Disaggregation Shear Stress Research Articles

Ultrasound scattering from silica fume clusters: in-line shear flow dynamics ofhydrophilic or partially hydrophobic silica fume fillers in melts of non-polar polymericsystems

Shear flow dynamics of fractal aggregates are investigated by ultrasound scattering tostudy shear induced disruption processes of hydrophilic polydisperse silica fume fillers inmelts of non-polar polymeric systems (polypropylene). A rheo-acoustical modelin the low frequency scattering regime only involving structural parameters isproposed. Flow-dependent changes of the ultrasound scattering power per unit ofvolume from hydrophilic silica fume aggregates during extrusion are analyzed inthe frame of the proposed rheo-acoustical model. In a second part, equilibriumstructures of polymer coated silica fume fillers are investigated to elucidate thecoupled intra- and inter-filler interactions. Surface chemistry of silica fume fillerswas modified by grafting amphiphilic molecules with the same hydrophobic tail(long alkyl chains) and various hydrophilic polar heads (amine, carboxylic acid orhydroxyl groups) to obtain a range of hydrophobic fumed silica units. The effectsof the surface layer of tethered chains and the extent of the coating level in anon-polar liquid dispersion are analyzed within the framework of the mismatch inchemical nature or solubility parameters between grafted chains and continuouspolymer phase. Lastly, the ability of the ultrasound scattering technique to give aquantitative estimate of the critical disaggregation shear stress mainly representative ofthe particle surface adhesive energy in relation to filler surface modification isshown.

Rheology and Ultrasound Scattering from Aggregated Red Cell Suspensions in Shear Flow

The shear flow dynamics of reversible red cell aggregates in dense suspensions were investigated by ultrasound scattering, to study the shear disruption processes of Rayleigh clusters and examine the effective mean field approximation used in microrheological models. In a first section, a rheo-acoustical model, in the Rayleigh scattering regime, is proposed to describe the shear stress dependence of the low frequency scattered power in relation to structural parameters. The fractal scattering regime characterizing the anisotropic scattering from flocs of size larger than the ultrasound wavelength is further discussed. In the second section, we report flow-dependent changes in the low-frequency scattering coefficient in a plane-plane flow geometry to analyze the shear disruption processes of hardened or deformable red cell aggregates in neutral dextran polymer solution. Rheo-acoustical experiments are examined on the basis of the rheo-acoustical model and the effective medium approximation. The ability of ultrasound scattering technique to determine the critical disaggregation shear stress and to give quantitative information on particle surface adhesive energy is analyzed. Lastly, the shear-thinning behavior of weakly aggregated hardened or deformable red cells is described.

Ultrasound and light scattering from a suspension of reversible fractal clusters in shear flow

Shear break-up of reversible fractal clusters is investigated by ultrasound and multiple light scattering in the low shear regime. We consider a dense suspension of Rayleigh scatterers (particles or clusters) with acoustic properties close to those of the surrounding liquid so that the attenuation of the ultrasonic coherent field is weak and multiple scattering is negligible. The concept of variance in local particle volume fraction is used to derive an original expression of the ultrasound scattering cross-section per unit volume for Rayleigh fractal clusters. On the basis of a scaling law for the shear break-up of aggregates, then we derive the shear stress dependence of the ultrasound scattered intensity from a suspension of reversible fractal clusters. In a second part, we present rheo-acoustical experiments to study the shear break-up of hardened red cell aggregates in plane-plane flow geometry and we examine both the self consistent field approximation and the scaling laws used in microrheological models. We further compare the ability of acoustical backscattering and optical reflectometry techniques to estimate the critical disaggregation shear stress and the particle surface adhesive energy. Finally, the microrheological model from Snabre and Mills [#!ref5!#] based on a fractal approach is shown to describe the non Newtonian behavior of a dense distribution of hardened red cell aggregates.

Erythrocyte disaggregation shear stress, sialic acid, and cell aging in humans.

Erythrocyte aggregation, which plays an important role in the physiological behavior of blood fluidity, was found to be enhanced in hypertension and hypercholesterolemia. While the role of macromolecule bridging force has been widely described, cellular factors related to membrane sialic acid content, which might contribute to the negative charge of cell surface causing the repulsion of erythrocytes, have been less studied. Cell age-dependent changes in membrane sialic acid content (in micromoles per gram of integral membrane protein) were investigated in 24 normotensive and 24 hypertensive matched subjects, each divided into 2 identical subgroups according to a cutoff of 6.2 mmol/L serum cholesterol. A progressive and significant (P<0.001) decrease in membrane sialic acid content associated with an increase (P<0.001) of disaggregation shear rate threshold (laser reflectometry in the presence of dextran) were observed with increased erythrocyte density (erythrocytes fractionated by density using ultracentrifugation) in both normotensive and hypertensive groups regardless of the cholesterol level. However, disaggregation shear rate threshold was significantly higher and sialic acid content was lower (P<0.001) in both hypertensive and normotensive subjects with hypercholesterolemia compared with either normotensive or hypertensive subjects with low cholesterol, respectively. A high membrane sialic acid content variance, beginning in the younger erythrocytes, was due mainly to triglyceride and LDL cholesterol levels (R2=0.49 for low, R2=0.43 for middle, and R2=0.54 for high densities, ie, young, mean, and senescent erythrocytes, respectively). We conclude that an early decrease in erythrocyte sialic acid content may influence the rheological properties of blood by increasing the adhesive energy of erythrocyte aggregates.

Hemorheological abnormalities in rats with experimental mild diabetes: improving effect of troxerutine and α-tocopherol

(Received 5.7.1993~ accepted 8.10.1993) We investigated the effect of intravenous 2mM Troxerutine and lOmM (l­ Tocopherol (Vitamin E) and their combination on erythrocyte aggregation and deformability, blood viscosity and leukocyte adhesivity in rats made mildly diabetic (fasting glycemia: lOmM vs 6mM in controls; HbA 1c: 6.5% vs 3% in controls) by a low dose of streptozotocin (35mg/kg) injected via the saphenous vein. In diabetic rats, as compared to controls, erythrocyte aggregation and blood viscosity were dramatically increased (disaggregation shear stress: 5.2 ± 0.4 dynes/cm2 vs 3.7 ± 0.3 dynes/cm2 in controls), whereas erythrocyte deformability and mesenteric blood flow were sharply decreased. Moreover, leukocyte adhesivity was significantly increased. In diabetic rats, the combination of Troxerutine and Tocopherol improved all the above cited parameters. These data show that mild diabetes in rats results in many hemorheological disorders that can be largely corrected by Troxerutine and (l -Tocopherol.

Effects of lisinopril and hydrochlorothiazide on platelet function and blood rheology in essential hypertension: a randomly allocated double-blind study

To compare the effects of an angiotensin converting enzyme (ACE) inhibitor and a thiazide diuretic on platelet function and haemorrheological variables, since these factors may contribute to the atherosclerotic and thrombotic complications associated with hypertension. Following a 2-week placebo period, 80 male and female patients with mild to moderate hypertension, aged 50 +/- 10 (mean +/- SD) years, were randomly allocated in a double-blind study to 4 weeks of treatment with the ACE inhibitor lisinopril at 20 mg once a day or the diuretic hydrochlorothiazide at 25 mg once a day. Venous blood was sampled before and at the end of the 4-week treatment period to assess platelet function and haemorrheological variables. Both treatments lowered systolic and diastolic blood pressure equally and had no significant effect on platelet counts and platelet aggregation in response to ADP and to arachidonic acid. Haematocrit plasma viscosity and blood filterability were not altered by either drug. Hydrochlorothiazide tended to increase and lisinopril tended to decrease whole blood viscosity at all shear rates but these changes did not reach statistical significance. Lisinopril increased the erythrocyte aggregation time (from 1.98 +/- 0.50 to 2.08 +/- 0.52 s) and decreased the disaggregation shear rate (from 159 +/- 46 to 153 +/- 40 s-1) and the disaggregation shear stress (from 705 +/- 257 to 659 +/- 204 mPa). Hydrochlorothiazide induced the opposite effects (2.00 +/- 0.47 to 1.92 +/- 0.39 s, 181 +/- 531 to 196 +/- 82 s-1 and 813 +/- 268 to 868 +/- 392 mPa, respectively) with a statistically significant (P < 0.05) intergroup difference. These findings suggest that chronic treatment with the ACE inhibitor lisinopril, but not the diuretic hydrochlorothiazide, may produce favourable effects on blood rheology, but the clinical relevance requires further investigation.

Aggregation of human red blood cells after moderate heat treatment

The aggregation behaviour of normal and heat treated (48.4 degrees C, 48.8 degrees C, 49.5 degrees C) red blood cells (RBCs) suspended in dextran-saline solutions (Dx 70, Dx 173) was investigated by a laser light reflectometric method over a wide range of bridging energies. The characteristic times of rouleau formation were found to be increased after RBC heat treatment. The disaggregation shear stress is not significantly different between normal RBCs and heat treated RBCs. The loss of cell deformability is nevertheless shown to improve slightly the dissociation efficiency of the flowing liquid in a shear flow resulting in a small reduction of the disaggregation shear rate after heat treatment. Heat treatment is also shown to alter the structure of RBC network at equilibrium. These results indicate that heat induced alterations of erythrocytes only affects the mechanical properties of the cell membrane without significant changes in the macromolecular bridging energy.

Effects of dextran polydispersity on red blood cell aggregation

B 512 F dextran fractions of different polydispersities were prepared by fractionnal precipitation and preparative elution chromatography. By combination of low angle laser light scattering (LALLS) and gel permeation chromatography (GPC), absolute average molecular weights (MWs) and molecular weight distribution functions (MWDs) were determined. Static and thermodynamical properties in terms of polymer dimensions and second virial coefficient of dilute solutions of dextran in water have been investigated. The results indicate that dextran macromolecules in water are rather compact and impenetrable coils. Measuring the disaggregation shear stress of dextran-induced red blood cell aggregates by laser light reflectometry, the macromolecular bridging energy was shown to depend upon dextran sample polydispersity. This reflects the weak and reversible character of red blood cell aggregation by dextran chains in physiological saline solution.