Clear observation of the formation of nanoparticles inside the ablation bubble through a laser-induced flat transparent window by laser scattering

Cavitation erosion by shockwave self-focusing of a single bubble

The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach.

A comprehensive review and outlook on the experimental techniques to investigate the complex dynamics of pulsed laser ablation in liquid for nanoparticle synthesis

Pulsed laser ablation in liquid (PLAL) has been established as one of the most efficient and impactful methods for producing pure and ligand-free nanoparticles (NPs). PLAL has successfully been utilized for the synthesis of metal NPs, semiconductor NPs, ceramic NPs, and even nanocomposites. A variety of NPs, including core-shell, nanocubes, nanorods, and many other complex structures, can be synthesized using PLAL. The versatility associated with PLAL has led to the synthesis of NPs that have found applications in the field of biomedicine, sensing technology, energy harvesting, and various industries. Despite all the aforementioned advantages, there has been an ambiguity in terms of conditions/parameters for the nanoparticle synthesis as reported by various research groups. This has led to a perception that PLAL provides little or no control over the properties of the synthesized NPs. The properties of the NPs are reliant on transient dynamics caused due to a high-intensity laser's interaction with the target material. To understand the process of nanoparticle synthesis and to control the properties of NPs, it is critical to understand the various processes that occur during PLAL. The investigation of PLAL is essential for understanding the dynamical processes involved. However, the investigation techniques employed to probe PLAL present their own set of difficulties, as high temporal as well as spatial resolution is a prerequisite to probe PLAL. Hence, the purpose of this Review is to understand the dynamical processes of PLAL and gain an insight into the various investigation techniques and their data interpretation. In addition to the current challenges, some ways of overcoming these challenges are also presented. The benefits of concurrent investigations with special emphasis on the simultaneous investigation by multiple techniques are summarized, and furthermore, a few examples are also provided to help the readers understand how the simultaneous investigation works.

Ultrashort pulse laser ablation in liquids: probing the first nanoseconds of underwater phase explosion

The ultrafast pump-probe microscopy has shed new light on the complex dynamics of laser-induced explosive phase transformations and highlighted the importance of close integration of experimental, computational, and theoretical efforts.

Design and perspective of amorphous metal nanoparticles from laser synthesis and processing

Amorphous metal nanoparticles (A-NPs) have aroused great interest in their structural disordering nature and combined downsizing strategies (e.g. nanoscaling), both of which are beneficial for highly strengthened properties compared to their crystalline counterparts. Conventional synthesis strategies easily induce product contamination and/or size limitations, which largely narrow their applications. In recent years, laser ablation in liquid (LAL) and laser fragmentation in liquid (LFL) as "green" and scalable colloid synthesis methodologies have attracted extensive enthusiasm in the production of ultrapure crystalline NPs, while they also show promising potential for the production of A-NPs. Yet, the amorphization in such methods still lacks sufficient rules to follow regarding the formation mechanism and criteria. To that end, this article reviews amorphous metal oxide and carbide NPs from LAL and LFL in terms of NP types, liquid selection, target elements, laser parameters, and possible formation mechanism, all of which play a significant role in the competitive relationship between amorphization and crystallization. Furthermore, we provide the prospect of laser-generated metallic glass nanoparticles (MG-NPs) from MG targets. The current and potential applications of A-NPs are also discussed, categorized by the attractive application fields e.g. in catalysis and magnetism. The present work aims to give possible selection rules and perspective on the design of colloidal A-NPs as well as the synthesis criteria of MG-NPs from laser-based strategies.

In situ speciation and spatial mapping of Zn products during pulsed laser ablation in liquids (PLAL) by combined synchrotron methods

Pulsed laser ablation in liquids is a hierarchical multi-step process to produce pure inorganic nanoparticle colloids. Controlling this process is hampered by the partial understanding of individual steps and structure formation. In situ X-ray methods are employed to resolve macroscopic dynamics of nanosecond PLAL as well to analyse the distribution and speciation of ablated species with a microsecond time resolution. High time resolution can be achieved by synchrotron-based methods that are capable of 'single-shot' acquisition. X-ray multicontrast imaging by a Shack-Hartmann setup (XHI) and small angle X-ray scattering (SAXS) resolve evolving nanoparticles inside the transient cavitation bubble, while X-ray absorption spectroscopy in dispersive mode opens access to the total material yield and the chemical state of the ejecta. It is confirmed that during ablation nanoparticles are produced directly as well as reactive material is detected, which is identified in the early stage as Zn atoms. Nanoparticles within the cavitation bubble show a metal signature, which prevails for milliseconds, before gradual oxidation sets in. Ablation is described by a phase explosion of the target coexisting with full evaporation. Oxidation occurs only as a later step to already formed nanoparticles.