gum and levan) in salt-free solutions was studied. Results showed that a higher polymer concentration increased the zero-rate viscosity and decreased the critical strain rate (Cross model fit) as well as increasing the elastic and viscous moduli. Those properties (zero-rate viscosity and critical strain rate) can be a suitable indicator of the effect of the Laponite® on the shear flow behavior for the different solutions. Specifically, the effect of the Laponite® predominates for solutions with large critical strain rate and low zero-rate viscosity, modifying significantly the previous parameters and even the yield stress (if existing). On the other hand, larger higher polymeric concentration hinders the formation of the platelet structure, and polymer entanglement becomes predominant. Furthermore, the addition of high concentrations of Laponite® increases the elastic nature, but without modifying the typical mechanical spectra for polymeric solutions. Finally, Laponite® was added to (previously crosslinked) gels of alginate and chitosan, obtaining different results depending on the material. These results highlight the possibility of predicting qualitatively the impact of the Laponite® on different polymeric solutions depending on the solutions properties.

Microphase separation/crosslinking competition-based ternary microstructure evolution of poly(ether-b-amide)

The temperature dependence of the rheological properties of poly(ether-b-amide) (PEBA) segmented copolymer under oscillatory shear flow has been investigated.

Normal Stress Differences of Human Blood in Unidirectional Large-Amplitude Oscillatory Shear Flow

This work analyzes normal stress difference responses in blood tested in unidirectional large-amplitude oscillatory shear flow (udLAOS), a novel rheological test, designed for human blood. udLAOS mimics the pulsatile flow in veins and arteries, in the sense that it never reverses, and yet also nearly stops once per heartbeat. As for our continuum fluid model, we choose the Oldroyd 8-constant framework for its rich diversity of popular constitutive equations, including the corotational Jeffreys fluid. This work arrives at exact solutions for normal stress differences from the corotational Jeffreys fluid in udLAOS. We discover fractional harmonics comprising the transient part of the normal stress difference responses, and both integer and fractional harmonics, the alternant part. By fractional, we mean that these occur at frequencies other than integer multiples of the superposed oscillation frequency. More generally, predictions from the Oldroyd 8-constant framework are explored by means of the finite difference method. Finally, the generalized versions of both the Oldroyd 8-constant framework and the corotational Jeffreys fluid are employed to predict the nonlinear normal stress responses for the model parameters fitted to udLAOS measurements from three very different donors, all healthy. From our predictions, we are led to expect less variation in normal stress differences in udLAOS from healthy donor to donor, than for the corresponding measured shear stress responses.