A Novel Method to Choose the Experimental Parameters in Large Amplitude Oscillatory Shear Rheology

Large Amplitude Oscillatory Shear (LAOS) rheology is a technique to analyze materials that are viscoelastic in nature. The raw values of stress and strain that were taken out from rheometer during the large amplitude oscillatory shear test are used in the constitutive models. The model parameters from the constitutive model are then analyzed on the materials being tested. Various test protocols and geometries will be used to analyze the materials of interest during LAOS rheological examination. The selection of test protocols and usage of geometry are less studied in testing various kinds of materials using LAOS. Cone and plate and parallel plate geometries are generally used for LAOS. The test protocols would be varying amplitude and varying frequencies. In the present work, quantification of relative performance of test protocols and geometry that have been used during the analysis of cross-linked poly vinyl alcohol hyaluronic acid (PVAHA) gels as material systems are studied using data envelopment analysis (DEA). The methodological approach using output oriented constant return to scale (CRS) and output oriented variable return to scale (VRS) are tested with the decision making units (DMU) as the geometry and test protocols used. These results are then combined with the Shannon's entropy to rank the efficient DMUs. Using Shannon's entropy combined with CRS and VRS, it is suggested that the use of parallel plate geometry with the test protocol of 0.5 rad/s and 50 frequency is best suitable for the cross-linked hyaluronic acid and poly vinyl alcohol gels examined.

Dynamics of Erythrocytes in Oscillatory Shear Flows: Effects of S/V Ratio

By combining a multiscale structural model of erythrocyte with a fluid-cell interaction model based on the boundary-integral method, we numerically investigate the dynamic response of erythrocytes in oscillatory shear flows...

The Normal Force Characteristic of a Novel Magnetorheological Elastomer Based on Butadiene Rubber Matrix Compounded with the Self-Fabricated Silly Putty

In this paper, a novel magnetorheological elastomer (MRE) was prepared by dispersing carbonyl iron particles (CIPs) into a composite matrix compounded by butadiene rubber (BR) and self-fabricated Silly Putty. The rate-sensitive and magneto-induced characteristics of normal force were experimental investigated to discuss the working mechanism. The results demonstrated that the normal force increased with the compression rate and the mass fraction of boron-silicon copolymer added to the composite matrix due to the formation of the more and more B-O cross bonds which could be blocked in the C-C cross-linked network of BR. Meanwhile, the magneto-induced normal force was positively correlated with the applied magnetic field strength and the compression strain due to the decreased gap between the centers of soft magnetic particles and the increased particle intensity of magnetization. Moreover, the magneto-induced normal force continued to enhance with the increase of compression strain because the CIP chains fixed in the C-C cross-linked network could bend to a radian and CIP chains in B-O cross-linked network could rupture to form more stable and intensive short-chain structures. Besides, a simplified model was deduced to characterize the mechanism of the generation of the magneto-induced normal force. Furthermore, the normal force varied stably with the oscillatory shear strain (less than 9%) at different magnetic induction intensities and suddenly reduced when the applied oscillatory shear strain was more than 9%.

Orientational arrest in dense suspensions of elliptical particles under oscillatory shear flows

We study the rheological response of dense suspensions of elliptical particles, with an aspect ratio equal to 3, under oscillatory shear flows and imposed pressure by numerical simulations. Like for the isotropic particles, we find that the oscillatory shear flows respect the Cox-Merz rule at large oscillatory strains but differ at low strains, with a lower viscosity than the steady shear and higher shear jamming packing fractions. However, unlike the isotropic cases (i.e., discs and spheres), frictionless ellipses get dynamically arrested in their initial orientational configuration at small oscillatory strains. We illustrate this by starting at two different configurations with different nematic order parameters and the average orientation of the particles. Surprisingly, the overall orientation in the frictionless case is uncoupled to the rheological response close to jamming, and the rheology is only controlled by the average number of contacts and the oscillatory strain. Having larger oscillatory strains or adding friction does, however, help the system escape these orientational arrested states, which are evolving to a disordered state independent of the initial configuration at low strains and ordered ones at large strains.

The Association Between Time-Varying Wall Shear Stress and the Development of Plaque Ulcerations in Carotid Arteries From the Plaque at Risk Study

Background and Purpose: Shear stress (WSS) is involved in the pathophysiology of atherosclerotic disease and might affect plaque ulceration. In this case-control study, we compared carotid plaques that developed a new ulcer during follow-up and plaques that remained silent for their exposure to time-dependent oscillatory shear stress parameters at baseline.Materials and Methods: Eighteen patients who underwent CTA and MRI of their carotid arteries at baseline and 2 years follow-up were included. These 18 patients consisted of six patients who demonstrated a new ulcer and 12 control patients selected from a larger cohort with similar MRI-based plaque characteristics as the ulcer group. (Oscillatory) WSS parameters [time average WSS, oscillatory shear index (OSI), and relative residence time (RRT)] were calculated using computational fluid dynamics applying the MRI-based geometry of the carotid arteries and compared among plaques (wall thickness>2 mm) with and without ulceration (Mann–Whitney U test) and ulcer-site vs. non-ulcer-site within the plaque (Wilcoxon signed rank test). More detailed analysis on ulcer cases was performed and the predictive value of oscillatory WSS parameters was calculated using linear and logistic mixed-effect regression models.Results: The ulcer group demonstrated no difference in maximum WSS [9.9 (6.6–18.5) vs. 13.6 (9.7–17.7) Pa, p = 0.349], a lower maximum OSI [0.04 (0.01–0.10) vs. 0.12 (0.06–0.20) p = 0.019] and lower maximum RRT [1.25 (0.78–2.03) Pa−1 vs. 2.93 (2.03–5.28) Pa−1, p = 0.011] compared to controls. The location of the ulcer (ulcer-site) within the plaque was not always at the maximal WSS, but demonstrated higher average WSS, lower average RRT and OSI at the ulcer-site compared to the non-ulcer-sites. High WSS (WSS>4.3 Pa) and low RRT (RRT < 0.25 Pa) were associated with ulceration with an odds ratio of 3.6 [CI 2.1–6.3] and 2.6 [CI 1.54–4.44] respectively, which remained significant after adjustment for wall thickness.Conclusion: In this explorative study, ulcers were not exclusively located at plaque regions exposed to the highest WSS, OSI, or RRT, but high WSS and low RRT regions had a significantly higher odds to present ulceration within the plaque even after adjustment for wall thickness.

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.