Time-resolved monitoring of cavitation activity in megasonic cleaning systems

The occurrence of acoustic cavitation in the cleaning liquid is a crucial precondition for the performance of megasonic cleaning systems. Hence, a fundamental understanding of the impact of different parameters of the megasonic process on cavitation activity is necessary. A setup capable of synchronously measuring sonoluminescence and acoustic emission originating from acoustically active bubbles is presented. The system also includes a high-speed-stroboscopic Schlieren imaging system to directly visualize the influence of cavitation activity on the Schlieren contrast and resolvable bubbles. This allows a thorough characterization of the mutual interaction of cavitation bubbles with the sound field and with each other. Results obtained during continuous sonication of argon-saturated water at various nominal power densities indicate that acoustic cavitation occurs in a cyclic manner, during which periods of stable and inertial cavitation activity alternate. The occurrence of higher and ultraharmonics in the acoustic emission spectra is characteristic for the stable cavitation state. The inertial cavitation state is characterized by a strong attenuation of the sound field, the explosive growth of bubbles and the occurrence of broadband components in the acoustic spectra. Both states can only be sustained at sufficiently high intensities of the sound field. At lower intensities, their occurrences are limited to short, random bursts. Cleaning activity can be linked to the cavitation activity through the measurement of particle removal on standard 200 mm silicon wafers. It is found that the particle removal efficiency is reduced, when a continuous state of cavitation activity ceases to exist.