Comparison of Holographic and Coulter Counter Measurements of Cavitation Nuclei in the Ocean

Abstract

Holographic and Coulter Counter detection techniques were jointly used to measure the concentration density distribution of cavitation nuclei in the ocean. Comparison of the two techniques indicates that Coulter Counter analysis measures particulate contents up to an order of magnitude smaller than indicated by the holographic method and may also produce a distorted concentration density distribution. Several possible explanations of the observed discrepancies are proposed and discussed, including fundamental differences between the in situ holographic samples and the collected samples examined with the Coulter Counter, differences between the unknown electrical conductivity of the measured particles in the sea water samples and the non-conductive polystyrene spheres used to calibrate the Coulter Counter, the rupture of aggregate particles in the flow through the Coulter Counter orifice, the effect of electronic noise on the Coulter Counter signal, and the influence of statistical sampling error. The particle number concentration density distributions decrease approximately with the fourth power of the particle radius in the observed size range of 10 to 50 μm radius. Both sets of data indicate an increase in particle concentration near the bottom of the thermocline, and the holographic bubble concentrations also indicate a similar behavior. Much higher concentrations of particles were detected in the ocean, according to the holographic analysis, than in typical cavitation test facilities. Consideration of the static mechanical equilibrium of individual air bubbles suggests that the average tensile strength of the ocean waters examined in this study is not larger than about 2000 Pa and occasionally as low as about 100 Pa.