SELF-PRESERVATION OF THE DROP SIZE DISTRIBUTION FUNCTION AND VARIATION IN THE STABILITY RATIO FOR RAPID COALESCENCE OF A POLYDISPERSE EMULSION IN A SIMPLE SHEAR FIELD

Coalescence of oil-in-water emulsion droplets in a simple shear flow p roduced by a Couette device is considered. A phase Doppler anemometer was used to measure the droplet size distribution as a function of tim e for shear rates ranging from 55 to 213 s(-1) and for sodium chloride salt concentrations from 0.095 to 0.6 M. The initial droplet size dis tribution was log-normal. During the coalescence process, the size dis tribution was self-preserving in accordance with D. L. Swift and S. K. Friedlander's analysis [J. Colloid Sci. 19, 621 (1964)]. In the limit ing case of negligible repulsive force due to the electric double laye r, the calculated stability ratios, corrected for droplet polydispersi ty, agree well with the theoretical analyses of G. R. Zeichner and W. R. Schowalter [AIChE J. 23, 243 (1977)] and D. L. Feke and W. R. Schow alter [J. Fluid Mech. 133, 17 (1983)] for the case of solid particle a ggregation. The good agreement between the stability ratios for the ca se of coalescence of droplets in the present study and those for aggre gation of solid particles indicates that resistance to film deformatio n and thinning present in the case of coalescence is not important com pared with the collision process. (C) 1998 Academic Press.