The acoustical signals produced by antibubble formation.

An antibubble is an unusual object: a submerged water drop encapsulated in a thin shell of air that is stable underwater for 10-100 s. They are often thought of as the inverse of a soap bubble because they are a spherical shell of air in water in contrast to a shell of water in air. Antibubbles may be formed when water droplets impact the surface of surfactant-covered water, within a limited range of drop radius and drop impact velocity. In this paper, the range of drop size and impact velocity over which large antibubbles (radius 1-3 mm) are generated by the impact of falling drops is characterised, and the relationship of these parameters to the size of the antibubble formed is shown. Measurements of the two acoustical signals that may be produced as an antibubble is formed by drop impact are reported, and their relationship to the antibubble radius and shell thickness is established. Acoustical measurements taken are interpreted in the context of a modified Rayleigh-Plesset equation that provides a good fit to the frequency data for air shells greater than 100 µm in thickness. However, these results highlight the need for future work on the damping mechanisms associated with these larger antibubbles.

Toward omnidirectional and automated imaging system for measuring oceanic whitecap coverage.

Accurate measurements of the oceanic whitecap coverage from whitecap images are required for better understanding the air-gas transfer and aerosol production processes. However, this is a challenging task because the whitecap patches are formed immediately after the wave breaks and are spread over a wide area. The main challenges in designing a whitecap-imaging instrument are the small field of view of the camera lens, processing large numbers of images, recording data over long time periods, and deployment difficulties in stormy conditions. This paper describes the design of a novel high-resolution optical instrument for imaging oceanic whitecaps and the automated algorithm processing the collected images. The instrument was successfully deployed in 2013 as part of the HiWINGS campaign in the North Atlantic Ocean. The instrument uses a fish-eye camera lens to image the whitecaps in wide angle of view (180°).

Improvements to the methods used to measure bubble attenuation using an underwater acoustical resonator.

Active acoustic techniques are commonly used to measure oceanic bubble size distributions, by inverting the bulk acoustical properties of the water (usually the attenuation) to infer the bubble population. Acoustical resonators have previously been used to determine attenuation over a wide range of frequencies (10-200 kHz) in a single measurement, corresponding to the simultaneous measurement of a wide range of bubble sizes (20-300 µm radii). However, there is now also considerable interest in acquiring measurements of bubbles with radii smaller than 16 µm, since these are thought to be important for ocean optics and as tracers for near-surface flow. To extend the bubble population measurement to smaller radii, it is necessary to extend the attenuation measurements to higher frequencies. Although the principles of resonator operation do not change as the frequency increases, the assumptions previously made during the spectral analysis may no longer be valid. In order to improve the methods used to calculate attenuation from acoustical resonator outputs, a more complete analysis of the resonator operation is presented here than has been published previously. This approach allows for robust attenuation measurements over a much wider frequency range and enables accurate measurements from lower-quality spectral peaks.

A candidate mechanism for exciting sound during bubble coalescence.

Coalescing bubbles are known to produce a pulse of sound at the moment of coalescence, but the mechanism driving the sound production is uncertain. A candidate mechanism for the acoustic forcing is the rapid increase in the bubble volume, as the neck of air joining the two parent bubbles expands. A simple model is presented here for the volume forcing caused by the coalescence dynamics, and its predictions are tested against the available data. The model predicts the right order of magnitude for the acoustic amplitude, and the predicted amplitudes also scale correctly with the radius of the smaller parent bubble.

The effect of coupling on bubble fragmentation acoustics.

Understanding the formation and evolution of bubble populations is important in a wide range of situations, including industrial processes, medical applications, and ocean science. Passive acoustical techniques can be used to track changes in the population, since each bubble formation or fragmentation event is likely to produce sound. This sound potentially contains a wealth of information about the fragmentation process and the products, but to fully exploit these data it is necessary to understand the physical processes that determine its characteristics. The focus of this paper is binary fragmentation, when turbulence causes one bubble to split into two. Specifically, the effect that bubble-bubble coupling has on the sound produced is examined. A numerical simulation of the acoustical excitation of fragmenting bubbles is used to generate model acoustic signals, which are compared with experimental data. A frequency range with a suppressed acoustic output which is observed in the experimental data can be explained when coupling is taken into account. In addition, although the driving mechanism of neck collapse is always consistent with the data for the larger bubble of the newly formed pair, a different mechanism must be driving the smaller bubble in some situations.

Contributions to the acoustic excitation of bubbles released from a nozzle.

It has recently been demonstrated that air bubbles released from a nozzle are excited into volume mode oscillations by the collapse of the neck of air formed at the moment of bubble detachment. A pulse of sound is caused by these breathing mode oscillations, and the sound of air-entraining flows is made up of many such pulses emitted as bubbles are created. This paper is an elaboration on a JASA-EL paper, which examined the acoustical excitation of bubbles released from a nozzle. Here, further details of the collapse of a neck of air formed at the moment of bubble formation and its implications for the emission of sound by newly formed bubbles are presented. The role of fluid surface tension was studied using high-speed photography and found to be consistent with a simple model for neck collapse. A re-entrant fluid jet forms inside the bubble just after detachment, and its role in acoustic excitation is assessed. It is found that for slowly-grown bubbles the jet does make a noticeable difference to the total volume decrease during neck collapse, but that it is not a dominant effect in the overall acoustic excitation.

A mechanism stimulating sound production from air bubbles released from a nozzle.

Gas bubbles in water act as oscillators with a natural frequency inversely proportional to their radius and a quality factor determined by thermal, radiation, and viscous losses. The linear dynamics of spherical bubbles are well understood, but the excitation mechanism leading to sound production at the moment of bubble creation has been the subject of speculation. Experiments and models presented here show that sound from bubbles released from a nozzle can be excited by the rapid decrease in volume accompanying the collapse of the neck of gas which joins the bubble to its parent.