Dispersion: Oil droplet size measurements at sea

Abstract

This paper presents the first measurements of oil droplet sizes at sea. A laser Phase Doppler Particle Analyzer (PDPA) has been developed which can distinguish between oil, air bubbles, suspended sediment and plankton. The resulting improvement in the signal to noise ratio has enabled us to measure oil droplet size in situ below an oil slick. Measurements in the North Sea indicate that the droplet size distribution for a variety of oils and oil-dispersant combinations are very similar with a number median around 20 microns. There are however major differences in the number density for these different combinations. The information on droplet sizes and number density has led us to propose a mechanism of oil dispersion which has implications for both the modelling of the process at sea and the design of laboratory dispersant tests.