Cinephotographic observations of particle removal from a surface by acoustic cavitation

It is commonly believed that ultrasonic cleaners remove particles from a surface through acoustic cavitation, presumably in which a pulsating bubble interacts directly with the particle. In this study, we have used a highspeed movie camera to observe the removal of biofilm attached to a solid surface during exposure to a cavitation field. The biofilm consisted of Streptococcus mutans, a common oral bacterium, grown on a glass slide and observed under magnification to be both thinly coating the glass surface and clustered in larger colonies. The cavitation field was created by an Ultreo™ toothbrush which combines both vibrating bristles and an ultrasound transducer with a waveguide, operating at a frequency of 324 kHz. When the waveguide was immersed in water containing bubbles from an ultrasound contrast agent (Optison™), visual observations could be made with the naked eye of biofilm removal. With high-speed microcinephotography, it was possible to observe bacteria removal by the direct interaction of a cavitation cloud (cluster of cavitation bubbles) and the colony. These observations will be presented along with our interpretations of the data.