Residual stresses in colloidal gels.

The spatial organization of particles in a colloidal gel controls the macroscopic properties, such as the rheological ones or the macroscopic gel stability. However, the suspension microstructure depends in a non-trivial manner on the colloidal interactions, the concentration and the flow history. Especially the effect of the latter is so far ill-understood and difficult to study in bulk suspensions. In the present work, well-characterized two dimensional suspensions are used to study the flow-induced structure in colloidal gels. Both a parallel band apparatus and a four roll mill are used to generate surface flows and the resulting steady state structures are analyzed using video microscopy. Because of the 2D nature of the suspensions, a high temporal and spatial resolution can be obtained, rendering the visualization for a wide range of surface coverages and flow rates possible. In this wide parameter space, two different regimes can be distinguished. At low surface coverages or high shear rates, only isolated flocs exist, whereas at high surface coverages or low shear rates, a transient network structure is encountered. In the two micro structural regimes the evolution of the characteristic length scales, the dominant orientation and anisotropy and the local aggregate or cluster densities is investigated. For individual aggregates, a simple analysis of hydrodynamic forces on the aggregates is used to evaluate the shear rate dependence of the aggregate size and aggregate density.

We examine the microstructural and mechanical changes which occur during oscillatory shear flow and reformation after flow cessation of an intermediate volume fraction colloidal gel using rheometry and Brownian Dynamics (BD) simulations. A model depletion colloid-polymer mixture is used, comprising a hard sphere colloidal suspension with the addition of non-adsorbing linear polymer chains. The results reveal three distinct regimes depending on the strain amplitude of oscillatory shear. Large shear strain amplitudes fully break the structure which results in a more homogenous and stronger gel after flow cessation. Intermediate strain amplitudes densify the clusters and lead to highly heterogeneous and weak gels. Shearing the gel to even lower strain amplitudes creates a less heterogonous stronger solid. These three regimes of shearing are connected to the microscopic shear-induced structural heterogeneity. A comparison with steady shear flow reveals that the latter does not produce structural heterogeneities as large as oscillatory shear. Therefore oscillatory shear is a much more efficient way of tuning the mechanical properties of colloidal gels. Moreover, colloidal gels presheared at large strain amplitudes exhibit a distinct nonlinear response characterized largely by a single yielding process while in those presheared at lower rates a two-step yielding process is promoted due to the creation of highly heterogeneous structures.

Segmental blood flow and rheological determinants in diabetic patients with peripheral occlusive arterial disease

Enhancement of flocculant-aided filtration performance of coal tailings under alternating low and high shear rates

Melt rheological properties of the blend of isotactic polypropylene (PP) and ethylene propylene diene rubber (EPDM) at varying ratios and of the glass fiber (GF) filled PP and PP/EPDM blend by varying both GF loading and blending ratio of the polyblend matrix are studied. Rheological measurements at 220°C in shear rate range 101−104s−1 were made on a capillary rheometer. Scanning electron micrographs of the extrudates are presented to show the morphology and the alignment of the glass fibers with respect to the flow direction. Variations of pseudoplasticity index, melt viscosity, and melt elasticity with EPDM content in PP/EPDM blend, and with varying GF content at any given composition of the matrix in PP/EPDM/GF ternary system, in the studied range are presented and discussed. Resultes on melt viscosity and melt elasticity show (i) reduced effect of GF at high shear rates on these properties and (ii) upward deviation of melt viscosity versus shear rate curve at low shear rates. A change in flow behavior in presence of GF is observed around a critical shear rate 2 × 103 s−1 and is attributed to the difference of interaction of GF and the dispersed rubber droplets at high and low shear rates. Elastic recovery showed nonequilibrium behavior at low shear rates.

The current investigations are related to presenting the characteristics of flow at very high and low shear rates along with the melting process over the geometry of Riga plate. Carreau nanofluid is most appropriate mathematical viscosity model that can deal with low and high shear rates. Transportation of energy is discussed through melting conditions, thermal radiation and viscous dissipation. In this study, electromagnetic hydrodynamic aspect of nanofluid is discussed by using electromagnetic surface (Riga plate) and this system generates Lorentz force parallel to wall that keeps its effect on velocity of nanofluid. Partial differential equations (PDEs) are moved in ordinary differential equations and numerical approach named Keller Box and Shooting scheme are exercised to fetch the numerical solution of the system. An augmentation in Q leads to an enrichment in an external magnetic field. The velocity as well as momentum boundary layer booms by the virtue of an improvement in an external magnetic field. The viscosity of the nanofluid increases by the virtue of a magnification in β. Nanofluid becomes more radiated and absorbs more heat as a result of magnification in Nr, which escalates the heat transfer rate.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Edward J. Stancik, Anne L. Hawkinson, Jan Vermant, Gerald G. Fuller; Dynamic transitions and oscillatory melting of a two-dimensional crystal subjected to shear flow. Journal of Rheology 1 January 2004; 48 (1): 159–173. https://doi.org/10.1122/1.1631425 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentThe Society of RheologyJournal of Rheology Search Advanced Search |Citation Search

Whole blood viscosity in microvascular angina and coronary artery disease: Significance and utility

Magnetorheological energy absorbers (MREAs) have been successfully deployed in occupant protection systems to protect against potentially injurious shock, crash and blast loads. These MREAs operate at shear rates upwards of 25 000 s-1, but magnetorheological fluids (MRFs) are typically characterized for shear rates up to 1000 s-1 in commercially available parallel counter-rotating disk rheometers. Because of the lack of availability of data at the required high shear rates, a Searle-type magnetorheometer (essentially a concentric cylinder rotating in a cup) was designed and fabricated at the University of Maryland. Using this magnetorheometer, two commercial MRFs were characterized over the shear rate range of 0-25 000 s-1 . It is shown that the rheometer was successful in replicating available characterization data at low shear rate, as well as quantifying high shear rate behavior as a function of applied field. In addition, it was shown that the Herschel-Bulkley constitutive model is appropriate and successfully characterized the apparent viscosity vs. shear rate behavior of the MRFs over this shear rate range. Experimental data demonstrate that an increase in field dependent yield stress can be realized over this entire shear rate range, so that MREAs can be designed using data taken with the magnetorheometer. Finally, the Mason number, which has been shown to be a useful non-dimensional number at low shear rates, also provides a useful physical interpretation at high shear rates.

The shear viscosity and normal stress difference of two filled and two unfilled thermotropic liquid crystal polymer (TLCPs) were studied. The rigid and rod like molecules of TLCPs orientate differently at different shear rates. Under low shear rate, the molecules tend to align in the direction of the flow but also tumble and wagging on their own axis. The abnormal orientation of the molecules also depends upon temperature, fillers contents, aspect ratio and elastic nature of LCP molecules. These behaviors lead to unusual rheological properties of LCPs, such as negative first normal stress difference for filled LCPs at low shear rates. But with the increase of shear rate, the molecules are oriented in the direction of flow, which lead to isotropic flow at high shear rates. The complicated rheological properties and characteristic anisotropic properties of LCPs are modelled by recently developed Leonov's viscoselastic constitutive equations. Simulation has been carried out using Mathematica software and the characteristic anisotropic properties of LCPs have been identified. The experimentally measured viscosities at high shear rate have been compared with model predictions. Moreover, the normal stress differences using at high shear rates have been estimated using Leonov's model, which is experimentally not accessible.

The electrorheology (ER) of suspensions based on polystyrene/polyaniline (PS/PANI) core/shell structured microspheres and those based on disk-like zeolite particles at different electric fields and particle volume fractions have been studied, respectively. Both types of ER fluids showed abrupt shear thickening under high electric fields and low shear rates, as well as shear thinning when the shear rate increased. A normalized method that considers the effects of electric field strength, shear rate and particle volume fraction was proposed to compare the rheological curves of the two ER fluids. The curves evaluated from the normalization method showed similar shear thinning at low shear rates and the hydrodynamic effect at high shear rates. Shear thinning represents the structure destroyed by shearing, and shear thickening at low shear regions indicates the dramatic structure change. The particle volume fraction and structure factor effects demonstrate that the mechanical contact between particles and the wall of the electrodes is crucial to the shear strength of ER fluids, indicating an electric/magnetic field modulated friction mechanism of the ER and magnetorheological (MR) effects.

Objective This article investigates the relationship of fractional flow reserve (FFR) with whole blood viscosity (WBV) in patients who were diagnosed with chronic coronary syndrome and significant stenosis in the major coronary arteries and underwent the measurement of FFR. Material and Method In the FFR measurements performed to evaluate the severity of coronary artery stenosis, 160 patients were included in the study and divided into two groups as follows: 80 with significant stenosis and 80 with nonsignificant stenosis. WBVs at low shear rate (LSR) and high shear rate (HSR) were compared between the patients in the significant and nonsignificant coronary artery stenosis groups. Results In the group with FFR < 0.80 and significant coronary artery stenosis, WBV was significantly higher compared with the group with nonsignificant coronary artery stenosis in terms of both HSR (19.33 ± 0.84) and LSR (81.19 ± 14.20) ( p < 0.001). In the multivariate logistic regression analysis, HSR and LSR were independent predictors of significant coronary artery stenosis (HSR: odds ratio: 1.67, 95% confidence interval: 1.17-2.64; LSR: odds ratio: 2.46, 95% confidence interval: 2.19-2.78). In the receiver operating characteristic (ROC) curve analysis, when the cutoff value of WBV at LSR was taken as 79.23, it had 58.42% sensitivity and 62.13% specificity for the prediction of significant coronary artery stenosis (area under the ROC curve: 0.628, p < 0.001). Conclusion WBV, an inexpensive biomarker that can be easily calculated prior to coronary angiography, was higher in patients with functionally severe coronary artery stenosis, and thus could be a useful marker in predicting the hemodynamic severity of coronary artery stenosis in patients with chronic coronary syndrome.

We present an experimental investigation of the rheological aspects of collective motion by the swimming Turbatrix aceti nematodes. We discover that these nematodes can significantly change the rheological properties of the suspension due to their body oscillations and form synchronized waves, which produce strong fluid flows. The strength of the collective state changes the shape of the interface where they swim in synchronization. We unravel that the effective viscosity of the nematode suspension at higher shear rates shows steady viscous behavior with time, where no significant effect of nematode activity is observed. For the first time, we have reported that at low shear rates, the activity effect is significant enough to generate oscillating viscous effects. In addition, we also measured the influence of the nematode concentration on suspension viscosity. This work opens a new way for understanding the rheological aspects of active matter under low and high shear rates. We illustrate these dynamics by showing that the force generated by these nematodes is sufficient to change the suspension rheology. The various aspects of nematodes, especially their large size and ease of culturing, make them a good model organism for experimental investigation as active fibers with oscillations. The oscillating behavior regulates the interfacial phenomenon and produces oscillatory rheological dynamics at low shear rates. The results of our work can be utilized to further study the novel metamaterials with negative viscosity, which have applications in healthcare and energy systems.

Relative hydraulic resistance, shear rate, and pressure in a vascular network integrate the network's architecture via fluid flow, and determine vein dynamics, with a time delay, in the prototypical organism Physarum polycephalum.

Mobility reduction is one of the critical parameters in polymer flooding. The EOR polymers have shear thinning bulk rheology, while core flood experiments with hydrolysed polyacrylamide (HPAM) in this paper show four different viscosity regimes. Through a systematic work, in which the molecular weight and degree of hydrolysis as well as the permeability and brine salinity were varied, the apparent viscosity was well-matched with theoretical models. The following four viscosity regimes were identified: (i) At low shear rates, the apparent Newtonian viscosity is less than the bulk viscosity; this effect is because of the inaccessible pore volume (IPV) with the polymer not entering the entire pore space. (ii) Shear thinning behaviour which is controlled by the polymers relaxation time, λ1. (iii) At high shear rates, the apparent viscosity increases by increasing the shear rate caused by elongation whose onset is controlled by a critical shear rate which depends on the relaxation time. (iv) At very high shear rates, the apparent viscosity decreases by increasing the shear rate because of mechanical polymer degradation caused by polymer rupture. The controlling parameter is the bulk relaxation time for the polymer. The critical shear rates for elongation and shear degradation increase when the effective molecular weight decreases. Typical injection shear rates in offshore matrix reservoirs exceed the critical shear rate for elongation and shear degradation. Consequently, high molecular weight HPAM will either have poor injectivity or cause fracturing. For injection into fractured wells, the shear rate is substantially reduced and the shear degradation can be avoided. Acrylamido-Propyl-Sulfonate (AMPS) co-polymers have similar apparent viscosity versus shear rate as the HPAM. However, the AMPS co-polymers seem to tolerate higher shear rates before degradation sets in. For comparison, core flood experiments with Xanthan have been performed. This polymer was shear stable and the shear thinning apparent viscosity was similar to the bulk viscosity. The polymers reduced the permeability and the permeability reduction is understood by the polymer size, i.e., the permeability reduction increases by increasing the molecular weight.