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