Non-intrusive determination of particle size distribution in a concentrated dispersion

Abstract

Concentration profiles of a 19% oil-in-water emulsion were determined non-intrusively (from a measure of the velocity of ultrasound through the emulsion at a series of heights) over a period of 63 days. The oil concentration gradually increased with height until a cream layer of ∼ 70% oil was reached. Such profiles are typical of a dispersion of polydisperse particles with very weak interactions. The size distribution (by weight) was calculated from the rise velocities of contours of constant volume fraction. The size ranged from 200 nm to 12 µm. The size distribution of the small particles was compared with fractionated sections of the creaming emulsions sized by dynamic light scattering. The large particles were compared with laser light diffraction studies. In both cases the particle size distribution by creaming agreed well with the optical studies. A delay in the creaming for the small droplets (diameter <0.5µm) is observed and discussed. The implications of such a delay for size determination from a single concentration profile are also discussed.