Interaction between acoustic cavitation bubbles and dispersed particles in a kHz-order-ultrasound-irradiated water

In the past studies, MHz-order ultrasound has been frequently employed in the ultrasound separation techniques and the flocculation mechanism of particles due to the acoustic radiation force has been reported. The previous separation technique, however, is applicable to particles with diameters similar to a wavelength of the irradiated ultrasound or with smaller diameters than that. Hence, particles which are larger than μm-order in diameter are difficult to be manipulated with MHz-band ultrasound. In the present study, to clarify an unknown flocculation mechanism of the particles in an ultrapure water under kHz-band ultrasound irradiation, we quantitatively discussed an interaction between the particle motion and the acoustic cavitation bubble motion based on the experimental results. First, we successfully captured the particle motion and acoustic-cavitation-oriented bubble motion simultaneously by using a high-speed video camera. Second, we measured the distribution of the sound pressure in the water phase and discussed the relationship between that of the sound pressure and the motion of the particle and the acoustic cavitation bubble. Finally, we investigated the effects of the gravity force, the acoustic radiation force and the spatial heterogeneity of the pressure acting on the particle. By combining the results, we found out that an acoustic-cavitation-oriented bubble adhered to the particle and the particles moved toward the pressure anti-nodes of the standing wave by the acoustic radiation force acting on the adhering acoustic-cavitation-oriented bubble.

► We newly developed a solid–liquid separation technique using kHz-band ultrasound.
► This technique can manipulate the particle in mm-order diameter.
► We found that the cavitation-oriented bubble was induced by acoustic cavitation bubbles.
► The cavitation-oriented bubble led the particle to flocculation position.
► The technique is strongly hopeful for particle classification by particle diameter.