Zimm-Crothers viscometer

Here T 0 means the viscosity of the solution at zero shear rate, r g the viscosity of the pure solvent, 6 the number of rods per unit volume and L the lengths of the rods. As long as L is still shorter than the mean spacing between the aggregates, the viscosity of the detergent solution is still very low and always small in comparison with the solvent viscosity. As a consequence, very accurate measurements are necessary to get some informations on the dimensions of the rodlike micelles. The Zimm-Crothers viscometer is a very sensitive instrument and it was possible to measure the viscosity of the dust-and airfree solutions with an accuracy of 0,2% at very low shear rates (15). 

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... 

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

For the present fluid-mechanical tests a homologous series of polyacrylamide (PAAm) samples was used whose mean weights, of the molar masses were determined by means of a low-angle laser-light-scattering photometer. The PAAm samples exhibit virtually the same molecular weight distribution Myy/Mn = 2.5 the intrinsic viscosity [ 7 1 was deter-minded in a Zimm-Crothers rotational viscometer, since the polymer solutions are subjected to a very low shear rate in this instrument. The porous media flow tests were carried out with the aid of an instrument such as described in Refs. [1, 2]. Reference is also made to these studies as regards the test procedure and evaluation of the measured data. 

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