Couette-type viscometers

It can be concluded that the Couette-type viscometer may well find its place in standard powder testing because it can measure the stress-strain relationships for aerated powders and thus provide a basis for design of equipment for handling of such powders. 

Calf thymus and salmon sperm DNA were used in this experiment. The length of calf thymus ranges from 10,500 to 1500 A. and that of salmon sperm DNA from 7400 to 1300 A. A 20-kc. sonic oscillation is applied to produce smaller DNA samples. Viscosity is measured with a Rao couette-type viscometer with varying shearing stress. It is obtained by extrapolating the consistency curve to zero shear. 

Viscosity Measurements. A Zimm-Couette type low shear viscometer was used. The intrinsic viscosities were estimated from single concentration viscosity measurements using the equations for the concentration dependence of the specific viscosity (5,6). The Mark-Houwink equation was used to determine My (5,6). 

Typical of this class of viscometer is the coaxial or Couette type of instrument described in Volume l, Section 3.7.4. The sample fluid is contained within the annular space between two coaxial cylinders, either of which may be rotated by a motor with the remaining cylinder suspended elastically in such a way that the torsional couple exerted on the latter can be measured. If the outer cylinder of radius r2 rotates with an angular velocity cou and the inner cylinder of radius r, is stationary, and the torque (or viscous drag) per unit length of cylinder exerted on the inner cylinder is T, then, for a Newtonian fluid(49) ... 

In order to model the flow behavior of molten silicate suspensions such as magmas and slags, the rheological behavior must be known as a function of the concentration of suspended crystals, melt composition, and external conditions. We have determined the viscosity and crystallization sequence for a Kilauea Iki basalt between 1250°C and 1149°C at 100 kPa total pressure and f02 corresponding to the quartz-fayalite-magnetite buffer in an iron-saturated Pt30Rh rotating cup. viscometer of the Couette type. The apparent viscosity varies from 9 to 879 Pa.s. The concentration of suspended crystals varies from 18 volume percent at 1250°C to 59 volume percent at 1149 C. The molten silicate suspension shows power-law behavior ... 

Because of the problem of sealing at both ends, this so-called ribbon viscometer can only be realized for materials of extremely high viscosity. The properties of a ribbon viscometer are shown to a good approximation by a rotation viscometer of the Couette type (see Section 9.5.2). In Couette viscometers, a rotor revolves around a stator (or vice versa). The viscous... 

To determine 97sp = 97rei — 1 for i7rei = 1.2 to an accuracy of 1%, the viscosity ratio must be determined to an accuracy better than 0.2%, and the viscosities themselves to an accuracy better than 0.1% (compounding of errors). Capillary viscometers are particularly suitable for such determinations. The usual rotation viscometer of the Couette type at best gives an accuracy of 1%. Falling-ball viscometers are even less exact. 

Since the effects of shear stress are particularly strong in the case of rodlike macromolecules, rotation viscometers are frequently used for measurements on substances such as deoxyribonucleic acid (Figure 9-23). With a sufficiently low rotational speed and a narrow gap between rotor and stator, a linear shear-rate gradient can be produced between the rotor and stator of a rotation viscometer. With such narrow gaps between rotor and stator, the centering of rotor in the stator (or, in some viscometers, vice versa) is particularly important. In rotation viscometers of the Couette type, centering is achieved by use of a mechanical axis. A much better centering system is used in the Zimm-Crothers viscometer. This utilizes... 

As these polymers are hygroscopic the effective concentration of their aqueous solution was determined from the total carbon analysis (apparatus Dohrman, Xertex DC 80). The latter was also used for the composition determination to establish the tie lines in the phase diagram. Viscometric determinations were performed with a Couette s type viscometer (Low shear 30 Contraves) at 25 °C. In the range of shear rate and concentration we used, the sample exhibited a Newtonian behavior. 

The second type of viscometer, which is fairly often used in colloid science, is the CouETTE type, which exists in many constructions and the principle of which is illustrated by Fig. 11. A hollow cylinder A is set in rotation by some mechanism D a cylinder B is suspended by a wire c (on which is a mirror d) coaxially with A. In the space between A and B is the liquid, for wMch one wishes to determine tj. Thanks to the internal friction the cylinder B experiences a rotation and the wire a torsion the angle of rotation can be determined with the aid of a beam of light oti the mirror c and a reading telescope. The couple exerted on the cylinder B is Kyfn, in which K is a constant for the apparatus and ol the ai ular velocity of cylinder A as a result the cylinder B experiences a rotation, characterized by the angle of torsion of the suspension wire. If the apparatus is first calibrated with water (subscript ly) and then the torsion angle for a sol (subscript s) is measured, the expression... 

Rotating cone viscometers are among the most commonly used rheometry devices. These instruments essentially consist of a steel cone which rotates in a chamber filled with the fluid generating a Couette flow regime. Based on the same fundamental concept various types of single and double cone devices are developed. The schematic diagram of a double cone viscometer is shown in... 

Figure 11 shows the reference floe diameter for viscometers as a function of shear stress and also the comparison with the results for stirred tanks. The stress was determined in the case of viscosimeters from Eq. (13) and impeller systems from Eqs. (2) and (4) using the maximum energy density according to Eq. (20). For r > 1 N/m (Ta > 2000), the disintegration performance produced by the flow in the viscosimeter with laminar flow of Taylor eddies is less than that in the turbulent flow of stirred tanks. Whereas in the stirred tank according to Eq. (4) and (16b) the particle diameter is inversely affected by the turbulent stress dp l/T, in viscosimeters it was found for r > 1.5 N/m, independently of the type (Searle or Couette), the dependency dp l/ pi (see Fig. 11). 

Viscometric manipulations are simplified considerably by using the modified Ubbelohde viscometer described by Davis and Elliott.26,126 This type of capillary viscometer can be modified to enable an estimate of the shear correction to be made,127 although a Couette viscometer128 is more satisfactory for this purpose. 

Two main types of viscometers are suitable for the determination of the viscosity of a polymer melt The rotation viscometer (Couette viscometer, cone-plate viscometer) and the capillary viscometer or capillary extrusiometer. The latter are especially suitable for laboratory use since they are relatively easy to handle and are also applicable in the case of high shear rates. With the capillary extrusiometer the measure of fluidity is not expressed in terms of the melt viscosity q but as the amount of material extruded in a given time (10 min). The amount of ex-trudate per unit of time is called the melt index or melt flow index i (MFI). It is also necessary to specify the temperature and the shearing stress or load. Thus MFI/2 (190 °C)=9.2 g/10 min means that at 190 °C and 2 kg load, 9.2 g of poly-... 

Continuous viscometers based upon the Couette principle are able to measure the viscosity of both Newtonian and non-Newtonian fluids over a wide range (Table 6.7). A typical instrument of this type is illustrated in Fig. 6.40<5J). 

In a rotational viscometer the solution is filled in the annulus between two concentric cylinders of which either the external (Couette-Hatschek type) or the internal (Searle type) cylinder rotates and the other, which is connected to a torsion-measuring device, is kept in position. Let R, and Ro be the radii of the inner and outer cylinders, h the height of the cylinder which is immersed in the solution or its equivalent height if end effects are present, a> the angular velocity of the rotating cylinder, and T the torque (or moment of force) required to keep the velocity constant against the viscous resistance of the solution. It can be shown that the shearing stress (see, for example, Reiner, 1960) ... 

Figure 9-23. Rotation viscometers of the Couette (left) and Zimm-Crothers (right) type. R, rotor S, stator St, steel plate M, magnet.

It should, however, be noted that there exist some hints that bicontinuous microemulsions behave elastically. This has been assumed to be due to the differences observed in measuring viscosities once in a Couette flow and in the other case by a capillary viscometer. Here it was observed that the values obtained with the capillary viscometer are markedly higher. It has been suggested that in capillary flow a component of elongational flow is observed and that in this type of flow elastic components can be observed much earlier than in shear flow [106,107]. 

The flow-property or rheological constants of non-Newtonian fluids can be measured using pipe flow as discussed in Section 3.5E. Another, more important method for measuring flow properties is by use of a rotating concentric-cylinder viscometer. This was first described by Couette in 1890. In this device a concentric rotating cylinder (spindle) spins at a constant rotational speed inside another cylinder. Generally, there is a very small gap between the walls. This annulus is filled with the fluid. The torque needed to maintain this constant rotation rate of the inner spindle is measured by a torsion wire from which the spindle is suspended. A typical commercial instrument of this type is the Brookfield viscometer. Some types rotate the outer cylinder. 

Numerous methods for measuring fluid viscosity exist, for example, capillary tube flow methods (Ostwald viscometer), Zahn cup method, falling sphere methods, vibrational methods, and rotational methods. Rotational viscometers measure the torque required to turn an object immersed or in contact with a fluid this torque is related to the fluid s viscosity. A well-known example of this type of system is the Couette viscometer. However, it should be noted that as some CMP slurries may be non-Newtonian fluids, the viscosity may be a function of the rotation rate (shear rate). An example of this is the dilatant behavior (increasing viscosity unda increasing shear) of precipitated slurries that have symmetrical particles [33]. Furthermore, the CMP polisher can be thought of as a large rotational plate viscometer where shear rates can exceed 10 s and possibly affect changes to the apparoit viscosity. The reader can refer to the comprehensive review of viscosity measurement techniques in the book by Viswanath et aL [34]. 

The shear mode of operation is the term generally given to the simple shearing of the fluid, as in a Couette rotational or parallel plate type of viscometer but with an electric field applied between the moving and the stationary electrodes of gap size h (Fig. 6.63). With zero voltage V = 0) applied, most ER fluids exhibit near-Newtonian properties. When an electric field E = V/h) is applied to the fluid, there is an increased resistance to its movement which must be overcome before motion can take place (see Fig. 6.64 which is an idealised representation). Conventional constant temperature O and speed lv Couette laboratory techniques can normally only encompass shear rates (7 = cuR/h) up to several hundred s although cooled purpose made industrial clutch-type devices of similar geometry may reach 6000 s. ... 

Many types of rotational rheometers and viscometers have been developed. The cone and plate, couette (coaxial cylinder), torsional, and disc spindle types are the most common. 

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