Vapor supersaturation in collapsing bubbles. Relevance to the mechanisms of sonochemistry and sonoluminescence

Small amplitude oscillations of gas bubbles in sonicated liquids are quasi- reversible events, but the sudden collapse of widely expanded bubbles resul ts in extensive vapor supersaturation. However, Hilgenfeldt et al. (Nature 1999, 398, 402) explain single-bubble sonoluminescence by assuming that col lapsing bubbles revisit their equilibrium radii, R-o, filled with vapor-sat urated gas ([gas]/[H2O] approximate to 31 at 300 K) before becoming imperme able and adiabatic en route to uniform greater than or equal to 25 kK tempe ratures. We find that the above assumption is physically untenable and seri ously in error. In contrast, we calculate robust [gas]/[H2O] less than or e qual to 0.12 ratios at R-o by using realistic alpha(H2O) less than or equal to 0.3 values for the mass accommodation coefficient of H2O molecules on l iquid water at 300 K, and by taking into account the diffusive resistance d eveloping within collapsing bubbles. Therefore, water vapor, rather than an y particular gas, is the main component of collapsing bubbles. Its large he at capacity and atomization energies preclude reaching uniform peak tempera tures exceeding 5 kK. We briefly analyze the consequences of this analysis and their relation to existing information.