45O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS If a yield value is observed equation (VIII) becomes (12) 1 v (s- So) n (IX) Equation (IX) reduces to Bingham's equation for plastic flow when n = 1. in all other cases • * is not the same as the true viscosity •q, nor does it have the dimensions of viscosity. When n • 1, the sample is said to exhibit "quasiflow." Many criticisms have been levelled against equations (VIii) and (IX) since they can be made to fit most curves by suitably adjusting the values of the constants. Furthermore, when S changes, n sometimes alters • * may also be affected. COHESION Interest in the stickiness, or tackiness, of materials led to the design of an instrument to measure the force required to pull a movable plate away from a stationary plate when a thin layer of sample is sandwiched between them (13,14). This principle has proved useful for measuring the increase in tackiness which occurs when lotions and ointments dry after rubbing into the skin (15). Two kinds of forces are involved in these measurements, viz. cohesion between the individual particles, and adhesion between the sample and plates. When cohesion exceeds adhesion, rupture occurs at the surface of the sample in contact with the movable plate, thus providing a measure of stickiness. Alternatively, if adhesion exceeds cohesion, rupture occurs within the sample itself, and one derives a measure of the internalstrength. Some materials show decreasing stickiness with increasing stiffness. Their consistency can be examined with an instrument (16) of the basic design shown in Fig. 4•. The sample is deposited in a Petri dish and the surface is carefully levelled and smoothed out. A circular steel plate of smaller diameter than the dish, which is connected by a vertical rod to two cords via metal rod junctions and an Oldak gauge, is brought carefully into contact with the upper surface of the sample. The upper cord is attached to an axle which applies an upward force to the steel plate when rotated by a constant speed motor. This force causes a metal plate, which passes through the lower metal rod junction, to bend in the centre since it is fixed towards its extremities over two knife edges. The deformation is recorded by the gauge. The maximum reading recorded, viz. when the upward acting force just exceeds the cohesion, is related to the strength of the material.

TECHNIQUES FOR ASSESSING RHEOLOGICAL PROPERTIES o 451 1%gure 4 St•ckiness meter. (a) Movable framework. (b) Gauge for measuring deflection of spring {d). (c) Adjustable knife edges. (d) Spring. (e) Grooves. (f) Upper cord connecting axle (k) to framework. (g) Lower cord connecting frame- work to test body (m). (h) Motor. (i) Gear. (j) Handle. (k) Axle. (1) Rod. (m) Test body. (n) Container for sample. (o) Scale. (p) Fixed frame. The sample draws out into a "neck" as the axle rotates and pulls up the cord. This neck progressively thins in the middle and eventually ruptures, leaving a layer of sample on the underside of the metal plate.