Serj
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I can't find it anywere, why does the bubble collapse? And could someone clarify acoustic levitation.
The buoyancy of such a small bubble is extremely slight. It would rise to the top very slowly, so the downward force similarly needs to be extremely slight. Note this is called "single bubble sonoluminescence". The phenomenon was originally found to occur when photographic plates were accidentally exposed to light emitted from large numbers of luminescent bubbles in an ultrasonic cleaning bath. It wasn't till decades later that a single bubble was isolated and held in position in the flask using sound.This bubble is then trapped at the center of the cylindrical flask, where the buoyancy force that would make the bubble rise to the top is balanced by the force of the sound waves.
So gasses that are disolved in the liquid can enter and exit the bubble, and can undergo chemical reactions. The force driving this is of course the pressure waves in the water which compress the bubble at roughly its resonant frequency. But how and why it collapses is also a function the chemical reactions taking place.A bubble pulsating in water containing dissolved air is thought to contain primarily argon, because the N2, O2 and H2O that diffuse into the bubble during expansion should burn off to form soluble products during bubble compression: the "dissociation hypothesis". The expected initial products of chemical reactions inside the bubble include OH and nitrogen oxides (NOx). OH will react with organic compounds in the water or dimerize to H2O2, and NOx will react with water giving nitrite and nitrate ions.