SELECTION OF SUSPENDING AGENTS 125 size and shape of the container and the air space in the container. No precise measurements have been made of these shearing stresses but they are believed to be approximately 100 to 600 dynes/cm. = and 25 to 150 dynes/cm. • for shaking and pouring, respectively. When the different conditions of shearing stress are understood, it be- comes readily apparent why a good suspension vehicle should be both thick and thin. Also, it is obvious that in formulating suspensions with the minimum of phase separation the conditions of low rates of shear should be of prime concern. This is where measurements of the theological behavior of suspension vehicles should be made. The higher rates of shear should also be considered, but the measurement of these conditions is unnecessary as long as the suspension can be shaken and poured with ease. IOO-- IO-- .IO-- .Ol • 2,0 # I,O • • • •o.i i I I0 I00 I000 Particle Radius(/•) Figure 1.--Shearing stresses of particles (spheres) settling under the influence of gravity.

126 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In summarizing the first portion of the discussion, we can say that the rheological requirement of a good suspension vehicle is (1) that it have infinitely high viscosity at low rates of shear or under the conditions when particle sedimentation would occur and (2) that it have low viscosity at the higher rates of shear or when the suspension is shaken and poured. With this in mind, it is possible to construct a hypothetical flow curve of shear- ing stress against rate of shear of a suspension vehicle having the desired theological properties. In doing so, however, we find that such a curve would be similar to that of ideal plastic flow. This is shown in Fig. 2. Rate of Shear Figure 2.--Flow curve of ideal plastic flow. An ideal plastic fluid is unique in that it will flow only after the applied shearing stress exceeds a minimum value. This minimum shearing stress is called the yield stress of the fluid and is indicated by the intercept f on the stress axis (Fig. 2). When referring to dispersed systems, yield stress may be defined as the resistive force of a fluid which prevents or retards the motion of particles in the fluid. It is obvious that a suspension vehicle with ideal plastic behavior is most desirable. In addition to fulfilling the viscosity requirements, it possesses a yield stress which has been found to contribute to the suspension quality of the vehicle. The search for such a vehicle would be fruitless, however, since ideal plastic behavior is seldom, if ever, found among the