42 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 10 0.9 0,8 0,7 0.1 ..16 rev rain -I .•. 32 rev min -I 0 I00 200 300 400 500 600 700 800 l:igttre 4. dp, /4m Plot: Particle size and kinetic coefficient of friction for sugar powders. value, all the curves show a definite downward trend indicating easier flowing conditions. With sugar powders, the trend downwards with average particle size is more definite and consistent over the limited range in- vestigated. Effect of drum speed (Fig. ,•) With both materials, very little effect of drum speed on the kinetic angle of repose is evident a slight trend of reduction in p with increase in

THE KINETIC ANGLE OF REPOSE OF POWDERS 43 J'0- 0,9- 0.8-- 0.7- 0,6- 0,5-- O, • elO25.P i••----"•' •.._ /'•, 725.P •"• '• -•---, ----.--'•= 5 I0 'P •"•.-'-"• e•'"'""•'-/ 1800'P 510 255.P 180,P "'" •.. . 360.S 255.S Average particle size = P N Pumice Sugar o x• I I I I 8 16 •- 32 N, rev rain '1 Figure 5. Plot: Drum speed and kinetic coefficient of friction for pumice and sugar powders. drum speed is noticeable which could possibly be detected by a more rigorous statistical appraisal of these results. Effect of material (Compare Figs. $ and The much lower values of ix obtained for the sugar powders indicate a definite effect. This means that over the common range of sizes investigated,