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

当大量的蒸汽气泡在过热的液体中成核时，就会发生“爆炸沸腾”或“相爆炸”。这种现象比较常见，在许多实际应用中起着关键作用，包括纳米颗粒和纳米材料的产生、表面清洁和纳米/微制造。尽管几十年来进行了广泛的实验和理论研究，但对相爆炸的条件和微观机制仍缺乏清楚的了解。该研究项目的目的是了解过热到热力学稳定性极限的亚稳态液体中爆炸相分解的机理和动力学。将使用大规模原子模拟和最先进的、时间分辨的相爆炸瞬变动力学探测相结合来跟踪该过程的所有阶段。将研究相爆炸动力学对环境、靶的几何形状和升温速率的依赖关系，以进一步深入了解能够在实际应用中控制过程的基本机制。该项目将揭示爆炸性汽化的基本机制，其量化长期以来一直难以捉摸，并将促进激光加工和制造方面的突破。准确和有效的激光烧蚀动力学预测将有助于材料加工和微/纳米加工的发展，以及具有定制尺寸、成分和性能的纳米结构的产生。通过实验和计算的紧密结合，可以深入了解相爆炸的微观机理和动力学。对于相同的材料系统、约束条件和激光参数进行的模拟和实验将最大限度地增加可靠解释实验观测和直接验证计算预测的机会。爆炸蒸发金属和合金的块体和薄膜形式以及金属纳米线将被研究在不同的环境背景压力条件下和在强约束下的盖层。通过泵浦-探测光学解调、时间分辨成像、快速测温和使用超薄嵌入式传感器进行温度测量，将研究相爆炸的时间演化。关于抛射纳米粒子的瞬时温度变化、光学散射分布、速度和内部温度的定量动态数据将直接关系到大规模原子模拟的预测。非原位分析表面形态、结晶度和缺陷结构，以及所产生的纳米颗粒的尺寸分布也将与计算预测有关。这些研究将提供一幅完整的多尺度图景，将最初的爆炸相转变与实际相关结果的影响联系起来，包括表面纳米结构和纳米粒子的产生。热能分配、传输和转换的基本原理将通过直接实验探测、辐照目标中的余热建模和烧蚀竖管的热发射相结合进行分析。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

“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.