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PubMed2004

Drag reduction by a linear viscosity profile.

Drag reduction by polymers in turbulent flows raises an apparent contradiction: the stretching of the polymers must increase the viscosity, so why is the drag reduced? A recent theory proposed that drag reduction, in agreement with experiments, is consistent with the effective viscosity growing linearly with the distance from the wall. With this self-consistent solution the reduction in the Reynolds stress overwhelms the increase in viscous drag. In this Rapid Communication we show, using direct numerical simulations, that a linear viscosity profile indeed reduces the drag in agreement with the theory and in close correspondence with direct simulations of the FENE-P model at the same flow conditions.

Journal Article↗
PubMed2005

Viscosity peaks at the cholesteric-isotropic phase transitions.

In this work, the effective viscosity of the cholesteryl myristate and cholesteryl nonanoate liquid crystals is studied as a function of temperature at the region of their cholesteric-to-isotropic phase transition, where blue phases are found. Using a change of scale it is shown that the viscosity peaks that characterize these phase transitions are shape invariant, which suggests that large-scale fluctuations on the two-point correlation function give an important contribution to the observed viscosity. The consequences of this fact are investigated and, from the experimental data, the critical exponents associated with the diverging two-point correlation function are calculated. The results found for both compounds are essentially the same, being also in good agreement with the known values of the corresponding critical exponents of nematic-isotropic phase transition.

Journal Article↗
PubMed2006

Domain growth dynamics and local viscosity in stratifying foam films.

We present a quantitative theory and experiments for the expansion dynamics of domains in stratifying foam films. Foam films containing micelles, colloidal particles or polymer-surfactant complexes often form layered structures and thin in a stepwise fashion: circular domains of lower thickness are formed and expand following a R(t) proportional variant t1/2 law. In the present paper the film is modeled by an incompressible three-dimensional fluid with incompressible surfaces. The film tension difference between the film and domains results in the formation of a rim at the domain boundary and a gradient in film thickness and pressure in the surrounding film. The material transport due to this gradient lets the domains grow. We present experiments utilizing the thin balance method to qualitatively confirm the thinning mechanism and to determine material parameters including local film viscosity of a film composed of 4400 ppm acrylamide/acrylamidomethylpropanesulfonate-copolymer and 0.006 mmol/l dodecyltrimethylammonium bromide solution. We found a film viscosity of about 60 times the bulk viscosity, consistent with previous measurement in the same system but using another method.

Journal Article↗
PubMed2006

Molecular dynamics comparative study of Lennard-Jones -6 and exponential -6 potentials: application to real simple fluids (viscosity and pressure).

In this work, using molecular dynamics simulation, the viscosity (dynamic property) and the pressure (static property) of spherical fluid particles interacting through Lennard-Jones -6 and exponential -6 potentials are computed. Simulations are performed for going from 10 to 20 for the Lennard-Jones potential and from 12 to 22 for the exponential one. Six different thermodynamic states are tested that cover a large range of conditions, from sub- to supercritical temperature and from low to high density. To compare in a consistent manner the results for the various potentials tested, the simulations are carried out for the same set of reduced thermodynamic conditions (using the critical point). It is found that a perfect corresponding-states formulation is not possible between these potentials. Then, these potentials are applied on real simple fluids (argon, oxygen, nitrogen, methane, ethane, and one mixture, air) and the calculated viscosity and pressure values are compared with reference values. It appears that, using the appropriate , both potential families lead to a good accuracy in pressure and viscosity using the same set of molecular parameters for both properties, the average absolute deviations being always lower than 5% for the studied states. In addition, it is shown that the exponential potential results do not outperform the Lennard-Jones ones. Furthermore, for all compounds except for methane, the best results are obtained for the Lennard-Jones 12-6 and the exponential 14-6 potentials. This result partly explains why, despite no theoretical background, the Lennard-Jones 12-6 potential is so widely used. Finally, it is shown that a van der Waals one-fluid model performs extremely well for the studied mixture (air).

Journal Article↗
PubMed2006

Effects of quenching rate and viscosity on spinodal decomposition.

Spinodal decomposition of deeply quenched mixtures is studied experimentally, with particular emphasis on the domain growth rate during the late stage of coarsening. We provide some experimental evidence that at high Péclet number, the process is isotropic and the domain growth is linear in time, even at finite quenching rates. In fact, the quenching rate appears to influence the magnitude of the growth rate, but not its scaling law. In the second part of the work we analyze the effect of viscosity on the growth rate. As predicted by the diffuse interface model, we do not find any effect of viscosity on the growth rate of the nucleating drops, although, as expected, the viscosity of the continuous phase does influence the settling speed and thus the total separation time.

Journal Article↗
PubMed2006

High viscosity gas fluidization of fine particles: An extended window of quasihomogeneous flow.

We explore the role of gas viscosity in the behavior of gas-fluidized beds of fine powders by means of experimental measurements using nitrogen and neon as fluidizing gases, and theoretical considerations. The existence of a nonbubbling fluidlike regime has been recently observed in beds of fine powders fluidized with nitrogen. Our experiments with neon reveal a discontinuous transition from heterogeneous fluidization to a highly expanded homogeneous fluidization state. We point out that increasing gas viscosity enhances the coherence of agglomerate swarms, which promotes a local void-splitting mechanism, thus improving the uniformity of fluidization. Our theoretical analysis predicts that further increase of gas viscosity would produce a full suppression of the bubbling regime, i.e., the uniformly fluidized bed would undergo a direct transition to a turbulent regime as seen in beds of nanoparticles fluidized by nitrogen and in liquid-fluidized beds of moderate-density beads.

Journal Article↗
PubMed2004

Viscosity at the dynamic crossover in o-terphenyl and salol under high pressure.

The viscosities of two prototypical glass formers, o-terphenyl and phenyl salicylate (salol), are shown to exhibit a change in their temperature and pressure dependences at a constant value of the viscosity. This is the first evidence of a dynamic crossover in the viscosity induced by pressure. The characteristic value associated with the change in dynamics is material dependent, but independent of temperature and pressure. These results are in accord with the previous finding, for other glass formers, that the dielectric relaxation time assumes a density-independent value at the dynamic crossover.

Journal Article↗
PubMed2004

Eddy viscosity for time reversing waves in a dissipative environment.

We present new results for the time reversal of weakly nonlinear pulses traveling in a random dissipative environment. Also we describe a new theory for calculating the eddy viscosity for weakly nonlinear waves propagating over a random surface. The turbulent viscosity is calculated from first principles, namely, without imposing any stress-strain hypothesis. A viscous shallow water model is considered and its effective viscosity characterized. We also show that weakly nonlinear waves can still be time reversed under weak dissipation. Incoherently scattered signals are recompressed, both for time reversal in transmission as well as in reflection. Under the weakly nonlinear, weakly dissipative regime, dissipation only affects the refocused pulse profile regarding its amplitude, but its shape is not corrupted. Numerical experiments are presented.

Journal Article↗
PubMed2005

Shear viscosity of two-dimensional Yukawa systems in the liquid state.

The shear viscosity of a two-dimensional (2D) liquid was calculated using molecular dynamics simulations with a Yukawa potential. The viscosity has a minimum at a Coulomb coupling parameter Gamma of about 17, arising from the temperature dependence of the kinetic and potential contributions. Previous calculations of 2D viscosity were less extensive as well as for a different potential. The stress autocorrelation function was found to decay rapidly, contrary to earlier work. These results are useful for 2D condensed matter systems and are compared to a dusty plasma experiment.

Journal Article↗
PubMed1990

Membrane viscosity correlates with alpha 1-adrenergic signal transduction of the aged rat cerebral cortex.

We investigated, using adult (2-month-old) and senescent (12- and 24-month-old) rats, the effects of aging on the relationship between the alpha 1-adrenergic coupling system and the membrane viscosity of the cerebral cortex. There was no age-related difference in the KD values of [3H]prazosin binding on the membranes. The Bmax values of [3H]prazosin binding were reduced with advanced age. Norepinephrine-induced formation of 3H-labeled inositol phosphates (3H-IPs) in the slices increased with advanced age. The EC50 values for norepinephrine to stimulate the formation of 3H-IPs at advanced age were lower than that at adult age. The cholesterol content in membranes increased with advanced age. No changes in the phospholipid content in membranes were observed with advanced age. Concomitantly, an increase of the molar ratio of cholesterol to phospholipids was observed with advanced age. The membrane viscosity as measured by 1,6-diphenyl-1,3,5-hexatriene increased with advanced age. These results indicate that the altered cholesterol content and/or viscosity in cortical membranes of the aged rat may account for the loss of alpha 1-adrenergic receptor density and/or compensatory changes in the receptor-phospholipase C coupling system.

Aging↗
PubMed2005

Mechanism for cavitation in the mechanical heart valve with an artificial heart: nuclei and viscosity dependence.

Until now, we have estimated cavitation for mechanical heart valves (MHV) mounted in an electrohydraulic total artificial heart (EHTAH) with tap water. However, tap water at room temperature is not a proper substitute for blood at 37 degrees C. We therefore investigated fluid characterization in studies of MHV cavitation associated with the viscosity and nuclei content of a testing fluid. We used the Medtronic Hall valve mounted in the mitral position of the EHTAH. As testing fluids, tap water, distilled water, and glycerin solution were used. The valve-closing velocity, pressure-drop measurements, and a high-speed video camera were employed to determine the cavitation intensity in MHV. Most of the cavitation bubbles were observed at the edge of the valve stop. Our analysis of the results indicates that squeeze flow is the major cause of cavitation in the Medtronic Hall valve. The cavitation intensity increased with increases in the fluid viscosity and the valve-closing velocity. Even if cavitation intensity in glycerin solution was greater, the cavitation occurrence probability was less in glycerin solution than in tap water. Our results suggest that tap water contains particles that cause an increase in the cavitation occurrence probability. We conclude that cavitation intensity is greatly affected by the nuclei concentration in the fluid and the fluid viscosity.

Cardiac Output↗
PubMed2004

Viscosity of fluids in subduction zones.

The viscosities of aqueous fluids with 10 to 80 weight percent dissolved silicates have been measured at 600 degrees to 950 degrees C and 1.0 to 2.0 gigapascals by in situ observation of falling spheres in the diamond anvil cell. The viscosities at 800 degrees C range from 10(-4) to 10(0.5) pascal seconds. The combination of low viscosities with a favorable wetting angle makes silicate-rich fluid an efficient agent for material transport at low-volume fractions. Our results therefore suggest that there may be a direct relationship between the position of the volcanic front and the onset of complete miscibility between water and silicate melt in the subducting slab.

Journal Article↗
PubMed1999

Viscosity near Earth's solid inner core

Anomalous splitting of the two equatorial translational modes of oscillation of Earth's solid inner core is used to estimate the effective viscosity just outside its boundary. Superconducting gravimeter observations give periods of 3.5822 +/- 0.0012 (retrograde) and 4.0150 +/- 0.0010 (prograde) hours. With the use of Ekman layer theory to estimate viscous drag forces, an inferred single viscosity of 1.22 x 10(11) Pascal seconds gives calculated periods of 3.5839 and 4.0167 hours for the two modes, close to the observed values. The large effective viscosity is consistent with a fluid, solid-liquid mixture surrounding the inner core associated with the "compositional convection" that drives Earth's geodynamo.

Journal Article↗
PubMed2002

Viscosity dependence of optical limiting in carbon black suspensions.

We measure the optical limiting behavior of carbon black suspensions in various viscosity solvents by using a 10-Hz repetition rate, 532-nm, 5-ns pulsed laser. We found that, for common solvents used in the past such as water and ethanol, the limiting behavior ceases after a few laser firings and a turnover in the limiting curve appears. This can be explained by depletion of the carbon black within the focal volume. This turnover shifts to lower energies as the viscosity of the solvent becomes greater. However, for low viscosity liquids, such as carbon disulfide or pentane, the limiting is unaffected by the repetition rate, at least for frequencies up to 10 Hz, because of diffusion of the carbon black particles. This diffusion allows fresh material to replace the irradiated volume within the time between pulses.

Journal Article↗
PubMed2003

Measurement of the bulk viscosity of liquid by Brillouin scattering.

The aim is to develop a rapid and direct method for measuring the bulk viscosity of a liquid as a function of temperature. Brillouin scattering of a laser beam in fresh water and salt water at different temperatures has been studied. The results show that there exists a close temperature-dependent relationship among the Brillouin frequency shift, the Brillouin linewidth, and the bulk viscosity of water. Thus the bulk viscosity of water can be determined directly from Brillouin-scattering measurements. The method has a high signal-to-noise ratio and high accuracy.

Journal Article↗
PubMed1997

Exeter and charnley arthroplasties with Boneloc or high viscosity cement. Comparison of 1,127 arthroplasties followed for 5 years in the Norwegian Arthroplasty Register.

During the years 1991-1994, the Norwegian Arthroplasty Register recorded 1,324 primary hip arthroplasties implanted with the Boneloc cement. We have compared the survival until revision due to aseptic loosening for charnley (n 955) and Exeter (n 172) prostheses. The Boneloc cemented hips were also compared with high viscosity cemented hips implanted during the same period. In the Boneloc cemented group, the estimated probability of survival at 4.5 years of a Charnley femoral component was 74% and for an Exeter femoral component 97% (p < 0.0001). Using a Cox regression model with adjustment for age, gender, type of cement, systemic antibiotic and stratified for diagnosis, an 8 times higher risk of revision was found in Boneloc cemented Charnley femoral components than in Exeter femoral components (p < 0.0001). For the acetabular components, the difference between the Charnley and Exeter components with Boneloc cement was not statistically significant. In both the Charnley and the Exeter prostheses, the high viscosity cemented components had significantly better survival than the Boneloc cemented components. The Cox regression model showed that a Boneloc cemented Charnley femoral component had a 14 times higher risk of revision than a high viscosity cemented component (p < 0.0001), and for Exeter femoral components a 7 times higher revision risk was found in the Boneloc cemented components (p = 0.003). Our results confirm the previously reported inferior results of Charnley prostheses implanted with Boneloc cement and inferior results of Boneloc cemented Exeter prostheses as well, but less pronounced than for Charnley prostheses.

Aged↗
PubMed1989

Membrane viscosity of lymphocytes and influence of phytohemagglutinin.

The membrane viscosity of peripheral blood lymphocytes (PBLs) of equine, bovine and canine was measured by the use of time-resolved fluorescence depolarization technique with 1, 6-diphenyl-1,3,5-hexatriene (DPH). The viscosity values were 0.55, 0.59 and 0.50 poise for equine, bovine and canine PBLs, respectively. These values were compared with steady-state anisotropies and order parameters measured from electron spin resonance (ESR) of 5-doxyl stearic acid. Both values were increased with increase of viscosity. The fluid property of the membranes stimulated with phytohemagglutinin-P (PHA) was measured with steady-state fluorescence anisotropy and ESR. Little change of membrane fluidity was recognized with both methods during the stimulation with PHA. It appears that PHA activation process for these lymphocytes does not included large increase of the membrane fluidity which significantly accelerate the diffusion velocity of receptors in the plasma membrane.

Animals↗
PubMed2003

Impaired plasma viscosity via increased cholesterol levels in peripheral occlusive arterial disease [correction of disase].

The aim of this study was to investigate the relationship between plasma viscosity and lipoprotein and apolipoprotein pattern in normo- and hypercholesterolemic patients with peripheral occlusive arterial disease (POAD). 40 patients with POAD have been selected (8 females and 32 males, mean age: 54+/-3.2 years) with clinically evident superficial femoral occlusive artery disease. They were separated into two groups as normocholesterolemic (plasma total cholesterol <200 mg/dl) and hypercholesterolemic (plasma total cholesterol >200 mg/dl). Plasma total cholesterol, high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), triglycerides, total protein, and albumin levels were determined by enzymatic methods using commercial kits. Levels of apolipoprotein AI (apo AI), and apolipoprotein B (apo B) were measured using a immunoturbidometric method. Plasma viscosity (PV) was measured by capillary viscometer. Classifying the patients with PAOD according to the cholesterol levels; hypercholesterolemic (mean total-cholesterol: 227.90+/-26.97 mg/dl) patients had significantly higher LDL-C, PV and triglyceride levels compared with nornocholesterolemic patients (p<0.001, p<0.001, p<0.001, respectively). HDL-C and apo B were significantly lower in hypercholesterolemic patients than in normocholesterolemic patients (p<0.001, p<0.001, respectively). PV was positively correlated with total cholesterol (r=0.485, p<0.05), atherogenic index (r=0.624, p<0.01), total-C/HDL-C ratio (r=0.624, p<0.05), and LDL-C/HDL-C ratio (r=0.707, p<0.001) in hypercholesterolemic patients with POAD. PV was higher in hypercholesterolemic patients with POAD than in normocholesterolemic patients with POAD. We suggest that POAD patients should be regarded as a heterogenous group with lipid and lipoprotein parameters in order to assess the microcirculation in the affected limb. In case of dyslipidemia in POAD patients an elevated plasma viscosity should be considered as coexisting risk factor.

Aged↗