Concentric cylinder viscometer Newtonian shear rate

From equation 3.7, as shown in Table 3-1, it can be deduced that for non-Newtonian foods the correction term to Newtonian shear rate depends on the extent to which a fluid deviates from Newtonian behavior and the size of gap between the inner and outer cylinders. To minimize errors in the calculated shear rates, it would be preferable to employ concentric cylinders with narrow gap between them. Therefore, some of the commercially available units have the ratio of the radii (n/ro) of about 0.95. However, it is not obvious that the software provided by viscometer manufecturers contains corrections for non-Newtonian flow behavior, so that, when ever possible, it is advisable to use narrow gap concentric cylinder systems for fluids that deviate considerably from Newtonian behavior. 

Other errors, which could influence the results obtained, are, for example, wall effects ( slipping ), the dissipation of heat, and the increase in temperature due to shear. In a tube, the viscosity of a flowing medium is less near the tube walls compared to the center. This is due to the occurrence of shear stress and wall friction and has to be minimized by the correct choice of the tube diameter. In most cases, an increase in tube diameter reduces the influence of wall slip on the flow rate measured, but for Newtonian materials of low viscosity, a large tube diameter could be the cause of turbulent flow. ° When investigating suspensions with tube viscometers, constrictions can lead to inhomogeneous particle distributions and blockage. Due to the influence of temperature on viscosity (see Section Influence Factors on the Viscosity ), heat dissipated must be removed instantaneously, and temperature increase due to shear must be prevented under all circumstances. This is mainly a constructional problem of rheometers. Technically, the problem is easier to control in tube rheometers than in rotating instruments, in particular, the concentric cylinder viscometers. ... 

For non-Newtonian liquids the capillary viscometer is inappropriate, although in principle capillary viscometers of different internal radii could be used to give data for different average shear rates. A very wide range of such averages would be needed. In practice a concentric cylinder or related rheometer is used instead. 

Some rotational viscometers employ a rotating disc, bar, paddle or pin at a constant speed (or series of constant speeds). It is extremely difficult to obtain tme shear stress, and the shear rate usually varies from point to point in the rotating member. In particular, the velocity field of a rotating disc geometry can be considerably distorted in viscoelastic fluids. Nevertheless, because they are simple to operate and give results easily, and their cost is low, they are widely used in the food industry. While they may be useful for quality control purposes, especially Newtonian foods, the reliability of their values should be verified by comparison with data obtained with well defined geometries (capillary/tube, concentric cylinder, and cone-plate). 

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. 

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-... 

Fignre 13.11 shows the apparent viscosity versus shear rate plots obtained by a coaxial cylinder viscometer for mixtures of PEO/SDS. The addition of SDS to pure PEO solution causes an increase in apparent viscosity. The solution shows a Newtonian behavior at low SDS concentrations. The solution becomes pseudo plastic shear-thinning at high SDS concentration. 

In steady state measurements one measures the shear stress (x)-shear rate (y) relationship using a rotational viscometer. A concentric cylinder or cone and plate geometry may be used depending on the emulsion consistency. Most cosmetic emulsions are non-Newtonian, usually pseudoplastic as illustrated in Fig. 1.11. In this case the viscosity decreases with applied shear rate (shear thinning behavior (Fig. 1.11)), but at very low shear rates the viscosity reaches a high limiting value (usually referred to as the residual or zero shear viscosity). 

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