Towards increasing the ablation efficiency of pulsed laser ablation in liquids by discrimination of cavitation bubble and nanoparticle effects
Over the past 25 years, the formation of colloidal solutions using laser ablation in liquids (LAL) has attracted considerable attention due to its advantages over nanoparticle synthesis by reduction of chemical precursors. Besides the almost unlimited number of feasible target materials and, respectively, nanoparticle compositions, LAL colloids exhibit surfaces free of any organic residuals. In recent years, the productivity of LAL was increased to the gram per hour regime, which was driven by advances in laser technology. However, the gap between ablation efficiency, and hence the final nanoparticle productivity, between laser ablation in air and in liquid medium is still large. Typically, productivity rates of nanoparticles for laser ablation in air are five to ten times larger compared to LAL. As LAL technology becomes applicable to industrial testing in a wide range of areas, the feasible productivity of nanoparticle generation becomes a critical issue due to increasing demand. Understanding shielding cross-effects is a prerequisite for maximal power-specific LAL. However, discrimination between cavitation bubbles (CB), nanoparticles (NP), and shielding, e.g., by the plasma or a transient vapor layer, is challenging.
Therefore, CB imaging by shadowgraphy is performed to better understand the plasma and laser beam-NP interaction during LAL. By comparing the CB volume for ablations performed with 1 ns and 7 ns pulses, Dittrich et al. observe smaller energy-specific CB volumes for 1 ns-ablation leading to a superior ablation efficiency. Furthermore, it is shown that a CB cascade occurs when the focal plane is shifted into the liquid. This effect is enhanced when NPs are present in the fluid. Even minute amounts of NPs trapped in a stationary layer decrease the laser energy significantly. However, this local concentration in the sticking film has so far not been considered. For the first time, the research group of Priv.-Doz. Dr. Bilal Gökce from the University of Duisburg-Essen shows in this study funded by the German Research Foundation (DFG) that nanoparticles trapped in the stationary layer present an essential obstacle in high-yield LAL, shielding already the second laser pulse that arrives. The article is entitled “Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease” and published in Opto-Electronic Advances Issue 1 2021.
Fig.1 The sketch illustrates the stationary liquid layer above the target and the first cavitation bubble of an image sequence. The formed NP are partially trapped in the stationary layer leading to satellite bubble formation (as shown in the sketch, and observed experimentally) and a decreased power-specific NP productivity.
About The Group
The Gökce Group is part of the Department of Technical Chemistry, chaired by Prof. Stephan Barcikowski. With more than 25 scientific researchers, this department is the largest group working in the field of nanoparticle synthesis by laser ablation in liquids. Dr. Gökce’s team explores the applications of laser-generated nanoparticles and their composites. Moreover, high power ultrafast lasers are utilized in combination with optical techniques to scale-up laser synthesis of colloids to an industrial scale. A special focus is placed on materials development for laser additive manufacturing.
Dittrich S, Barcikowski S, Gökce B. Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease. Opto-Electron Adv 4, 200072 (2021).