The response of microscopic bubbles to sudden changes in the ambient pressure

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Abstract
This paper focuses on the résponse of bubbles to sudden changes in the ambient pressure. Trains of bubbles with radii varying between 45 and 200 μm were exposed to various pressure steps, and their response was monitored by pulsed laser holography. The experiments were performed in a specially constructed chamber, allowing generation of pressure steps ranging from 0.1 to 20 times the initial value. Air, CO2, helium and hydrogen bubbles were selected, providing a range of mass, thermal diffusivities, solubilities in water and isentropic constants. The changes in the bubbles’ diameters were determined by reconstructing the holograms, magnifying the images and measuring the sizes of individual bubbles. Most of the experiments were performed with pressure changes at the rate of 20 KPa/ms, and most data were recorded in less than 20 ms. The results confirm that, within the present range of test conditions, the bubbles can respond instantaneously to changes in the ambient pressure. The experiments also demonstrate that the response of the bubbles at the present timescales can be assumed to be isothermal (polytropic constant of 1.0), irrespective of the bubble content or size. Repeated measurements with different pressure waveforms, but with the same final pressure resulted in identical results, demonstrating that bubbles can be used as pressure sensors. Variations in timescales up to a few hundred milliseconds still resulted in the same response, confirming the isothermal assumption. The dissolved gas content had a noticeable effect on the behaviour of the CO2 bubbles, the most soluble of the gases tested, and had no detectable effect on the behaviour of air bubbles. The paper also includes a detailed error evaluation of the present experiments and an estimate of the expected error when the bubbles are utilized as pressure sensors.

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