Brian's right, don't ever use combustible/flammable liquids in any sonic type machine.
Those bubbles, called cavitation bubbles are propagated by high frequency pressure sound waves agitating the water. The agitation produces high forces (pounding) on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates deep within passages, cracks, and recesses.
One great benifit is that you don't have to disassemble all the parts of a carb to clean using ultra sonic cleaning.
Most Important: DO NOT REST OBJECTS TO CLEAN ON ANY TANK SURFACE. TOUCHING WILL NOT ALLOW CAVITATION.
Contamination cleaned: Dirt, oil, pigments, rust, grease, algae, fungus, bacteria, lime scale, polishing compounds, flux agents, fingerprints, soot wax and mold release agents, biologics like blood are easily removed.