INTRODUCTION TO THE RHEOLOGY OF DISPERSE SYSTEMS 345 The degree to which such non-Newtonian properties are conferred decreases markedly as the dispersing power of the continuous phase for the disperse phase is increased. Thus, a stiff, rigid paste decreases in plastic viscosity, curvature of the line, and in rigidity, when a deflocculator is added. The rate of thixotropic regain after deflocculation by shear increases with decrease in viscosity in the dispersing medium, but this again is modified by the degree of the deflocculating power of the medium, thixotropic regain being very slow when this degree is high. Finally, a few words on the application and interpretation of rheological data. In the first place, rheological figures afford, perhaps, a unique means of classifying the properties of plastic bodies, checking possible variation from a known standard, and observing stability on storage by periodical testing. Thus, changes in residual viscosity and rigidity clearly indicate internal structural changes arising from coalescence, coagulation, gel formation, etc. A study of the rheologicM data of materials subjectively recognised to exhibit marked characteristics of shortness, heavy body, stickiness or tack, etc., often permits of short cuts to formulation of those systems, the subjective properties of which depend not only on quantitative but on structural com- positions. The rheological curve has also proved useful in determining the efficacy of different types of mixers, grinders and emulsifiers. APPENDIX Whilst the interpretation of rheological data into basic units demands somewhat involved mathematical treatment, the older de Waele-Ostwald equation • can be usefully employed for most practical purposes. It may be stated that in spite of the publication of the writer's later dimensionally sound equation, • many rheologists still prefer to use the earlier treatment as yielding usefully indicative information. Reduced to its simplest iorm, the de Waele-Ostwald equation is ß Shearing stress (Effiux velocity)4 = O' 4 being a positive exponent less than unity, (1 -- 4) being a measure of the non-Newtonianism of the material and •' a coefficient of flow. •', not possessing the dimensions of a true viscosity •, must not be regarded as anything but a coefficient of flow. It is thus only necessary to plot log shearing stress against effiux velocity, when ß /k log shearing stress • log effiux velocity

346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS where • is the slope of the upper linear portion of the plot, and (log stress) -- (4) (log effiux velocity) ---- log •' An account of the applications of this treatment may be found in ('). The treatment is, naturally, applicable to rheological data from either rotary or capillary effiux instruments. REFERENCES • "Viscometry and Plastometry," de Waele, A., J. Oil & Colour Chemists Assoc., 1925, 6, $3-69. "Change of Viscosity with Rate of Shear in Disperse Systems" (in German), de Waele, A., Koll.-Zeits., 1925, 36, 6, 332-33. "Plastometric Studies on the Structure of Interfaces" (in German), de Wade, A., and Lewis, G. L., Koll.-Zeits., 1929, 48, 2, 126-41. • "The Measurement of Plasticity," A. de Wade, The Science of Petroleum (Oxford University Press), 1935, 1106-17. "The Double Mobility of some non-Newtonian Fluids," de Waele, A., and Dinnis, G., Physics, 1936, ?, 426-$1. a "Rheology for the Technical Man," de Waele, A., Paint Technology, 1949, 14, 161, 205-14. "The Rheological Diagram," de Waele, A., and Lewis, G. L., (in English), Koll.-Zeits., 1953, 133, 2/3, 86-91. TECHNICAL ASPECTS OF AEROSOL PACKAGING A Lecture ddivered to the Society on Thursday, April 26th, 1956 A. HERZKA, B.Sc.* The considerations of practical impor[ante in [he planning of Aerosol produc[s and [he chief formula[ion difficulties usually encoun[ered are described and illus[ra[ed. A• aerosol, by definition, is a dispersion of fine particles in a gas for a period of time. Some pressurised packs, as will be discussed below, give rise to true aerosols, while the majority of packs give rise to anything but aerosols. Because of this and also to avoid any possible misconceptions, the terms "pressurised dispenser" or "fingertip dispenser" have been widely used in this country in the past. To-day, however, it is already possible to talk of an aerosol container, and everyone realises that a method of packing rather than a particular spray or emission is being discussed. According to Shepherd, • Charlie Griebauher, who packed ethyl chloride in glass containers in the 1890s, was the father of the aerosol industry. During the late 1920s German patents protected the use of carbon dioxide •' * Research Division, The 1V[etal Box Co., Ltd., London, W.3.