Rheometers Couette

Couette rheometer Couette-Strdmung Couette flow Craze (Pseudobruch)... 

Baumwoll-Tupfer cotton fiber Baumwollfaser cotton gloves Baumwollhandschuhe cotton oil Baumwollsaatbl cotton stopper Wattestopfen Couette flow Couette-Strbmung Couette rheometer Couette-Rheometer... 

The first drag flow rheometer, Couette s concentric cylinders shown in lugure 5.1.1, was a controlled strain device. Couette fixed the angular velocity of the outer cup and measured the torque on his inner cylinder by the deflection of a suspending wire. 

Table 5. Results from shear degradation measurements in two different rheometers (capillary, Couette-type) compared with model predictions of narrowly distributed polystyrene samples in toluene... 

The relative viscosity rj and storage modulus G were determined by Cirkel and Okada in experiments using a rheometer in oscillatory rotational mode and Couette sample geometry as a function of Nafion volume fraction, cp, and angular frequency, a>, for the acid and sodium forms at 25 °C. Parallel experiments... 

Experimentally, the dynamic shear moduli are usually measured by applying sinusoidal oscillatory shear in constant stress or constant strain rheometers. This can be in parallel plate, cone-and-plate or concentric cylinder (Couette) geometries. An excellent monograph on rheology, including its application to polymers, is provided by Macosko (1994). 

It was of interest to compare the results obtained with the FRAP technique with those obtained with classical surface rheological techniques. Our detailed knowledge of properties of solutions of /3-lg containing Tween 20 made this an ideal system on which to compare the methods. Firstly, surface shear viscosity measurements were performed on the Tween 20//3-lg system [47] using a Couette-type torsion-wire surface rheometer as described previously [3,48]. All the experiments were carried out at a macroscopic n-tetradecane-water interface at a fixed protein concentration of O.Olmg/ml. In the absence of Tween 20, the surface shear... 

The Couette rheometer. Another rheometer commonly used in industry is the concentric cylinder or Couette flow rheometer schematically depicted in Fig. 2.48. The torque, T, and rotational speed, 0, can easily be measured. The torque is related to the shear stress that acts on the inner cylinder wall and the rate of deformation in that region is related to the rotational speed. The type of flow present in a Couette device is analyzed in detail in Chapter 5. 

In the cone and plate rheometer, a cone-shaped bob is placed against a flat plate so that the fluid to be studied may be placed into the gap between the lower face of the cone and the upper face of the plate. Again, in the Searle method, the cone is rotated while in the Couette method the plate turns. In each case, the torque on the cone is measured. Figure 6.5 shows a Searle-type cone and plate arrangement. For this arrangement the shear stress is given by ... 

Normally the coordinate system is chosen in such a way that T13 = T31 = T23 = T32 = 0 In general, use is made of normal stress differences, N1 and N2, because they do not include undetermined hydrostatic pressures that are always present but not affect the material properties (as long as they are not too high). In Table 15.1, also the possibilities to determine the normal stress differences or combinations are depicted. In the modem rheogoniometers also normal stress differences can be determined but. They follow from measurements of normal forces, Fn, or normal stresses, T22, as is also depicted in Table 15.1. For the measurements of the normal stresses T22 pressure gauges have to be mounted in the Couette cylinders, in the capillary of the capillary rheometer (in both cases quite difficult to mount) and in the plate of a cone and plate instrument at several distances from the axis (not that difficult). Sometimes use is made of a slit rheometer instead of a capillary rheometer, because pressure gauges are much easier to mount (Te Nijenhuis, 2007, Chap. 9.1.2). 

Riljanski, T. 1989. A mefliod for correction of the wall-slip effect in a Couette rheometer. Rheol. Acta 28 61-64. 

GAP-DEPENDENT APPARENT SHEAR RATE. Indirect evidence of slip, as well as a measurement of its magnitude, can be extracted from the flow curve (shear stress versus shear rate) measured at different rheometer gaps (Mooney 1931). If slip occurs, one expects the slip velocity V (a) to depend on the shear stress a, but not on the gap h. Thus, if a fluid is sheared in a plane Couette device with one plate moving and one stationary, and the gap h is varied with the shear stress a held fixed, there will be a velocity jump of magnitude Vs(ct) at the interfaces between the fluid and each of the two plates. There will also be a velocity gradient >(a) in the bulk of the fluid thus the velocity of the moving surface will be y = 2V,(a) + y (a)/i. The apparent shear rate V/h will therefore be... 

A plot of yapp against 1 / h will then be a straight line with slope 2Vs. This method has been used to measure the slip velocity for polyethylene melts in a sliding plate (plane Couette) rheometer by Hatzikiriakos and Dealy (1991). Analogous methods have been applied to shearing flows of melts in capillaries and in plate-and-plate rheometers (Mooney 1931 Henson and Mackay 1995 Wang and Drda 1996). 

Problem 1.5 Suppose you shear a polymer melt in a plane-Couette rheometer at a shear stress a of 0.1 MPa and measure the apparent shear rate = V/h, where V is the velocity of the moving plate (the other is stationary) and h is the gap between the plates. At a gap h of 2 mm, you measure an apparent shear rate of 1.5 sec- aXh — mm, you measure 2 sec- and at h = 0.5 mm. you measure 3 sec. What is the slip velocity V, and the true shear rate y at this shear stress What is the extrapolation length bl... 

The advantage of the vane technique is that material is allowed in between the prongs of the vane and in effect shears the material upon itself (rather than inducing shearing between a mechanical fixture and the material, as would be seen if a standard cup-and-bob or Couette rheometer were used). This eliminates the potential for slippage between a mechanical fixture and the material. A typical response is given in Figure 4.5. 

For dynamic mechanical and steady shear measurements, the Rheometric Scientific RFSII rheometer was used equipped with the sensitive range force rebalance transducer and couette geometry or parallel plate tooling. 

Figure 2-9. A number of simple flow geometries, such as concentric cylinder (Couette), cone-and-plate, and parallel disk, are commonly employed as rheometers to subject a liquid to shear flows for measurement of the fluid viscosity (see, e.g., Fig. 3-5). In the present discussion, we approximately represent the flow in these devices as the flow between two plane boundaries as described in the text and sketched in this figure.

Figure 3-5. Typical rheometer geometries (a) parallel disk, (b) concentric cylinder (Couette) geometry, (c) cone-and-plate. Either the angular velocity is set and one measures the torque required to produce this rotation rate, or the torque is set and one measures the angular velocity. We analyze the Couette device in this section.

Figure 13.13. Comparison of the behavior predicted from Equation 13.35 with the data tabulated by de Kruif et al [43] for the viscosity of dispersions of sterically stabilized hard silica spheres in cyclohexane. There are no adjustable parameters in Equation 13.35. Relative viscosity denotes r (dispersion)/r (cyclohexane). Relative volume fraction denotes 0/0. Couette and parallel refer to measurements with a Couette rheometer and a parallel plate rheometer, respectively. Zero and infinite refer to the limits y —>0 and y- < >, respectively.

Couelle Flow The flow of liquid in the annulus between two concentric cylinders that rotate at different speeds. In the Couette rheometer, one cylinder rotates, and torque is measured at the other. See also Rheometer. 

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