Electrification of Sonoluminescing Single Bubble.

The charge of microbubbles in water is considered to be negative at the liquid-bubble interface. We investigated, for the first time, the charge of a single bubble sonoluminescence (SBSL bubble) that exhibits spatiotemporally stable light emission. When negative DC voltage was applied, the SBSL bubble was attracted to a hot electrode. Conversely, the SBSL bubble was repelled by the hot electrode when positive DC voltage was applied. The translation of the SBSL bubble under an electric field suggests that it is positively charged, and the bubble moved to an equilibrium position to balance the primary Bjerknes force and the electrostatic force. The amount of bubble translation under an electric field depended on the elapsed time of sonoluminescence, suggesting that the products generated inside the SBSL bubble affect the mechanism of bubble charging. Furthermore, we measured the electric field effects on bubble expansion and contraction by a light scattering technique. Applying a positive voltage decreased the maximum bubble diameter and also the intensity of the SBSL. Conversely, applying a negative voltage increased the maximum bubble diameter and also the intensity of the SBSL. The present study revealed that a SBSL bubble is positively charged.