Viscometer, Couette

Cottrell equation Cottrell unit Couchman equation Couette flow Couette viscometers Cough drops Coughlozenges... 

Daucus carota Couette viscometer laminar regrowth mitochondrial activity membrane integrity cell lysis (cell number) non-growth (sterile) [57, 69]... 

Fig. 8. Sustained damage in Daucus carota suspensions, as a function of total energy expended, under laminar flow conditions in a Couette viscometer. Redrawn from Dunlop et al. (1994) Effect of fluid shear forces on plant cell suspensions. Chem Eng Sci 49 2263 - 2276, with permission of Elsevier Science...

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-... 

The viscosities were measured on a Contraves LS 30 low shear couette viscometer at a shear rate of 1.3 s thermostatted at... 

By taking into account that v = cor, the velocity profile of the fluid layers in the Couette viscometer with rotating inner cylinder is given in Figure 13.18. Equation (13.70) in combination with the first boundary condition in Eq. (13.69) gives... 

Dynamic crystallization under precise control of shear and temperature was studied in a prototype apparatus specially developed. The whole cell, similar to a Couette viscometer, was made out of glass. The inner cylinder rotated at a controlled speed, CO, while the outer wall was fixed. A double-mantel with a circulation of water allowed precise control of the temperature. Temperature of cocoa butter in the cell was measured with a chromel-alumel thermocouple. Measures were recorded with a data acquisition system. Shear rate imposed to cocoa butter in the system could be estimated from the rotation speed of the inner cylinder, assuming that the fluid is Newtonian and incompressible. There is no normal speed, only tangential speed. The shear rate, y> in the specimen is a single function of the radiu. In the rest of this work, shear in the cell was characterized by its average value, y, calculated by integration over the cell thickness (7). 

Problem 3-24. Planar Couette Viscometer with Time-Dependent Boundary Motion. It... 

Finally, there are industrial rheological instruments in common use that allow determination of viscosities for liquids to within a few percent, which is sufficient for many purposes. These involve measuring the torque required to maintain a given velocity for a fluid confined in the annulus between two concentric cylinders, one of which is rotated (Couette viscometer), or a fluid confined between a flat plate and a rotating cone (cone-and-plate viscometer). 

The apparatus with a cylinder rotating in liquid in a fixed cylinder is usually called the Couette viscometer in the Pochettino viscometer6 the two cylinders rotate in opposite directions. In Searle s7 apparatus (Fig. 6.VIIIE) a brass cylinder C of radius a is supported on a vertical spindle A pivoted at its ends, and dips to a depth / in a liquid in a cylinder B of radius b. The cylinder C is rotated by weights mg in scale pans which pull on cords passing round a drum of diameter d attached to the spindle, on the top of which is a disc a used with an index b to measure the period of rotation, t0. Then ... 

Figure 4-10. Concentric cylinder rotational viscometer. (a) Basic scheme of operation. (b) Couette viscometer with guard rings and liquid seal.

A nearly identical flow pattern exists in the annular space between two concentric cylinders, one rotating and the other stationary, provided the width of the annulus, B, is small compared to the diameters of the cylinders. A device that makes use of this is the Couette viscometer. By measuring the torque required to rotate one cylinder at a known speed, the viscosity may be readily calculated from Eq. (5.70). 

The cone-and-plate viscometer is one of the rotational methods of measuring the polymer viscosity. It consists of a fiat horizontal plate and a cone with an obtuse angle. The cone touches the plate at its tip and rotates at a constant speed. The melt is charged into the gap forming between the horizontal plate and the cone. The rotational velocity determines shear rate and the torque applied gives shear stress. Shear rate is constant across the gap, thus it eliminates the need for non-Newtonian behavior of the melt. In a plate-plate viscometer, the cone is replaced by a second flat plate. The Couette viscometer is comprised of two concentric cylinders where one can be rotated at a constant speed. 

Shear dependence of the limiting viscosity nvunber is probably only significant for values of [i ] of several hundred, but this has always to be confirmed. Measurements can be made in a modified capillary viscometer. Details of construction of one for this purpose have been given, and the instrument has been used to study dextran fractions. Shear dependence can also be measured in a rotating cylinder (Couette) viscometer, and a simplified model has been described. ... 

Let us now take the planes y, and — yj to be the walls of the Couette viscometer so that yi + y2 = and U is the relative velocity of the walls. When the viscometer is filled with pure liquid, the shear stress t is given by the subscript 0 referring to the particle-free state. However, with particles in the fluid the planes move with the new relative velocity... 

These facts indicate that, from the chemical point of view, there is only a single actomyosin complex of composition three parts of myosin to one of actin and that mixtures in any other proportion contain one component in excess. If this is so, the maximum in true viscosity should be at the 3 1 ratio, a point which should be tested in the Couette viscometer. Since artificial actomyosin solutions contain several very rapidly sedimenting fractions (H. H. Weher, 1947 Snellman and Gelotte, 1950 Johnson and Landolt, 1950), and since their viscosities are very variable (Jaisle, 1951 see Section III, 4d), it would have to be assumed that the same 3 1 complex forms threads of very vaiiable length and thickness. This may be so, but the problem requires further elucidation. It is possible that actomyosins with different physical properties represent sharply defined stages in the interaction of the two components. 

It was assumed that the solutions were Newtonian at the shear rates in the capillary. This assumption was assessed with Couette viscometer measurements of these dilute solutions over a range of shear rates and was reasonable. Deviations were found for solutions at the higher concentrations, as indicated by negative departure from linearity of the reduced-viscosity-concentration plots these values were not used for intrinsic viscosity and Huggins constant determination. 

Merrill EW, Smith KA, Shin H, Mickley HS (1966) Study of turbulent flows of dilute polymer solutions in a couette viscometer. Trans Soc Rheol 10 335-351... 

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... 

Rotational methods are particularly suitable for studying the flow of non-Newtonian liquids. An example is the concentric cylinder (or Couette) viscometer. The liquid is sheared between concentric cylinders, which are moving relative to one another. The outer cylinder can be rotated (or oscillated) at a constant rate and the shear measured in terms of the deflection of the inner cylinder, which is suspended by a torsion wire. Altema-... 

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