Near-wall dynamics of single cavitation bubbles imaged by total internal reflection fluorescence

This study introduces total internal reflection fluorescence (TIRF) as a method for investigating the dynamics of a single laser-induced cavitation bubble in water within a thin layer near a solid surface. TIRF, supported by a planar waveguide, was employed to image cavitation bubble collapses near the liquid-solid interface at 50 thousand frames per second (kfps). Simultaneously, traditional side-view background illumination at 150 kfps captured the bubble dynamics. Dilution series and thickness estimation were conducted to determine interfering fluorescence signal contributions. The results confirm that most fluorescence is excited by the evanescent field rather than surface scattering or bulk scattering. The lifecycle of cavitation bubbles with moderate non-dimensional stand-off distances, γ = 1.15, 1.35, and 1.6, are discussed. We find that a thin liquid film persists between the bubble and the solid surface throughout the entire collapse process, except for randomly distributed micron-sized bubbles attached to the surface between the first and second collapse. Shock waves were captured with TIRF during the second collapse, centered around regions of the toroidal cavity exhibiting locally stronger dynamics. These regions, termed gas-filled strong collapse areas (SCAs), were also where microcracks formed during the second collapse of a single cavitation bubble. While not all SCAs resulted in microcracks, every observed microcrack could be traced back to a preceding SCA. To our knowledge, this is the first application of TIRF to observe cavitation bubble collapses with sub-millisecond time resolution.