CHANGES DURING EVAPORATION OF EMULSION 17 original amount will be presented and the microscopy photos of the layers observed. Thirdly, the number distribution of drop size will be given to establish the degree of flocculation and coalescence. The emulsion per se (Figure la) shows a "normal" distribution of drop sizes, and does not contain any birefringent material (Figure lb). The determination of the drop sizes by manual measurement on the microphoto shows a predominance of drops of a diameter of 1. 75 µm, with substantial tails towards both larger and smaller drops (Figure 2). Centrifugation of the emulsion did not give complete separation into clear phases. Instead, three layers were found (Figure 3). The calculation of the volume of the three layers is complex. Due to the geometry and details of the calculation, the reader is referred to two appendices (Appendix I and Appendix II). The volumes of the layers are presented in Table I. The dispersion structure in the different layers is disclosed by the microscopy photos (Figure 4). The first feature to be observed is the absence of birefringent material the photos taken with the sample between crossed polarizers were all black and have not been included. The top emulsion layer is similar to the entire emulsion (Figure 1). The drop size distribution is similar, but the emulsion, as expected, contains a signifi­ cantly greater amount of dispersed material, and there are strong indications of frequent flocculation. Obviously the emulsion shows good stability against coalescence. The middle layer contains a dilute emulsion with characteristics similar to those of original emulsion, but now, in addition, there are particles present. These are easily distinguished from potential coalesced drops because of their shape, with discrete edges. These particles are more prevalent in the bottom emulsion layer (Figure 4c). These results give essential information about the emulsion. It consists of an oil phase that is lighter than water, and in addition, it contains particulate matter of a density slightly greater than that of water. This information is now related to the changes during evaporation. These changes during evaporation are of two kinds. The first one is found in the initial stage of evaporation, while the second appears first, when a predominant part of the emulsion is already evaporated. The first stage is characterized by an increase in the number of drops, as illustrated by the difference between the features of the emulsion in Figure 1 and those in Figure 5. There is an obvious increase in the number of drops, mainly smaller drops or even particles. As far as the larger drops are concerned, the size distribution remains the same (Figure 2). The alteration at later stages of evaporation is exemplified by the modification of the features in Figure 1 to those in Figure 6. The decisive changes are initiated in the emulsion after 70% of evaporation (Figure 6a,b). The original features of individual drops are now replaced by a pattern of heavily coalesced drops only, at less evaporation (Figure 6a), and in addition there are small amounts of anisotropic material found (Figure 66). These new emulsion traits are the predominant ones after 80% evaporation, in contrast to those found at 40% evaporation (Figure 2) and in the original emulsion, with well-defined drops now replaced by an assembly of irregular drops or particles. In addition, anisotropic material is now prevalent (Figure 6d). As to the structure of the anisotropic material, the photo does not provide an answer it may be a crystal or a liquid

18 JOURNAL OF COSMETIC SCIENCE (a) Figure 1. Microphotographs of the original emulsion. (a) Without crossed polarizers. (b) With crossed polarizers. Magnification: (a) 400x (b) l00x.