Resolving size distributions of bubbles with radii less than 30 μm with optical and acoustical methods

Abstract

[1] Many studies have investigated bubble size distributions in the ocean, but the measured size range does not normally extend to bubbles with a radius below 20 mm. Bubbles smaller than this are thought to have a significant effect on the optical properties of the ocean, potentially affecting remotely sensed measurements of ocean color and the optical detection of particulates and dissolved matter. Such optical data are becoming the major source of oceanic information about algal blooms, primary productivity, sediment loading and the spread of pollutants. The challenges associated with measuring these bubbles are difficulty of calibrating sensors with independent bubble size measurements and lack of knowledge about the organic coating on the bubbles. This paper describes simultaneous oceanic measurements of these small bubbles using independent optical and acoustical techniques. These measurements agree well, and an investigation of the bubble coating parameters was made. Both the optical and acoustical properties of bubbles are affected by this organic coating, and a comparison of these measurements narrows down the choice of possible coating parameters. Our results suggest that the bubbles measured in this study were likely to have a coating with a thickness of 10 nm and a refractive index of 1.18, and that the coating thickness is the more important parameter for optical inversions. The research described here is the first attempt to constrain these parameters in the ocean using two independent techniques and suggests that further studies of this type could result in significant insight into oceanic bubble coatings.

Biogeochemical origins of particles obtained from the inversion of the volume scattering function and spectral absorption in coastal waters

“…The VSF-inversion method has been evaluated in several studies with promising results. For waters south of Hawaii, the size distribution of microbubbles inverted from the measured VSFs agreed well with concurrent acoustical results (Czerski et al, 2011). In the surf zone off of Scripps Pier, the wave-injected bubble and suspended sediment populations derived from the VSF-inversion were found to be consistent with acoustical measurements of bubbles and video observations of mineral particles, respectively (Twardowski et al, 2012).…”

“…For example, the backscattering ratio (fraction of scattered light into the backward directions) has been used to infer the bulk refractive index of particles Boss et al, 2004). Measurements of VSFs at near forward angles, from commercial instruments, have been used to derive the size distribution of larger particles (∼ 2 to 200 µm) (Chin et al, 1955;Coston and George, 1991;Knight et al, 1991;Riley and Agrawal, 1991). Building on earlier studies (Gordon and Brown, 1972;Zaneveld et al, 1974), Zhang et al (2011) developed an inversion method that uses the full angular range of VSFs to retrieve the size and composition of particles.…”

Effect of soluble surfactant on bubble persistence and bubble‐produced aerosol particles

“…Bubbles in this size range are known to produce jet drops [Lee et al, 2011] that could contribute to the size range of particles considered in Figure 7. Although only a few drops at most would be generated per bubble, whitecaps contain many more bubbles of this size than they do millimetric bubbles, particularly if the smallest bubbles are stabilized by organic coatings [Czerski et al, 2011], such that the overall contribution of these jet drops to the total particle flux may be significant. However, the scarce direct measurements available suggest that jet drops do not contribute significantly to submicrometer SSA populations [Woolf et al, 1987].…”

Micro-bubble morphologies following drop impacts onto a pool surface

“…Wanninkhof et al (2009) and Czerski et al (2011). The bursting of a submicron gas film is also important to understanding the fundamental mechanisms involved in film breakup (Reiter & Sharma 2001;Neitzel & Dell'Aversana 2002;de Gennes, Brochard-Wyart & Quéré 2004;Reyssat & Quéré 2006) and the micro-bubbles generated can enhance nucleate boiling (Dhir 1998).…”