Increase In Effective Viscosity Research Articles

High-temperature deformation of volcanic materials in the presence of water

We describe an experimental apparatus used to perform deformation experiments relevant to the volcanological sciences. The apparatus supports low-load, high-temperature deformation experiments under dry and wet conditions on natural and synthetic samples. The experiments recover the transient rheology of complex (melt ± porosity ± solids) volcanic materials during uniaxial deformation. The key component to this apparatus is a steel cell designed for high-temperature deformation experiments under controlled water pressure. Experiments are run under constant displacement rates or constant loads; the range of accessible experimental conditions include: 25–1100 °C, load stresses 0 to 150 MPa, strain rates 10 −6 to 10 −2 1/s, and fluid pressures 0–150 MPa. The apparatus is calibrated against standard values of viscosity using constant-load experiments on cores of NIST (NBS) 717a borosilicate glass. We also report results of constant-displacement rate (~10 −6 m/s) experiments on porous (~70%) sintered cores of ash from the Rattlesnake Tuff. The cores of volcanic ash were deformed in experiments under ambient (“dry”) and elevated water pressures (“wet”). Dry experiments at ~870 °C show an increase in effective viscosity (10 9.5 to 10 10.4 Pa·s) with increasing strain (~30%) due to porosity reduction during compaction. Experiments under ~1–3 MPa P H 2 O recover lower values of apparent viscosity (10 9.2 to 10 9.4 Pa·s) despite being run at lower temperatures (640–665 °C). The wet experiments also do not show a rise in viscosity with increased strain (decreasing porosity) as observed in dry experiments. Rather, the presence of an H 2 O fluid phase expands the window of viscous deformation and delays the onset of strain hardening that normally accompanies porosity reduction.

Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing

Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing

Post-seismic motion following the 1997 Manyi (Tibet) earthquake: InSAR observations and modelling

SUMMARY On November 8 1997 a Mw 7.6 earthquake occurred in the Manyi region of northern Tibet, near the western end of the Kunlun Fault. Over 7 m of left-lateral slip occurred on a 200-kmlong fault. Here we use InSAR observations of post-seismic surface deformation following the Manyi earthquake to investigate possible causal mechanisms. Time-series of deformation are constructed from 26 interferograms, covering the entire length of the fault for nearly 4 yr after the earthquake. Three different modelling approaches are used to try and understand the observed variations in surface displacement. First order poroelastic models predict displacement fields which do not match those observed. Modelling of viscoelastic stress relaxation in a half-space with standard linear solid rheology beneath an elastic lid provides a reasonable fit to the observations, and demonstrates that two relaxation times are needed to characterize the post-seismic transient. The best-fitting viscosity is 4 £ 10 18 Pa s, and the fully-relaxed shear modulus is 32 per cent of the purely elastic value. A model with a Maxwell viscoelastic halfspace underlying the lid cannot explain the observations: at later times, larger viscosities are required than at earlier times. This increase in effective viscosity with time may be consistent with stress relaxation occurring in a power-law rheology. The time-series are also inverted for distributed afterslip on an extension of the coseismic rupture. Along-strike correlation of coseismic and post-seismic slip maxima suggests that afterslip is a plausible mechanism. The maximum slip in the afterslip model is 0.72 m after 3 yr, and the equivalent moment release is approximately 20 per cent of the coseismic moment. This study shows that it is possible to rule out certain mechanisms as the dominant post-seismic process, but with the current InSAR data set it is difficult to distinguish between other plausible options.

Numerical simulation of the dynamic flow behavior in a bubble column: A study of closures for turbulence and interface forces

Numerical simulations of the bubbly flow in two square cross-sectioned bubble columns were conducted with the commercial CFD package CFX-4.4. The effect of the model constant used in the sub-grid scale (SGS) model, C S , as well as the interfacial closures for the drag, lift and virtual mass forces were investigated. Furthermore, the performance of three models [Pfleger, D., Becker, S., 2001. Modeling and simulation of the dynamic flow behavior in a bubble column. Chemical Engineering Science, 56, 1737–1747; Sato, Y., Sekoguchi, K.,1975. Liquid velocity distribution in two-phase bubble flow. International Journal of Multiphase Flow 2, 79–95; Troshko, A.A., Hassan, Y.A., 2001. A two-equation turbulence model of turbulent bubbly flows. International Journal of Multiphase Flow 27, 1965–2000] to account for the bubble-induced turbulence in the k – ε model was assessed. All simulation results were compared with experimental data for the mean and fluctuating liquid and gas velocities. It is shown that the simulation results with C S = 0.08 and 0.10 agree well with the measurements. When C S is increased, the effective viscosity increases and subsequently the bubble plume becomes less dynamic. All three bubble-induced turbulence models could produce good solutions for the time-averaged velocity. The models of Troshko and Hassan and Pfleger and Becker reproduce the dynamics of the bubbly flow in a more accurate way than the model of Sato and Sekoguchi. Based on the comparison of the results obtained for two columns with different aspect ratio ( H / D = 3 and H / D = 6 ), it was found that the model of Pfleger and Becker performs better than the model of Troshko and Hassan, while the model of Sato and Sekoguchi performs the worst. It was observed that the interfacial closure model proposed by Tomiyama [2004. Drag, lift and virtual mass forces acting on a single bubble. Third International Symposium on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, 22–24 September] performs better for the taller column. With the drag coefficient proposed by Tomiyama, the predicted slip velocity agrees well with the experimental data in both columns. The virtual mass force has a small influence on the investigated bubbly flow characteristics. However, the lift force strongly influences the bubble plume dynamics and consequently determines the shape of the vertical velocity profile. In a taller column, the lift coefficient following from the model of Tomiyama produces the best results.

Analogue modelling of syntectonic leucosomes in migmatitic schists

Migmatites from the Cap de Creus tectonometamorphic belt display a wide variety of structures, from those formed when the leucosomes were melt-bearing, to those developed during solid-state deformation. The observed field structures have been modelled by means of analogue experiments. The materials used in the models are layered plasticine as a schist analogue, and chocolate as analogue of the crystallizing leucosome. A model for the development of syntectonic migmatites is proposed in which initial melt-bearing patches, preferentially formed within fertile pelitic layers, progressively evolve towards lens-shaped veins. Furthermore, heterogeneous deformation of anisotropic metasediments facilitates formation of extensional sites for further melt accumulation and transport. Melt crystallization implies a rapid increase in effective viscosity of leucosomes producing a reversal in competence contrast with respect to the enclosing schists. During the whole process, deformation localizes around crystallizing veins, giving rise to different and contrasting structures for melt-bearing and for solid-state stages.

Oscillatory Dissipation of a Simple Confined Liquid

We present a sensitive measurement of the dissipation and the effective viscosity of a simple confined liquid (octamethylcyclotetrasiloxane) using an atomic force microscope. The experimental data show that the damping and the effective viscosity increase and present oscillations as the gap between the cantilever tip and the surface is diminished. To our knowledge, the damping and the viscosity modulation are reported here with such good accuracy for the first time. Such an experimental result is different from what has been reported earlier where only a continuous increase of the damping and the viscosity are observed.

Polyacrylamide and Water Quality Effects on Infiltration in Sandy Loam Soils

Slow infiltration rates constrain effective and economical irrigation in some sandy loam soils in California. Polyacrylamide (PAM) has increased soil infiltration in some areas, especially in soils high in clay or silt. Field trials near Fresno, CA, with PAM failed to show improved infiltration. Laboratory experiments were conducted to investigate PAM effect on infiltration of various quality waters in sandy loam soils. Two formulations of a high molecular weight PAM, a liquid emulsion and a granular, were evaluated on a Hanford sandy loam soil (coarse‐loamy, mixed, superactive, nonacid, thermic Typic Xerorthents) in packed soil column experiments. Applying PAM continuously in the infiltration water always decreased infiltration for all PAM concentrations tested (5–20 mg PAM L−1). Final infiltration rates of 5 mg PAM L−1 relative to infiltration rate of deionized water were 65% for emulsion PAM and 36% for granular PAM and these ratios decreased with increasing PAM concentration. Reduction of infiltration rates when PAM was applied with water containing Ca (applied as gypsum) was less than with PAM solution containing Na. Permeability tests of PAM solutions through uniform sands showed a decrease of permeability with increased concentrations, due to an apparent increase in effective viscosity of the solution. The decrease in infiltration rates in this study was likely due to this increase in viscosity when PAM is added to water. This research concluded that PAM applied in irrigation water will reduce infiltration unless the material improves surface soil aggregate structure and sustains pores sufficient to mask the effect of solution viscosity.

Fluidity of Water Confined Down to Subnanometer Films

A surface force balance with extremely high sensitivity and resolution for measuring shear forces across thin films has been used to investigate directly the dynamic properties of salt-free water (so-called conductivity water) in a gap between two atomically smooth solid surfaces. Our results reveal that no shear stress can be sustained by water (within our resolution and shear rates) down to films of thickness D = D0 = 0.0 +/- 0.3 nm. At short range (D < 3.5 +/- 1 nm), an attractive van der Waals (vdW) force between the surfaces causes a jump into a flat adhesive contact at D0, at which the surfaces rigidly couple. Analysis of the jump behavior reveals that the viscosity of water remains within a factor of 3 or so of its bulk value down to D0. This contrasts sharply with the case of confined nonassociating liquids, whose effective viscosity increases by many orders of magnitude at film thicknesses lower than about five to eight monolayers. We attribute this to the fundamentally different mechanisms of solidification of organic liquids and of water. In the former case, the density increase induced in the films by the confinement promotes solidification, while, in the case of water, such densification (due to vdW attraction between the liquid molecules and the confining walls), in agreement with bulk behavior, suppresses the tendency of the water to solidify.

Atomic Force Microscopy at Solid−Liquid Interfaces

An atomic force microscope (AFM) operating in force modulation mode is used to study solvation forces at the interface between a graphite (HOPG) surface and the liquids octamethylcyclotetrasiloxane (OMCTS) and n-dodecanol. Simple analytical models are used that adequately describe the response of the cantilever as the modulation frequency and tip−sample interaction change. The analysis of AFM force curves yields the tip−sample interaction stiffness and damping. Hydrodynamic damping is significant for all the levers used and at present this limits the sensitivity of detecting weak tip−surface damping effects. The main results are: (i) Confinement of liquid between two surfaces can lead to oscillatory structural forces even when one of the surfaces has very high curvature. This could influence topographic images at the atomic level in liquids. In these experiments the typical radius for a sharp AFM tip is measured as Rtip ≈ 14 nm. (ii) The effective viscosity increases by ∼4 orders of magnitude for a sharp tip interacting with the OMCTS solvation layers nearest the surface. (iii) The AFM data are compared to published results obtained using the surface force apparatus (SFA). The AFM data for the interaction stiffness and damping are qualitatively similar to the SFA results, but the magnitude of the effects is smaller. This most likely arises from the limited interaction area over which the confined molecules can exhibit cooperative behavior (∼100 nm2). (iv) Oscillatory solvation forces can also be observed with very blunt tips (Rtip ≈ 350 nm), and the data suggest that in this case tip microasperities dominate the tip−sample interaction.

Liquid-to-solid transitions in thin liquid films induced by confinement

The mechanical properties of confined thin films of simple liquids were measured via a surface force balance as a function of the film thickness. We find an abrupt transition from liquid-like to a solid-like behaviour at a confinement alone and does not require external applied pressure. At the transition the effective viscosity increases by at least seven orders of magnitude. It is proposed that this is a cooperative effect, reminiscent of a Lindemann-like mechanism, originating in the suppression of the freedom of the confined molecules to move.

Atomic Force Microscopic Imaging in Liquids: Effects of the Film Compressed between the Substrate and the Tip

Measurements are made of the forces acting on the tip of an atomic force microscope when the sample and cantilever are in air and also immersed in polar solvents like water and DMSO. For large tip/substrate separations ( >1000 nm) the liquid drag force can be modeled using a classical hydrodynamic drag force expression. For separations between the tip and substrate smaller than around 100 nm in a water medium, the force due to the effective viscosity increase of the compressed films for high tip/substrate relative velocity is comparable to the contribution of the double-layer repulsion and the van der Waals attraction. After tip/substrate contact these compressed films produce an attractive force that is a function of the liquid medium. In the DMSO medium the attractive force between tip and substrate shows an adhesive force that has at least two components indicating a multilayered structure between the tip and substrate during the tip/substrate separation. The scanning of a surface immersed in water and DMSO with a tip in contact produces distinct AFM images which depend on the liquid. These images show diverse symmetries and spacings between features which presumably correspond to the solvated atomic structure and are only obtained with atomic resolution for a scanning speed of 2 nm/s. It is possible to estimate the viscous relaxation time of the compressed layer value to be 250 ms from the observed resolution of substrate structures as a function of scanning speed.

Role of neutrals in the phase transition model

The phase transition model for the transition from the low-confinement regime (L mode) to high-confinement regime (H mode) is extended to incorporate the effect of the neutral particles on the transition threshold. For usual edge plasma parameters, the increase of effective poloidal viscosity through charge exchange damping and reduction of the effective fluxes by both ionization and charge exchange increases the threshold power required for the transition. For plasmas with the effective energy flux smaller than the convective energy flux, the transition may be triggered by a neutral influx.

Sedimentation of Bidisperse, Uncharged Colloidal Sphere Suspensions: Influence of Viscosity and Irregular Surfaces

The sedimentation velocity of uncharged, nonaggregated silica spheres under gravity is strongly reduced after addition ofsmallamounts of nonsedimenting small spheres. This reduction is largely due to surface irregularities on a nanoscale of the large spheres at which a limited number of small spheres adsorbs, leading to an increase of the hydrodynamic friction per particle. This adsorption also screens effectively any weak Van der Waals attraction between the large spheres, which despite its weakness, significantly influences the concentration dependence of settling in a pure solvent. The concentration dependence and magnitude of the large-sphere sedimentation velocity in a more concentrated dispersion of small spheres agree with the prediction by Batchelor that the small particles mainly manifest themselves as an effective viscosity increase.

Development of a helical‐ribbon impeller bioreactor for high‐density plant cell suspension culture

A double helical-ribbon impeller (HRI) bioreactor with a 11-L working volume was developed to grow high-density Catharanthus roseus cell suspensions. The rheological behavior of this suspension was found to be shear-thinning for concentrations higher than 12 to 15 g DW . L(-1). A granulated agar suspension of similar rheological properties was used as a model fluid for these suspensions. Mixing studies revealed that surface baffling and bottom profiling of the bioreactor and impeller speeds of 60 to 150 rpm ensured uniform mixing of suspensions. The HRI power requirement was found to increase significantly for agar suspensions higher than 13 g DW . L(-1), in conjunction with the effective viscosity increase. Oxygen transfer studies showed high apparent surface oxygen transfer coefficients (k(L)a approximately 4 to 45 h(-1)) from agar suspensions of 30 g DW . L(-1) to water and for mixing speeds ranging from 120 to 150 rpm. These high surface k(I)a values were ascribed to the flow pattern of this bioreactor configuration combined with surface bubble generation and entrainment in the liquid phase caused by the presence of the surface baffles. High-density C. roseus cell suspension cultures were successfully grown in this bioreactor without gas sparging. Up to 70% oxygen enrichment of the head space was required to ensure sufficient oxygen supply to the cultures so that dissolved oxygen concentration would remain above the critical level (> or =10% air saturation). The best mixing speed was 120 rpm. These cultures grew at the same rate ( approximately 0.4 d(-1)) and attained the same high biomass concentrations ( approximately 25 to 27 g DW . L(-1), 450 to 500 g filtered wet biomass . L(-1), and 92% to 100% settled wet biomass volume) as shake flask cultures. The scale-up potential of this bioreactor configuration is discussed.

Changes in haemorheology in the racing greyhound as related to oxygen delivery

Arterial blood samples were obtained from six greyhounds during rest, immediately before, and after a 704-m (7/16th mile) race. Measurements were made of various haematological (red cell count, haemoglobin, packed cell volume, white cell count, plasma proteins) and haemorheological variables. Blood and plasma viscosity were determined at high wall shear stresses (67-200 dynes.cm-2, 670-2000 microN.cm-2) in a 20-microns glass capillary device which was designed to take the diameter dependence of blood viscosity (Fahraeus-Lindqvist effect) into account. Compared to values at rest, substantial haemoconcentration occurred before the race, mainly due to splenic discharge of red cells. Additional haemoconcentration was found after the race. The increase of effective blood viscosity caused by elevation of packed cell volume was greater than the increase in O2 binding capacity resulting from the elevated haemoglobin concentration, suggesting that the haemoconcentration observed in the exercising greyhound does not enhance O2 delivery to skeletal muscle. The main physiological effect of red cell discharge from the contracting spleen appeared to be a consequence of the volume rather than the composition of the circulating blood.

Rotational Viscosities of Polymer Solutions in a Low Molecular Weight Nematic Liquid Crystal

Abstract Polystyrene polymers (M.W. - 2700) were dissolved in a low molecular weight nematic mixture E63 (E. Merck) in different concentrations, and studied over a broad temperature range. The rotational viscosities, γb of the mixtures were measured by using the rotating magnetic field method. Contrary to the theoretical indications and some earlier experimental results, it was found that the rotational viscosity is practically not influenced due to the polymers, which suggests that the polymer conformation is rather spherical in the anisotropic fluid. Preliminary investigations of the flow properties (capillary flow) show that the effective shear viscosity increase due to the added polymer is similar to that of isotropic solutions.

MHD turbulence and accretion disks

The viscosity in accretion disks derived from observations is many orders of magnitude larger than the molecular viscosity can account for. MHD turbulence may cause a drastic increase of effective viscosity. Using the cascade model we simulate magnetic turbulence in a differentially rotating disk. The magnetic field allows for an energy flow from small to larger length scales. The turbulent stress due to the combined action of turbulent velocity- and magnetic fields show large fluctuations around the mean. Bursts in the total turbulent energy are large contributors to the time averaged stress and thus to the effective viscosity. The simulations show that the shear force is sufficient to sustain the turbulence.

Turbulent drag reduction by polymers: A quantitative theory.

By use of the recently proposed model of the dynamics of a macromolecule in transient extensional flows, the effective viscosity increase in turbulence due to unraveling macromolecules is calculated, and a relation between this viscosity increase and a drag reduction parameter (the slope increment) is derived. The predictions of the theory are in good agreement with experiment.

Increase of effective viscosity of molten GaAs and InSb under an axial magnetic field

We have measured the effective dynamic viscosity of molten GaAs and InSb as a function of axial magnetic field by the oscillating vessel method. Effective viscosity of these semiconductor melts is observed to increase with the axial magnetic field intensity in the range of 0 to 5 kG.

The behaviour of magnetic fluids under strong nonuniform magnetic field in rotating seal

The time dependent local increase of magnetization and effective viscosity are determined for various magnetic fluids subjected to the action of a strong nonuniform magnetic field. Their rate of increase strongly depends on carrier liquid viscosity, particle concentration and dimensional distribution.