Abstract. Breaking waves result in very different populations of bubbles in fresh water and seawater. The differences in sizes and numbers of bubbles in the two media cause significant differences in many natural processes including gas transfer and generation of aerosols as well as affect sound and light propagation and ambient sound production. Observed differences in bubble populations in seawater and fresh water have in the past been attributed to coalescence inhibition in seawater, but bubble-shattering studies described here suggest that other factors may be important. When air bubbles are expanded through an orifice at constant pressure drop, they can be made to shatter into clouds of smaller bubbles. We report here measurements showing that the number and size distribution of resulting populations depend on the physical and chemical characteristics of the water samples. In particular, bubbles were found to break up into 4-5 times more small bubbles in seawater than in fresh water. The number of bubbles produced in shattering was found to be a function of salt concentration but was especially sensitive to the types of ions present. In addition, medium from the marine diatom Phaeodactylum tricornutum was found to further double production numbers, and a decrease in temperature from 20øC to 3øC was found to increase bubble production in seawater by nearly 50%. These effects appear to be separate from coalescence inhibition, the factor usually cited for differences in bubble populations in fresh water and seawater.

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Bubble surface tension (BST) is measured by spinning single bubbles in a rotating cell and relating the resulting bubble deformation to the Centrifugal number Ce. Using this method bubbles in "clean" fresh water (distilled, deionized, passed through activated charcoal and filtered at 0.2 !•m) are found to have a BST similar to that reported for the "clean" air-water interface (72.4 mN/m), whereas adsorption of organic films are observed to decrease the BST, with the effect increasing as the bubbles are aged. In natural coastal sea water (North West Arm, Halifax Harbour (10 m)) the BST decreases to 70.5 mN/m and in phytoplankton culture (Nitzschia pungens) decreases to 67 mN/m. Saturated stearic acid solution shows a BST of 57 mN/m. Coalescence of two bubbles with equilibrium surface film is accompanied by a rapid decrease in the surface tension, a decrease that we attribute to compression of the merging surface films at the newly created smaller interface. The decrease is most pronounced in water rich in organic surfactants.

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