446 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are interpreted by Stokes's law for a sphere descending through a liquid at constant speed. If the sphere is large in comparison with the diameter of the tube through which it falls a more complex equation has to be used. Recently, Scott-Blair and Oosthuizen (4) have shown that several tests can be made on a single sample provided very small spheres are used. By rotating the tube carefully between tests unsheared material becomes available for further tests. Systems deviating only slightly from Newton- ian behaviour can also be examined by using spheres of different radii and densities (5). The ultrasonic viscometer operates on principles which are different from those of other viscometers. A probe and an electronic computer are the essential components. At the end of the probe is a thin alloy steel blade which is excited by a short electrical impulse, so producing ultrasonic shear waves in the medium around the probe. The energy involved in shearing the sample is translated into viscosity units by the computer. Whilst this instrument lends itself to automatic control of h, and detection of deviations therefrom, its application is limited to Newton- ian fluids since only one rate of shear is available. Table III summarizes the equations required for calculating v, S, h, and yield value, from data acquired with the principal types of viscometers discussed in this section. Corrections for "end effect," kinetic energy, etc., should be introduced where necessary. In capillary viscometers both S and v vary from 0 at the centre of the capillary to a maximum at the wall. When considering non-Newtonian flow it is customary to base the calculation on conditions prevailing at the capillary wall. Co-axial and cone-plate viscometers are particularly useful for studying the consistency of semi-solids since they offer a wide range of v extending down into a region of very low shear. The latter, i.e. 1 sec% is suitable for studying stationary structure. For many practical purposes the Haake "Rotovisko" is the most versatile instrument as it comprises both coaxial and cone-plate viscometers. The Weissenberg rheogoniometer offers facilities for both rotational and oscillatory shear. Oscillatory shear is particularly useful for studying structure since small amplitudes affect structure to a lesser extent than complete rotation. Study of "stationary" structure Rheological studies are not restricted to measuring •. Other para- meters can be derived for a disperse system by studying the creep behaviour in a low shear coaxial cylinder viscometer. In one form of the instrument

TECHNIQUES FOR ASSESSING RHEOLOGICAL PROPERTIES 447 Figure 8 The Calculation of Creep Compliance (Fig. 3) the surfaces of both cylinders are finely ribbed to prevent sample slippage. The outer cylinder is kept stationary while the inner cylinder is rotated slowly by a pulley-weight arrangement. Angular rotation of