40 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS repose (M and the positional angle of the flight (0) are taken from each photograph. For any one set of conditions (i.e. powder, particle size and drum speed), these readings are analysed in a computer programme which computes the value of I• giving the least mean standard deviation from the theoretical angles of repose as predicted from equation (I) EXPERIMENTAL REAI)INGS AND RESULTS Two separate materials were investigated with sizes as follows: Pumice BSS Sieve Size 7+ 14 --14+ 18 --18+ 25 --•25+ 36 --365- 52 --52+ 72 --72 + 100 Sugar BSS Sieve Size --18+ 25 ---25+ 36 --36+ 52 --52+ 72 For the pumice, drum speeds of 8, 16, 24 and 32 rev min- l were investigated for the sugar, 16 and 24 rev min- 1. The results of this investigation are tabulated in Table 1. Table I Values of IX for pumice and sugar powders rev min- 1 B.S. sieve Ave. particle size s,ze Pumice Sugar 8 16 24 32 16 24 7+ 14 1 800 1.00 0.88 0.89 0.94 -- -- ---14.4. 18 1 025 0.98 0.97 0.93 1.01 -- -- --18+ 25 725 1.00 0.92 0.95 0.96 0,94 0.91 ---25.4- 36 510 0.97 0.91 0.93 0.95 0.81 0.80 ---36.4- 52 360 0.96 0.90 0.87 0.77 0.49 0.42 52 -4- 72 255 0.87 0.86 0.79 0.77 0.44 0.37 - -72+100 180 0.82 0.79 0.76 0.63 These results are shown graphically in Figs. 3--5.

THE KINETIC ANGLE OF REPOSE OF POWDERS 41 10-- i i i I 500 IOOC• 1500 2000 rev min-t rev rain-1 24 rev rain -I I6 rev rnm-I dp, pm Figure 3, Plot: Particle size and kinetic coefficient of friction for pumice powders. DISCUSSION From the results, the following observations may be immediately made with regard to the values of p. •[fect of particle size (Figs. 3 and 4) The results indicate a falling off in the value of p with a decrease in the average particle size. In the case of pumice, this effect is only true for powders whose average particle size is less than about 500 pm above this value, and at all drum speeds, p is reasonably constant below this critical