Laser-induced shock-wave-expanded nanobubbles in spherical geometry.

Area of Science:

• Acoustics
• Fluid Dynamics
• Biomedical Optics

Background:

• Laser-induced breakdown in aqueous media generates shock waves.
• Secondary cavitation can occur following the primary breakdown event.
• Understanding cavitation is critical for applications involving laser-tissue interaction, such as ophthalmic procedures.

Purpose of the Study:

• To investigate the mechanisms of secondary cavitation generation after laser-induced breakdown in a spherical geometry simulating the human eye.
• To identify the key factors influencing the onset and location of secondary cavitation.
• To compare numerical simulations with experimental results for validation.

Main Methods:

• Numerical simulation of shock wave propagation to predict pressure fields.
• Experimental investigation using a high-precision, multiple-illumination technique.
• Analysis of peak-pressure maps and cavitation bubble formation.

Main Results:

• Numerical simulations accurately predicted the location of secondary cavitation onset.
• Large transient negative pressures were observed at the acoustic image of the shock wave source, distinct from the optical focus.
• Experimental results confirmed that pre-illumination by the laser pulse conditions the medium, reducing the secondary cavitation threshold.
• Evidence suggests repetitive negative pressure pulses may also pre-condition water for cavitation.

Conclusions:

• Secondary cavitation is initiated by negative pressures at the acoustic image of the shock wave source.
• Laser pulse pre-conditioning significantly reduces the threshold for secondary cavitation.
• A multi-stage process involving repetitive negative pressure pulses may contribute to secondary cavitation onset.