Microparticle laser fragmentation in liquids: mechanisms, energetics, and efficiency quantified with single-pulse, single-particle precision

Microparticle laser fragmentation in liquids has emerged as a promising approach to generate nanoparticles with high efficiency. Despite its advantages, the underlying fragmentation mechanisms, their connection to the nanoparticle size distribution, and the energy efficiency of the process remain poorly understood. In this study for the first time, microparticle fragmentation is investigated in single-pulse, single-particle experiments on Au microparticles. Determining the absorbed peak fluence enables assessment of the process energetics. Pump-probe microscopy identifies photomechanical fracture of the molten microparticle volume and photothermal phase explosion of its superheated surface as the fragmentation mechanisms. We find that 83% of the absorbed laser energy is converted into cavitation bubble formation, while only 1% contributes to the surface energy of the generated nanoparticles. Despite this small fraction, MP-LFL outperforms laser ablation in liquids. The surface energy generated per absorbed energy is 10 times higher, and the overall energy efficiency is 14 times higher. This gain originates from the confined microparticle geometry, which minimizes energy losses and enhances photomechanical fragmentation via pressure focusing. These results position microparticle fragmentation in liquids as a fundamentally more energy-efficient approach for scalable, laser-based nanoparticle production than laser ablation in liquids.

💡 Research Summary

This paper presents a groundbreaking investigation into Microparticle Laser Fragmentation in Liquids (MP-LFL), a promising technique for nanoparticle synthesis. The study’s central achievement is the development and application of a single-pulse, single-particle experimental methodology, which for the first time allows precise quantification of the process’s fundamental mechanisms, energetics, and efficiency, free from ensemble averaging effects.

Using gold microparticles (~1.2 µm diameter) as a model system, the researchers established a precise fragmentation threshold, finding that an absorbed peak fluence of 37 mJ/cm² led to a 50% probability of fragmentation. The probabilistic nature of the threshold region was attributed to inter-particle variations in morphology and internal stress.

Through sophisticated pump-probe microscopy with nanosecond to second temporal resolution, the fragmentation dynamics were visualized in real-time. The analysis revealed two concurrent primary fragmentation mechanisms: (1) photomechanical fracture within the molten core of the microparticle, driven by stress confinement and pressure focusing, and (2) photothermal phase explosion at the superheated surface. The temporal evolution of the accompanying shockwave and cavitation bubble was quantitatively tracked.

A pivotal contribution of the work is the comprehensive energy balance analysis. It was determined that a mere 1% of the absorbed laser energy is converted into the surface energy of the newly generated nanoparticles. The vast majority, 83%, is channeled into the mechanical work of cavitation bubble formation, with the remaining 16% lost to thermal conduction into the surrounding liquid.

Despite this seemingly low direct conversion, the study demonstrates that MP-LFL is fundamentally more energy-efficient than the benchmark technique, Laser Ablation in Liquids (LAL). MP-LFL exhibits a 10-times higher “surface energy generated per absorbed energy” ratio and a 14-times higher overall “photon-to-nanoparticle” conversion efficiency. The authors attribute this superior performance to the confined geometry of the microparticle. This geometry minimizes energy losses to the environment and enhances the photomechanical fragmentation pathway by focusing laser-induced pressure waves within the particle volume.

In conclusion, this research transcends prior productivity-focused studies by providing a foundational physical understanding of MP-LFL. It proves that MP-LFL is not merely a scaled-down version of LAL but a distinct, inherently more energy-efficient process due to its unique mechanisms enabled by particle confinement. The established single-particle precision methodology sets a new standard for probing laser-matter interactions in liquids.