Effects of air oversaturation and wettability on ultrasonication-generated surface microbubbles and their implications for froth flotation

Surface microbubbles generated during ultrasonic pretreatment can enhance froth flotation. However, the effective utilization of these microbubbles requires air-oversaturated conditions in the liquid after ultrasonication. To verify this requirement, dissolved-air concentrations were measured before and after ultrasonication at different sonication durations, and the evolution of ultrasonication-generated surface microbubbles was monitored on microcavity-containing substrates with different wettabilities. Bubble pick-up and collision experiments were further conducted to evaluate the role of ultrasonication-generated surface microbubbles in particle flotation. Dissolved-air measurements and imaging observations indicate that ultrasonication generates surface microbubbles through two pathways: (i) nucleation of bulk bubbles followed by attachment to solid surfaces, and (ii) in situ nucleation from surface microcavities. The post-ultrasonication evolution of surface microbubbles demonstrates that air oversaturation is the dominant factor controlling the stable surface-microbubble population, whereas microbubbles cannot remain stable under non-oversaturated conditions. By contrast, increasing sonication duration has only a limited effect on the stable bubble population, although it slightly increases the initial number of bubbles. Surface microbubble formation also exhibits strong selectivity toward hydrophobic surfaces. Bubble pick-up and collision experiments demonstrate that ultrasonication-generated surface microbubbles enhance the attachment of hydrophobic particles to carrier bubbles and increase particle loading only under air-oversaturated conditions.