PSYCHO-RHEOLOGY 249 (a) (b) (c) (d) Figure 1. Typical shear stress-shear rate curves. (a) For a Newtonian liquid. (b) For a non-Newtonian liquid without a yield stress. (c) For a fluid where the yield stress is a very important factor. (d) For a fluid where the yield stress is present but not very important. and Fig. l(d) a fluid where a yield stress is present but not very im- portant. The shear stress-shear rate diagram can be used to answer four questions, namely How will the product appear in the container, when it may be tilted, prodded or gently shaken'?. How will it stand up to transport and storage? How will it transfer from the container to its use situation? How will its properties change and at what rate when the product is used ?

250 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The initial impression This is very important to the saleability of any product. A shear rate of 50-100 s -• corresponds to that which the product would suffer when tilted, gently shaken or prodded. At this shear rate a measured viscosity of less than 100 cp would be easily pourable as the viscosity increased the product would become gradually stiffer until at 1000 cp it would appear solid under these conditions. The importance of looking at the viscosity at the appro- priate shear rate is obvious from Fig. l(c). Some products appear solid at low shear rates but liquid at high ones. Storage and transport properties Since cosmetics will often be stored for several months or even years before use, it is important that they are stable. This simply means that the physical properties chosen as being of importance to acceptability must remain within some specified limits during the expected lifetime of the pro- duct. The acceptable limits are defined by sensory evaluation. It must be ascertained by physical testing whether a product can stand up to the shear- ing it will receive in transport so that a product arrives on the shelf in a suitable condition. This does not apply of course to thixotropic products since they regain all their original structure when allowed to rest after shearing. The impression on removing from the container The yield stress has an important influence on the. way in which a pro- duct will leave the container. Its position on the i-3• diagram defines at what stage of usage (at what shear rate) breakdown will occur, how much force is needed (what shear stress) to initiate breakdown, and how much of the total structure is destroyed when the yield stress is exceeded. A simple example illustrates its importance. When the end is removed from a tooth- paste tube the paste must not run out it must be held in place by its yield stress. The pressure needed to exceed the yield stress must be reasonably low, otherwise either the tube would burst or the sink would be full of toothpaste! When the yield stress has been exceeded the product must still retain most of its structure, so that it will remain intact on the brush and not drip off before it can be used. Given these requirements it is possible to predict the shape of a toothpaste flow curve without knowing any of the numerical values. Yield properties are varied by the dispersed phase concentration and the types and amounts of stabilizers used. Some gums and gelling agents give non-recoverable breakdown at 10w shear stresses, while clays can