Dissolution of microbubbles generated in seawater obtained offshore: Behavior and surface tension measurements

Ocean water samples were collected from Saint Andrew Bay and the Gulf of Mexico near Panama City, Florida, on board the R/V Athena. Optical microscopy observations of the dissolution of microbubbles generated in these water samples showed that the microbubbles remained spherical and their surfaces remained smooth during dissolution. Comparatively, these bubbles behaved like bubbles coated with expanded phase surfactants, as we show by directly observing the dissolution of sodium dodecyl sulfate (SDS) bubbles and citing our previous work involving dissolution of short‐chain lipid‐coated microbubbles. We apply Epstein and Plesset's model for the dissolution of a 15 to 25 μ m radius microbubble to determine surface tension, appropriate for a dissolving bubble coated with expanded phase surfactant. Average surface tensions of dissolving bubbles obtained in multiple locations and from two depths (from the sea‐air surface and 10 feet (1 foot = 0.3048 m) below the sea‐air surface) range between 21 and 37 dyn/cm with a resulting dissolution time of at least 2 times that of a clean microbubble at an initial radius of 15 μ m. We never observed a remnant particle upon completion of dissolution, consistent with the observed smooth dissolution. These visual observations are in contrast to several studies involving visual observations of dissolution of seawater microbubbles. Reasons for these differences are postulated. Generally, our results are consistent with microbubbles coated primarily with short‐chain and unsaturated fatty acids or lipids, but with a minority fraction of a long‐chain component, all remaining in an expanded phase. Similarities in occasional dimpled morphology and surface tension with SDS/1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine–coated bubbles are used to come to this conclusion.