Forced oscillation dynamics of surface nanobubbles

Removal of surface-attached micro- and nanobubbles by ultrasonic cavitation in microfluidics

Surface-attached micro- and nanobubbles are known for their resistance to external forces. This study experimentally and theoretically investigates their response to strong ultrasonic fields. Surface-attached micro- and nanobubbles with contact radii from 2 μm to 20 μm are generated in a microchannel and exposed to ultrasound through a vibrating glass substrate. At a driving frequency over 200 kHz up to 2 MHz tested, no significant response from the micro- and nanobubbles is observed. By contrast, at 100 kHz-200 kHz, ultrasonic cavitation bubbles appear in the microchannel and migrate toward the surface micro- and nanobubbles. Then the surface micro- and nanobubbles merge with the ultrasonic cavitation bubbles, detach from the substrate, and become free gaseous nuclei susceptible to further cavitation. Notably, the removal process leaves no observable residue. Theoretical analysis suggests that the directional migration of cavitation bubbles is driven by mutual acoustic radiation forces. This work demonstrates that ultrasonic fields can effectively remove surface micro- and nanobubbles, transforming them into free gaseous cavitation nuclei.

Thermal Oscillations of Nanobubbles

Nanobubble cavitation is advancing technologies in enhanced wastewater treatment, cancer therapy and diagnosis, and microfluidic cleaning. Current macroscale models predict that nanobubble oscillations should be isothermal, yet recent studies suggest that they are adiabatic with an associated increase in natural frequency, which becomes challenging when characterizing nanobubble sizes using ultrasound in experiments. We derive a new theoretical model that considers the nonideal nature of the nanobubble's internal gas phase and nonequilibrium effects, by employing the van der Waals (vdW) equation of state and implementing a temperature jump term at the liquid-gas interface, respectively, finding excellent agreement with molecular dynamics (MD) simulations. Our results reveal how adiabatic behavior could be erroneously interpreted when analyzing the thermal response of the gas using the commonly employed polytropic process and explain instead how nanobubble oscillations are physically closer to their isothermal limit.

Shock-induced collapse of surface nanobubbles

The collapse of cavitation bubbles often releases high-speed liquid jets capable of surface damage, with applications in drug delivery, cancer treatment, and surface cleaning. Spherical cap-shaped surface nanobubbles have previously been found to exist on immersed substrates. Despite being known nucleation sites for cavitation, their collapsing dynamics are currently unexplored. Here, we use molecular dynamics simulations to model the shock-induced collapse of different surface nanobubble sizes and contact angles. Comparisons are made with additional collapsing spherical nanobubble simulations near a substrate, to investigate the differences in their jet formation and resulting substrate pitting damage. Our main finding is that the pitting damage in the surface nanobubble simulations is greatly reduced, when compared to the spherical nanobubbles, which is primarily caused by the weaker jets formed during their collapse. Furthermore, the pit depths for surface nanobubble collapse do not depend on bubble size, unlike in the spherical nanobubble cases, but instead depend only on their contact angle. We also find a linear scaling relationship for all bubble cases between the final substrate damage and the peak pressure impulse at the impact centre, which can now be exploited to assess the relative damage in other computational studies of collapsing bubbles. We anticipate the more controlled surface-damage features produced by surface nanobubble cavitation jets will open up new applications in advanced manufacturing, medicine, and precision cleaning.