THE SUBHARMONIC OSCILLATIONS AND COMBINATION-FREQUENCY SUBHARMONIC EMISSIONS FROM A RESONANT BUBBLE - THEIR PROPERTIES AND GENERATION MECHANISMS

Sizing bubbles using a two frequency technique which examines the coup ling of a resonant subharmonic oscillation to an imaging beam is not p rone to some of the inaccuracies and ambiguities of other methods. Thi s combination frequency subharmonic signal is parametric in nature, an d the amplitude onset threshold has been quantified for thirty tethere d air bubbles in water. This paper details work which aims to investig ate the generation mechanism of a bubble's subharmonic oscillations, a nd presents results from theoretical consideration of three potential mechanisms: (i) The possibility that at high amplitudes a bifurcation appears in a bubble's volumetric pulsations; (ii) the potential for bu bbles driven at twice their resonant frequency to pulsate with a compo nent at their resonant frequency (which is a subharmonic of the drivin g field); (iii) that surface waves are responsible. The first two mech anisms are examined using a numerical solution to the Gilmore-Akuliche v model, and the third using plane surface theory. Whilst all three me chanisms show the potential far generating subharmonic oscillations, t he first two demonstrate onset thresholds four and three orders of mag nitude higher than the experimental threshold respectively. However, t he threshold predicted using surface wave theory shows very good agree ment with the experimental results.