The relationship between bubble motion and particle flocculation pattern under 20-kHz-ultrasound radiation in water

- Dispersed particles with sub-millimeter and millimeter diameters in water were flocculated by irradiating kHz-band ultrasound.
- The flocculating patterns varied with the components and concentration of the dissolved gases.
- We evaluated the forces acting on the particle flocculation by visualizing both the particle flocculation and liquid motion.
- The particle flocculation was dominantly influenced by the acoustic radiation force acting on the ACOBs.
- We also discussed the motion of the acoustic cavitation-oriented bubble adhering to the particle surface.

Dispersed particles with sub-millimeter and millimeter diameters in water were flocculated by a kHz-band-ultrasound radiation. This phenomenon was caused by the acoustic cavitation-oriented bubbles (ACOBs). The flocculating pattern varied with the components and concentration of the dissolved gases. The relationship between the gas conditions and the particle flocculation pattern is discussed along with the estimation of the force balance acting on the particle flocculation. First, the particle flocculation was visualized. The liquid motion in the vessel was then measured via particle image velocimetry. The flocculation patterns were mainly classified into two types: spherical flocculation and chain-like flocculation. Spherical flocculation occurred when air-dissolution water was used, and chain-like flocculation occurred when CO2 concentration-controlled water was used. Both patterns were dominantly influenced by the acoustic radiation force acting on the ACOBs. At the occurrence of spherical flocculation, the ratio of the acoustic radiation force to the other forces was greater than that at the occurrence of chain-like flocculation. We analyzed the motions of ACOBs and particles by high-speed visualization to clarify the interaction between the particles and ACOBs. The ACOBs adhering to the particle surface in CO2 concentration-controlled water oscillated with the irradiation frequency and synchronized with other ACOBs. The member particles of the chain-like flocculation were tenaciously held by the Bjerknes force. In contrast, in the air-saturated water, the diameter of the spherically flocculated particle swarm oscillated due to the unstable motion of ACOBs. We suspect that the diameters of the spherically flocculated particle swarm fluctuated.