On-demand removal and nucleation of surface micro- and nanobubbles using ultrasound

Surface micro- and nanobubbles are spherical cap-shaped gaseous domains on smooth surfaces, which are well-known for their stability under external forces, yet their response to ultrasonic fields remains largely unexplored. This study experimentally and theoretically investigates the behaviors of surface micro- and nanobubbles in a microchannel exposed to ultrasound via a vibrating substrate. At driving frequencies of 100–200 kHz, ultrasonic cavitation bubbles emerge from the channel sidewall, migrate toward the surface micro- and nanobubbles, and merge with them, ultimately detaching the surface bubbles from the substrate with minimal residue. In contrast, at higher frequencies up to 30 MHz, free microbubbles in the bulk are attracted to the substrate, nucleating new surface bubbles or merging with existing ones to induce growth. Theoretical analysis reveals that the directional motion of bulk bubbles toward the substrate and surface micro- and nanobubbles is driven by Bjerknes forces. These findings demonstrate the ability to achieve on-demand removal or nucleation of surface-attached micro- and nanobubbles, with potential applications in electrochemistry, surface cleaning, drag reduction, and medical imaging.