MEASUREMENT OF PARTICLE SIZE DISTRIBUTION IN AEROSOLS* By Mop, p, IS A. FsiER ?lrmour Research Foundation of Illinois Institute cf Technology, Chicago, Illinois TH• •r•R• ^•V. OSOL in its general sense refers to a dispersion of small solid or liquid particles in a gas. Aerosols of various types are very commonly encountered, both in nature and as a result of industrial or general human activities. We think immediately of natural clouds and fogs, as well as dust clouds, mists, sprays and various kinds of smoke. Although there are many different aerosols, it is readily recognized that they all have certain special features in common. The unique properties of aerosols are obviously a direct result of the fine state of subdivision of the solid or liqt•id materials in the aerosol. When bulk matter is subdivided into fine particles a number of marked changes occur in the physical and chemical behavior of the material. Thus, fine particles are easily airborne, and once suspended in a gas settle rather slowly. The exposed surface area increases rapidly with subdivision, and this has far reaching consequences. The rate of vaporization, chemical reactivity and catalytic activity may be greatly increased. Further, the fine particles exhibit unique light scattering effects and often carry strong electrical charges. The extent to which these special properties are mani- fested depends, of course, on the size of the particles. It thus becomes im- portant to develop the relationships between particle size and the proper- ties of particles. Although particle size measurements are basic to the study of all types of aerosols, the present discussion will be concerned primarily with the problems of measuring the particle size distribution of self-propelled aerosol products. These products which have come into widespread use during the past few years, employ special packaging and generating methods to make a variety of useful materials available in aerosol form. Ideally, the aerosol product package should take advantage of the unique proper- ties of aerosols in order to improve the effectiveness as well as the con- * Presented at the September 15-16, 1955, Seminar, New York City. 77

78 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS venience and availability of the product. Aerosol characteristics, and es- pecially particle size distribution, thus become important factors in prod- uct design. Controlling the particle size distribution of a spray product often presents great difficulties, in view of the many other factors which must be considered in formulating a specific product. Nevertheless, the importance of particle size distribution is so great that it cannot be by- passed. As technical progress is achieved in the measurement and con- trol of particle size, the chemist will not only improve the efficiency of pres- ent products, but will also acquire greater latitude in the development and formulation of new products. The range of desirable particle sizes in different aerosol product sprays will depend greatly on the nature of the product, its formulation and in- tended uses. However, in general, aerosol product spray particle sizes will cover the middle range of diameters--from a few microns to perhaps 50 or 60 microns for most of the particles. A certain proportion of particles may fall outside of this range into the extremely small or large sizes, but generally this involves at best an inefficient use of aerosol material. Ex- tremely fine aerosol particles, less than one micron in diameter, tend to approach gas molecules in their physical behavior, and thus deviate con- siderably from the characteristic aerosol particle properties. On the other end of the scale, large particles of the order of 100 microns and greater in diameter will, of course, approach the physical properties of bulk ma- terials. The methods available for measuring particle size distribution may be divided into two broad classes. The first of these includes the direct methods, in which the actual dimensions of particles are measured. Micro- scopic examination of individual particles is the outstanding method of this kind. Relatively coarse solid particles are also measured directly by siev- ing, where the dimensions of a particle are directly compared with the size of opening in a screen. Such direct methods yield particle size data which are relatively independent of other variables such as density or color. This may be an advantage when the properties of an aerosol can be directly correlated with particle dimensions, or when additional useful information can be obtained by microscopic observation of the particles. On the other hand, microscopic measurements are often difficult to carry out and are time consuming. The second class of particle size measurements includes the indirect methods which do not measure the dimensions of particles as such, but are based on measuring some other property of the particles which is related to their size. Thus the size of aerosol particles may be determined by measur- ing their settling velocity which is a function of particle size, but de- pends also on the particle density and viscosity of the medium, as will be discussed. Other examples of indirect measures of particle size are light