Measuring the oil droplet sizes produced when oil is dispersed at sea is a critical stage in understanding the process of dispersion. A laser Phase Doppler Particle Analyzer (PDPA) with a measurement range 1–300μm has been developed which can distinguish between oil and the background signal from air bubbles, suspended sediment and plankton at sea. 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 distributions for a variety of oils and oil-dispersant combinations have a number median around 20 microns and are independent of the oil/oil-dispersant combination. Although the droplet size distribution is similar what is very different for these different oil-dispersant combinations is the number of droplets in the 1–70μm size range. n the basis of the measured size distribution and model predictions of the vertical movement of these droplets at sea we define dispersed droplets ⩽ 70μm and suspended droplets > 70μm. Efficient dispersants produce a large number of droplets in the 1–70μm size range while poor dispersants or an untreated slick produces a smaller number of oil droplets in this size range.
Comparison of the oil droplet size distribution of naturally and chemically dispersed oil indicates that the rate determining step in the process of dispersion is the splitting of large droplets (70–300μm diameter) to produce oil droplets in the size range 1–70μm. The consistency of the oil droplet size distribution for a variety of oil and oil-dispersant combinations suggests that the splitting process is via the shear forces imparted by microscale turbulence. The proposed mechanism has implications for both the modelling of the process at sea and the design of laboratory dispersant tests.