How the re-irradiation of a single ablation spot affects cavitation bubble dynamics and nanoparticles properties in laser ablation in liquids

Fundamental theoretical and experimental studies on the formation of nanoparticles and cavitation during laser synthesis of colloids usually employ single-pulse conditions, whereas studies of the properties of nanoparticles naturally require prolonged ablation. We explored how a defined number of pulses changes a silver target’s surface geometry and thereby the dynamics of the laser-induced cavitation bubble and the resulting properties of the nanoparticles. The shape of the cavitation bubble transforms from hemispherical to almost spherical. The indirectly calculated mass concentration in the cavitation bubble follows a decay with the number of laser pulses. Surprisingly, the ablated mass does not set the volume of the extended cavitation bubble, as one would expect, because of the linear dependency of both the volume of the bubble and the ablation mass per pulse on the laser fluence. No influence of the altered cavitation bubble on the nanoparticles was identified. Instead, clear evidence of a high share of silver nanoclusters (d < 3 nm) with improved instrumentation (ultracentrifuge) was found in all samples at our low concentration conditions. The influence of these reactive species on the final particle size was found to be much larger than the cavitation bubble variations caused by prolonged surface ablation. In addition, no correlation was observed between the size of the primary particles (∼8 nm) and the mass concentration in the cavitation bubble.