Couette Device Research Articles

The measurement of the shear-induced particle and fluid tracer diffusivities in concentrated suspensions by a novel method

The shear-induced particle self-diffusivity in a concentrated suspension (20%–50% solids volume fraction) of non-colloidal spheres (90 μm average diameter) was measured using a new correlation technique. This method is based on the correlation between the positions of tracer particles in successive images and can be used to determine the self-diffusivity in non-colloidal suspensions for different time scales. These self-diffusivities were measured in the velocity gradient and vorticity directions in a narrow-gap Couette device for values of the strain γΔt ranging from 0.05 to 0.5, where γ is the applied shear rate and Δt is the correlation time. In both directions, the diffusive displacements scaled linearly with γΔt over the range given above and the corresponding diffusivities were found to be in good agreement with the experimental results of Leighton & Acrivos (1987a) and of Phan & Leighton (1993), even though these earlier studies were performed at much larger values of γΔt. The self-diffusivity in the velocity gradient direction was found to be about 1.7 times larger than in the vorticity direction. The technique was also used to determine the shear-induced fluid tracer by measuring the mean square displacement of 31.5 μm diameter tracer particles dispersed in concentrated suspensions (30%–50% solids volume fraction) of much larger spheres (325 μm average diameter). These fluid diffusivities were found to be 0.7 times the corresponding particle diffusivities when both were scaled with γ a2 (2a = 325 μm).

Forces on a slowly rotating, rough cylinder in a Couette device containing a dry, frictional powder

In the present work, a fine, dry powder was sheared in a Couette device: i.e., sheared between concentric vertical cylinders. The torque generated on the rough, inner cylinder was measured as this inner wall was rotated. Our experiments provided evidence that, in a column of granular material undergoing continuous shearing, normal and shear stresses increase linearly with depth. In other words, Janssen’s analysis ceases to apply if granular material is continuously sheared.

Self-Preservation of the Drop Size Distribution Function and Variation in the Stability Ratio for Rapid Coalescence of a Polydisperse Emulsion in a Simple Shear Field

Coalescence of oil-in-water emulsion droplets in a simple shear flow produced by a Couette device is considered. A phase Doppler anemometer was used to measure the droplet size distribution as a function of time for shear rates ranging from 55 to 213 s−1and for sodium chloride salt concentrations from 0.095 to 0.6 M. The initial droplet size distribution was log-normal. During the coalescence process, the size distribution was self-preserving in accordance with D. L. Swift and S. K. Friedlander's analysis [J. Colloid Sci.19,621 (1964)]. In the limiting case of negligible repulsive force due to the electric double layer, the calculated stability ratios, corrected for droplet polydispersity, agree well with the theoretical analyses of G. R. Zeichner and W. R. Schowalter [AIChE J.23,243 (1977)] and D. L. Feke and W. R. Schowalter [J. Fluid Mech.133,17 (1983)] for the case of solid particle aggregation. The good agreement between the stability ratios for the case of coalescence of droplets in the present study and those for aggregation of solid particles indicates that resistance to film deformation and thinning present in the case of coalescence is not important compared with the collision process.

Ristocetin- and Thrombin-induced Platelet Aggregation at Physiological Shear Rates: Differential Roles for GPIb and GPIIb-IIIa Receptor

We recently reported that washed platelets (WP) activated with ADP and expressing surface-bound vWF aggregated in flow through small tubes or in a cylindrical couette device at physiological shear rates of G = 300 s(-1)-1000 s(-1) in the absence of exogenous ligands, with GPIb-vWF partially, and activated GPIIb-IIIa totally required for the aggregation. We have now extended these studies to aggregation of platelets "activated" with ristocetin or thrombin. Washed platelet suspensions with added soluble vWF and ristocetin (0.3-0.75 mg/ml), or activated with thrombin (0.01-0.5 U/ml) but no added ligand, were sheared in a coaxial cylinder device at uniform shear rate, G = 1000 s(-1). The collision capture efficiency (alphaG) with which small aggregates form (= experimental/calculated initial rates of aggregation) was correlated with vWF platelet binding assessed by flow cytometry. The vWF-GPIb interaction was exclusively able to support ristocetin-mediated shear aggregation of metabolically active platelets, with very few vWF monomer equivalents bound per platelet (representing < or = 10 molecules of 10 million Da) required to yield high capture efficiencies (alphaG = 0.38+/-.02; n = 11), suggesting rapid and stable bond formations between vWF and GPIb. However, platelet surface-expressed vWF, generated by addition of thrombin to washed platelets, was found to mediate platelet aggregation with alphaG = 0.08+/-.01 (n = 6), surprisingly comparable to that previously reported for WP and ADP activation. Blocking the GPIIb-Illa receptor decreased alphaG by 95+/-3% (n =3), while a monoclonal antibody to the vWF site on GPIb caused a 49+/-7% (n = 8) decrease in alphaG. The partial role for GPIb thus appears to reflect a facilitative function for increasing contact time between flowing platelets, and allowing engagement of the GPIIb-IIa receptor to yield stable attachment.

Shear-induced radial segregation in bidisperse suspensions

In this paper, we discuss experimental evidence for radial particle segregation in a parallel-plate geometry. The motion of coloured tracer particles of a size different from the bulk suspension is followed as a function of time. The tracer particles are seen to experience a constant drift velocity independent of their radial position. This is in addition to the random-walk motion arising from their interactions with other particles in the suspension. These observations are found to be consistent with the shear-induced migration model of Leighton &amp; Acrivos (1987 a,b) as well as the tracer diffusivity measurements of Phan &amp; Leighton (1996). In the experiments of Abbott et al. (1991), it was observed that larger particles migrated radially outward to regions of lower shear stress in a wide-gap Couette device. In our experiments large tracers were also observed to migrate radially outward. In this case, however, the radial migration resulted in migration to regions of higher shear stress, contrary to expectations. This apparent discrepancy is explained in terms of a model that incorporates both the stress-induced migration of earlier studies and a curvature-induced migration flux (which in turn is shear-induced) as an opposing effect.

Experimental measurement of shear-induced diffusion in suspensions using long time data

A method based upon Taylor dispersion theory is used to determine the shear-induced diffusion coefficient in concentrated suspensions. The experiments are performed in a cylindrical Couette device with a suspension consisting of polystyrene spheres in a density-matched solution of glycerin and water. A sequence of several hundred transit times for a single tagged sphere to complete successive orbits within the device is measured. The data are analyzed to compute the azimuthal Taylor dispersion coefficient from which the coefficient of shear-induced diffusivity is obtained. In our experiments the particle Reynolds numbers are 𝒪(10−1). The experimental results are compared to the existing measurements of the shear-induced diffusion coefficient obtained at lower particle Reynolds numbers and based upon short-time data. We find a shear-enhanced diffusion coefficient D⊥/γ̇a2=𝒪(0.1) for a volume fraction of φ≊0.25; this is comparable to existing results from previous low particle Reynolds number studies (ℛ&amp;lt;10−3).

A new procedure for assessing the performance of flocculants

Laboratory batch cylinder tests are commonly used to assess the effectiveness of flocculants to enhance the settling rate of suspensions. In this paper, the performance obtained from batch cylinder tests is compared to that from a new flocculation system based upon vertically mounted, concentric rotating cylinders (Couette geometry). It has been shown that the results from batch cylinder tests are very dependent upon factors such as the number of inversions and the cylinder diameter. These limitations are overcome by conducting flocculation in a continuous mode under reproducible mixing conditions such as prevail within a mechanically driven Couette device. Results obtained with this equipment are found to be of much higher reproducibility than from cylinder tests conducted under set conditions.

Viscosity of deoxygenated haemoglobin-A in response to calcium ions and to 2,3-bisphosphoglycerate

Couette device was used for the examination of the flow behaviour exhibited by haemoglobin-A (Hb-A) solutions; the applied shear rates ranged from 0.624 x 10-3 through 4.59 s-1. The degree of O2 - saturation (Soz) was either above 90% (oxyhaemoglobin-A or OxyHb-A) or below 10% (deoxyhaemoglobin­ A or DeoxyHb-A). Besides O2 ; K+, ea2+ and 2,3-bisphosphoglycerate (2,3- BPG) were used as additional effectors of haemoglobin-A Thus, it has been possible to demonstrate: 1) The flow properties of OxyHb-A are strict Newtonian and are independent of any other effector added to the solutions. 2) The reduction of S02 does not alter the linear torque - shear rate relationship as known for OxyHb-A 3) K+ at 0.3 mmol/g Deo~Hb-A does not interfere with the Newtonian behaviour of DeoxyHb-A 4) ea2+ at 4 j.tmol/g DeoxyHb-A has no measurable influence on DeoxyHb-A rheology; at 24 j.tmol/g DeoxyHb-A, however, a fourfold rise in the viscosity of DeoxyHb-A was observed. 5) The addition of 2,3-BPG at fractions of 20 and 30 j.tmol/g DeoxyHb-A led to a 1.4 - fold increase in DeoxyHb-A viscosity for solutions with a DeoxyHb-A concentration below 32 g/100 ml; above 33 g/100 ml, however, the increase was 2.3 - fold if 2,3-BPG was present at the high relative concentration. 6) Micros­ copically, particle formation has been ascertained for the cases of increased viscosity in 4) and 5). This development of heterogeneity proved to be a reversible process on reoxygenation.

The viscosity and sedimentation of aggregating colloidal dispersions in a Couette flow

In order to interpret the time dependence of the measured torque in a steady shear experiment on an aggregating dispersion, a microrheological model has been used in which two existing models are integrated. In this microrheological model, a theory for fractal aggregation in shear flow is combined with a theory for the sedimentation and resuspension of non‐colloidal hard spheres. The former theory describes the viscosity as a function of shear rate, while the latter predicts the stress increase in a Couette device due to sedimentation. The connection between the two theories is made by identifying the aggregate parameters with the hard sphere parameters (size and volume fraction). The parameters of the aggregates as a function of shear stress are obtained by measuring a flow curve before sedimentation effects become significant and fitting this curve with the fractal aggregation theory. During the sedimentation, the aggregate size and volume fraction become a function of both time and position in the rheometer. Taking this into account, the predicted stress increase as a function of time is found to describe the experiments rather well. This result corroborates both of the existing theories.

Apparent wall slip velocity coefficients in concentrated suspensions of noncolloidal particles

The apparent wall slip velocity coefficient of concentrated monodisperse suspensions of 90 μm PMMA particles in a viscous Newtonian fluid was determined experimentally in a narrow gap Couette device by measuring the particle velocities across the gap using a laser doppler anemometer system and then extrapolating the results to the two walls. The slip coefficients thereby obtained were found to be insensitive to the magnitude of the applied shear rate and to equal, approximately, λ/8 for particle concentrations φ in the range 0.45≤φ≤0.52, where λ refers to the relative effective viscosity of the suspension with respect to that of the pure fluid.

Delayed slip after step strains in highly entangled polystyrene solutions

Apparent slip within a few micrometers of a glass surface is visualized with a microscope using small glass spheres as tracers suspended in a highly entangled 20% solution of polystyrene of molecular weight 8.42 million, confined between glass plates in a plane Couette device. After step shears of 0.5–5 strain units, particles within a few micrometers of each surface move by 70–500 μm, enough to unload more than half of the imposed shear strain at intermediate strains. The slip displacement, when plotted logarithmically against time, show a peculiar ‘‘kink’’ in the vicinity of the ‘‘Rouse’’ time of 100 s, similar to the kink reported for the shear stress. These results show that the peculiar rheological properties reported for highly entangled solutions are produced by an effective slip phenomenon that occurs either at the wall or in a thin disentanglement layer near it.

Flow birefringence experiments showing a shear-banding structure in a CTAB solution

Experiments of flow birefringence have been performed on a concentrated solution of CTAB submitted to shear in a Couette cell. Qualitative observations of the flow in the annular gap of the Couette device let us clearly see that the flow is divided into two layers when the shear rate exceeds a critical value. This shear banding structure is shown on a set of photographs. The extinction angle χ and the retardation ϕ were measured as a function of the shear rate\(\dot \gamma \) for different temperatures. The results are in agreement with the predictions of the shear banding theory.

Droplet break-up in simple shear flow in the presence of emulsifiers

The break-up of droplets in an inhomogeneous flow is a key process in emulsion manufacturing and manipulation. The break-up of droplets in a steady simple shear flow in the absence of emulsifiers has been studied extensively, and is by now well understood. In the presence of emulsifiers the interfacial tension is lowered, which promotes emulsification. In addition, however, the interface may acquire viscoelastic properties. These are known to be important for the prevention of coalescence, but their influence on droplet break-up has not been thoroughly established. In this paper we report a fairly simple phenomenological approach, which links the droplet break-up dynamics to the interfacial viscoelasticity, identifying the latter with the elasticity modulus of a deforming planar interface. Experiments were performed in a Couette device, in which break-up conditions for water droplets in oil were determined as a function of emulsifier concentration in the droplet phase. Below the critical micelle concentration (CMC) break-up requires a higher stress than above or far below the CMC. Over a viscosity ratio range of three decades the results have been found to agree with the model, which only uses independently known quantities derived from equilibrium interfacial tension properties.

EFFECTS OF STARTER CULTURES ON LINEAR VISCOELASTIC AND PHYSICAL PROPERTIES OF YOGURT GELS

ABSTRACTSet‐style yogurts were produced from cow's milk under standard conditions by using 11 commercially available starter cultures. Samples were prepared in control beakers and within the Couette device of a dynamic rheometer. Repeatability of rheometer yogurt fermentations proved to be satisfactory. Within the linear viscoelastic region a significant inverse relationship between storage modulus and tan δ of yogurt samples was found. Specific properties of starter cultures clearly affected response in dynamic frequency sweeps and in stress relaxation experiments. Dynamic and relaxation data showed good agreement. Relaxation spectra H(τ) were obtained from dynamic measurements and stress relaxation by approximation methods. Both the shape of H‐τ curves and the magnitude of H was affected by starter cultures. As compared to literature data on model casein gels and microstructural findings on yogurt the results imply that (a) a permanent network exists in yogurt, (b) interactions with long relaxation times contribute less to viscoelasticity in the case of yogurt produced with viscous starter cultures, and (c) attachment of mucogenic bacteria to the protein matrix via exogenic polysaccharides decreases yogurt firmness.

On the measurement of the relative viscosity of suspensions

The relative viscosity of a suspension of rigid, noncolloidal particles immersed in a Newtonian fluid was measured in a Couette device and was found to be shear thinning even for values of the solids fraction as low as 20%. Although such behavior was reported previously, no satisfactory explanation appears to have been given thus far. It shall be shown presently, however, that, at least for our systems, this shear‐thinning effect was due to a slight mismatch in the densities of the two phases. Moreover, the apparent relative viscosities measured in our apparatus were found to be in excellent agreement with those predicted theoretically using a model, originally proposed by Leighton and Acrivos [Chem. Eng. Sci. 41, 1377–1384 (1986)], to describe viscous resuspension, according to which the measured relative viscosity should depend on the bulk particle concentration and on the dimensionless Shields number A, and should attain its correct value for a well‐mixed suspension only as A→∞. The predictions of this...

Shear-Induced Size Segregation Phenomena in Bidisperse Suspensions

The behavior of tracer particles in a concentrated suspension of non-colloidal spheres sheared in a parallel plate viscometer at low Reynolds number was investigated. The tracer particles were shown to migrate radially, with the tracer particles larger than those making up the suspension migrating radially outward. The migration velocity of the large tracer particles was measured, and found to be consistent with a shear-induced migration scaling (Leighton and Acrivos, 1987b), however the direction of the migration was the reverse of that suggested by the experiments of Abbot, et al. (1991) in a wide gap Couette device. The particle migration and segregation described here are interpreted in terms of the curvature of the flow geometry.

An improved flocculator design for use in particle aggregation experiments

Experimental studies of aggregate formation usually require knowledge of the rate of fluid shear. A type of Couette device which consists of a fixed inner and a rotating outer cylinder provides an easily quantifiable two-dimensional laminar flow in the annular space between the cylinders. Early designs of such devices had several drawbacks, however, including the sedimentation of particles in vertically oriented devices and difficulties in the ease and repeatability of sampling in horizontal units. The new horizontal design presented here addresses these problems through the use of a movable endcap and sampling port. This device has been shown experimentally to minimize the effects of sedimentation and also to allow multiple samples to be taken without the inconveniences associated with previous horizontal units. Use of the device is illustrated by repeated measurements, over a period of 34 d, of the coagulation efficiency (a) of the diatom Skeletonema costatum.

Shear-induced resuspension in a couette device

The viscous resuspension of an initially settled bed of particles resulting from the application of a laminar shear flow was studied experimentally in a narrow-gap Couette device. The measured height of the resuspended layer was found to be in excellent agreement with that predicted theoretically using a model developed by Leighton & Acrivos, in which the downward gravitational particle flux is balanced by a corresponding upward flux due to shear-induced particle diffusion.

Shear stress effects on growth and activity of Lactobacillus delbrueckii subsp. bulgaricus

Cells are frequently submitted to shear stresses during industrial processes. Shear stress can be either beneficial or detrimental to the cells depending on the organism and on the level of intensity. The present work was designed to study the effect of shear stress on cell activity of a widely used lactic acid bacterium, Lactobacillus delbrueckii subsp. bulgaricus ( L. bulgaricus). A constant shear stress bioreactor, based on Couette device, was developed and used to control shear stress from 0 to 72 Pa during a 4-h cultivation of a supplemented whey permeate medium with L. bulgaricus at 42°C. In order to reach high shear stresses and to perform experiments within the laminar flow range where hydrodynamic conditions are accurately defined, the medium was thickened with carboxymethylcellulose (CMC). pH, cell counts, optical density, lactose and lactic acid concentrations were monitored during culture, and cell activity was evaluated after culture and after a freezing treatment at −80°C, using a standardized activity test based on optical density measurement. Cell metabolism was significantly improved by intermediate shear stress levels (36 and 54 Pa) during culture. Furthermore, biomass concentration, evaluated by optical density, was clearly higher at 36 Pa. Cell lengthening was observed, which was mainly related to the presence of CMC and partly to shear stress level, especially at 36 Pa. Hydrodynamic conditions during culture could affect the membrane permeability of the cell and its resistance to freezing. Cells cultivated at 72 Pa were certainly weakened by shearing forces, and these cultures exhibited lag times twice as long after freezing as those grown at 36 Pa. Furthermore, after freezing, cultures grown at 36 Pa had shorter lag times than at 0 Pa (1.1 and 1.3 h, respectively) and higher specific growth rates (1.24 and 0.99 h −1, respectively).

A rheo-optical study of shear-thickening and structure formation in polymer solutions. Part I: Experimental

Simultaneous measurements of the optical and theological response of solutions of highly fractionated polystyrenes have been made, in-situ, to ascertain the connection between flow-induced structure formation and the phenomenon of shear-thickening. Transient and steady state viscosity, dichroism, birefringence and the associated orientation angles were measured in decalin and bromobenzene in the semi-dilute region using a couette device capable of shear rates up to 8,000 s−1. A one-to-one correlation has been found between the occurrence of maxima in the dichroism and minima in the viscosity. While the size and shape of the shear-thickening structures could not be directly determined, results suggest they are intermediate in size between a cluster of entangled chains and a completely phase-separated liquid. For solutions exhibiting shear-thinning alone, no maximum in dichroism was observed, the signal instead showed a saturation behavior at high shear rates. Birefringence was found to be insensitive to the structure formation and attributable to that of the dissolved chains or entanglement regions. The kinetics of the structuring process leading to shear-thickening are instantaneous and completely reversible and there is a concentration window, above and below which only shear-thinning occurs.