Acoustic radiation by cavitation bubbles near solid boundaries with modified wettability

This study investigates the impact of surface wettability on the acoustic emissions of cavitation bubbles generated through low-voltage discharge. The behavior of single cavitation bubbles near surfaces with varying wettability is analyzed using high-speed cameras and hydrophones. Cavitation bubble dynamics is observed near three different surfaces: a nearly hydrophilic untreated bare plate, a commercially available Glaco paint-coated plate with hydrophobic properties, and a Sharklet film-coated plate, a commercially available thin film with unique anisotropic wettability. The effects of different wettability conditions on bubble dynamics and the resulting acoustic emissions are compared comprehensively. The dimensionless stand-off distance parameter, defined as the ratio of the stand-off distance from the point of nucleation to the maximum bubble radius in a free field, is found to be crucial in determining the interaction between cavitation bubbles and nearby surfaces, thereby influencing the acoustic signature. Key observations revealed that when the nucleation point is close to the wall, a wall vortex with a unique kink formed due to the hydrophobic coating, while at greater distances, a free vortex is weakened by the hydrophobic nature of the surface. Hydrophobic surfaces, particularly those coated with Sharklet films, amplified sound pressure levels when the nucleation point is close and reduced emissions when farther away. The findings suggest that wettability-modified hydrophobic plates, particularly those with Sharklet film-coatings, may have the potential to manage acoustic emissions from cavitation bubbles effectively.