Collaborative Research: Microscopic mechanisms and kinetics of laser-induced phase explosion

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“Explosive boiling” or “phase explosion” occurs when a massive number of vapor bubbles nucleate in a superheated liquid. This phenomenon is relatively common and plays a key role in numerous practical applications including the generation of nanoparticles and nanomaterials, surface cleaning, and nano/microfabrication. Despite decades of extensive experimental and theoretical studies, a clear understanding of the conditions and microscopic mechanisms of the phase explosion is still lacking. The objective of the research project is to understand the mechanisms and kinetics of the explosive phase decomposition in a metastable liquid superheated up to the limit of its thermodynamic stability. A combination of large-scale atomistic simulations with state-of-the-art, time-resolved probing of the transient dynamics of the phase explosion will be used to track all stages of the process. The dependence of the dynamics of the phase explosion on the environment, geometry of the target, and heating rate will be investigated to gain further insights into the fundamental mechanisms that would enable control over the process for practical applications. This project will unveil the fundamental mechanisms of explosive vaporization, whose quantification has long been elusive, and will foster breakthroughs in laser processing and manufacturing. Accurate and verified predictions of laser ablation dynamics will contribute to the advancement of material processing and micro/nanofabrication, as well as the generation of nanostructures with tailored size, composition, and properties.Insights into the microscopic mechanisms and kinetics of the phase explosion will be obtained through the close integration of experimental and computational studies. Simulations and experiments performed for the same material systems, confinement conditions, and laser parameters will maximize the opportunities for reliable interpretation of experimental observations and direct verification of the computational predictions. The explosive vaporization of metals and alloys in the bulk and thin film forms as well as metal nanowires will be studied under various ambient background pressure conditions and under strong confinement by capping layers. The temporal evolution of the phase explosion will be studied by pump-probe optical interrogation, time-resolved imaging, fast pyrometry and temperature measurement using ultrathin embedded sensors. Quantitative dynamic data on the transient temperature variation, optical scattering distributions, speed and internal temperature of ejected nanoparticles will be directly related to the predictions of large-scale atomistic simulations. Ex situ analysis of the surface morphology, crystallinity, and defect structures, as well as the size distribution of produced nanoparticles will also be related to the computational predictions. These studies will provide a complete multiscale picture connecting the initial explosive phase transformation to the implications for practically relevant outcomes, including surface nanostructuring and nanoparticle generation. The fundamentals of the thermal energy partitioning, transport and transformations will be analyzed through a combination of direct experimental probing, modeling of the residual heat in the irradiated targets and the thermal emission of the ablation plume.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.