Vapor Supersaturation in Collapsing Bubbles. Relevance to the Mechanisms of Sonochemistry and Sonoluminescence

Abstract

Small amplitude oscillations of gas bubbles in sonicated liquids are quasi-reversible events, but the sudden collapse of widely expanded bubbles results in extensive vapor supersaturation. However, Hilgenfeldt et al. (Nature 1999, 398, 402) explain single-bubble sonoluminescence by assuming that collapsing bubbles revisit their equilibrium radii, R_o, filled with vapor-saturated gas ([gas]/[H_2O] ≈ 31 at 300 K) before becoming impermeable and adiabatic en route to uniform ≥25 kK temperatures. We find that the above assumption is physically untenable and seriously in error. In contrast, we calculate robust [gas]/[H_2O] ≤ 0.12 ratios at R_o by using realistic α_(H_2O) ≤ 0.3 values for the mass accommodation coefficient of H_2O molecules on liquid water at 300 K, and by taking into account the diffusive resistance developing within collapsing bubbles. Therefore, water vapor, rather than any particular gas, is the main component of collapsing bubbles. Its large heat capacity and atomization energies preclude reaching uniform peak temperatures exceeding 5 kK. We briefly analyze the consequences of this analysis and their relation to existing information.