Squeezing alcohol into sonoluminescing bubbles: The universal role of surfactants

We conduct an experimental study of the dependence of single bubble sonoluminescence intensity on the concentration of various alcohols. The light intensity is reduced by one-half at a molar fraction of ethanol of 2.5 3 1025; butanol achieves the same reduction at a concentration 10 times smaller. We account for the results by a theoretical model in which the alcohols are assumed to be mechanically forced into the bubble at collapse, modifying the adiabatic exponent of the gas. The increasing hydrophobicities of the alcohols lead to decreasing effective adiabatic exponents, and thus to less heating and therefore less light. Support for this model is obtained by replotting the experimental light intensity values vs the calculated exponents, yielding a collapse of all data onto a universal curve.