Collaborative Research: Probing Cavitation Inception in Dielectric Liquids with Sub-Nanosecond Precision

空化是指由于压力降低而在液体中形成气泡(或空穴)。空化临界条件的准确确定对流体机械设计和靶向药物输送等许多技术领域具有重要意义。到目前为止，空化门槛压力的实验结果并不一致。一个突出的因素是，现有的技术不能可靠地捕捉到空化开始的非常早期的阶段。最近的研究表明，由于纳秒脉冲电场产生的负压，介电液体中可以形成纳米级的空穴。由于这一过程可以通过电子方式同步和控制(现有方法不是这样)，这暗示了一种新的技术，可以在时间上以亚纳秒的精度“精确地定位”空化开始，从而能够更准确地确定空化阈值。本研究项目将探索该方法的潜力，并将其用于测量水和其他液体的空化阈值，这对于验证空化理论和模型将是非常有价值的。这项跨学科研究将产生重大的教育成果，重点是为代表不足的少数族裔学生提供STEM丰富的创新方法。PIS将开发和提供与研究相关的主题的新学习模块，如日常生活中的放电和流体力学。该项目的目的是通过实验表征简单介质液体以及含有分散粒子的液体中电致伸缩空化的初始阶段，并从理论上阐明亚ns时间尺度下潜在的物理过程。由于相关的理论分析仅限于过于简单的情景，实验证据很少，而且远未系统，因此实现这一目标的方法是通过协同使用不同发展阶段的诊断测量和实际实验情景的数值模拟。该研究计划有两个具体目标：(1)检测超短脉冲电场下液体中的空化引发；(2)研究液体中分散相对空化的影响。光学诊断，包括纹影、瑞利散射和散斑成像，结合电学和材料特性测量，将被用来探测在不同电极几何形状下，含有和不含有分散颗粒的各种介质液体中的早期空化。对于水等简单液体，实验数据将与系统数值模拟的结果进行比较，以确定空化引发的临界条件。对于两相分散，将建立实验条件与光学诊断结果之间的关联，这将为分析和模拟分散颗粒附近的电致伸缩空化提供信息，从而加深对所涉及的物理过程的理解。这项研究的预期结果将包括确定在各种液体中引发空化的关键参数，以及描绘随后空化发展的物理图景和分散相的影响。该项目将在亚纳秒时间尺度上弥合空化引发的知识鸿沟，并有助于推进与空化以及纳秒/亚纳秒液体等离子体放电相关的技术。该奖项反映了美国国家科学基金会的法定使命，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。

Cavitation is the formation of bubbles (or cavities) in liquids due to decreased pressure. Accurate determination of the critical conditions of cavitation is of fundamental interests to many technological fields such as fluid machinery design and targeted drug delivery. So far, the experimental results of cavitation threshold pressures have not been consistent. One prominent factor is that the existing techniques cannot reliably capture the very early stage of cavitation inception. Recent works have shown that nanoscale cavities can form in dielectric liquids due to negative pressure generated by nanosecond-pulsed electric fields. Since this process can be synchronized and controlled electrically (which is not the case for existing methods), it hints at a novel technique to “pinpoint” cavitation inception with sub-nanosecond precision in time and therefore enable a more accurate determination of cavitation thresholds. This research project will explore the potential of the proposed method and use it to measure the cavitation thresholds of water and other liquids, which would be very valuable for the validation of cavitation theories and models. This interdisciplinary research will have substantial educational outputs, with an emphasis on innovative approaches to STEM enrichment for underrepresented minority students. The PIs will develop and deliver new learning modules on topics related to the research, such as electrical discharges and fluid mechanics in everyday life. The objective of this project is to experimentally characterize the initial stages of electrostrictive cavitation in simple dielectric liquids as well as liquids with dispersed particles and theoretically elucidate underlying physical processes at the sub-ns timescale. As relevant theoretical analyses have been limited to oversimplified scenarios and experimental evidence is rare and far from systematic, the approach to accomplish the objective is through synergistic use of diagnostic measurements at different developmental stages and numerical modeling of the actual experimental scenarios. The research plan has two specific aims: (1) detection of cavitation initiation in liquids under ultra-short pulsed electric fields; and (2) examination of the effects of the dispersed phase in liquids on cavitation. Optical diagnostics including Schlieren/shadowgraph, Rayleigh scattering, and speckle imaging, combined with electrical and material characterization measurements, will be used to probe early-stage cavitation in various dielectric liquids, with and without dispersed particles, under different electrode geometries. For simple liquids such as water, the experimental data will be compared with the results from systematic numerical simulations to determine the critical conditions of cavitation initiation. For two-phase dispersions, the correlations between the experimental conditions and optical diagnostic results will be established, which will provide the information for the analysis and modeling of electrostrictive cavitation near dispersed particles, and by doing so, deepen the understanding of the physical processes involved. The expected outcomes of the research will include the determination of critical parameters for cavitation initiation in various liquids and a delineation of the physical picture of the subsequent developments of cavitation and the effects of the dispersed phase. This project will bridge the knowledge gap of cavitation initiation at sub-nanosecond timescale and be instrumental in advancing the technologies related to cavitation as well as nanosecond/sub-nanosecond plasma discharge in liquids.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.