The dynamics of ultrafast cold cavitation in liquids

相变，如蒸发或沸腾，涉及分子机制，当相变发生缓慢时，我们很好地理解了这些机制。然而，在许多自然现象和技术过程中，相变发生得非常迅速。一个重要的例子是空化，这是由于液体中的低压甚至负压而形成的气泡。空化始于气泡的成核和快速生长。这个过程很难在三维样品中进行表征，因为它可以在任何地方发生，并且在纳秒内完成。经典成核理论（CNT）有助于理解成核的动力学，但在一些重要的情况下，它会失效，比如在室温附近纯水中的空化，这种空化发生在与CNT预测的压力非常不同的压力下。完善理论模型需要精确的实验数据。此外，极端条件下的数据对于解决现代柴油发动机等应用中的空化问题非常重要，现代柴油发动机使用高压燃油喷射来实现高效率和低排放。该项目将应用和改进激光烧蚀技术，以在纳秒级时间尺度上在大负压下诱导空化，并利用超快的光和x射线来捕捉纳秒级和纳米级空化气泡的演变。先进的图像分析和计算流体动力学（CFD）模拟将用于提高空化开始时压力测量的准确性。这项工作将在大学实验室和x射线自由电子激光器（XFEL）设施中进行，并将与慕尼黑工业大学的Nikolaus Adams教授博士合作进行CFD模拟。该项目将培养研究生和本科生，并将使他们接触到大学研究、大型科学设施和国际合作。该项目将使用脉冲x射线激光烧蚀和脉冲光学烧蚀，在时间和长度尺度上研究沸点以下的水和柴油中的空化（冷空化），这代表了该领域的实验前沿（纳秒和纳米，或“超快”）。在纳秒-纳米尺度上诱导空化的技术最近得到了证实，但它们必须加以改进，而且高精度测量空化压力和成核速率尚不可能。因此，表征超快空化动力学的新技术将得到发展：(1)利用先进的图像分析与图像模拟相结合，直接从液滴和射流的高精度光学图像中提取详细的流体动力学参数，如压力波运动学和振幅；(2)将高精度图像数据与高精度计算流体力学（CFD）模拟相结合，测量出非常高的空化率与压力的关系；(3)利用飞秒x射线激光散射成像快速演化的纳米气泡。此外，将设计并使用液体射流烧蚀实验来量化发生在纯水中的一种鲜为人知的空化类型。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知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Phase transitions, such as evaporation or boiling, involve molecular mechanisms that are well understood when the transition occurs slowly. However, in many natural phenomena and technological processes, phase transitions occur very rapidly. An important example is cavitation, which is the formation of bubbles due to low or even negative pressures in liquids. Cavitation starts with the nucleation and rapid growth of bubbles. The process is difficult to characterize in three-dimensional samples because it can occur anywhere and is completed in nanoseconds. Classical nucleation theory (CNT) helps understand the dynamics of nucleation, but there are important cases where it fails, such as cavitation in pure water near room temperature, which occurs at very different pressures than those predicted by CNT. Refining theoretical models requires accurate experimental data. Furthermore, data under extreme conditions is important for addressing cavitation in applications such as modern diesel engines, which use high pressure fuel injection to achieve high efficiency and low emissions. This project will apply and refine laser ablation techniques to induce cavitation at large negative pressures on nanosecond time scales, and ultrafast flashes of light and X-rays to capture the evolution of cavitation bubbles over nanoseconds and at nanometer dimensions. Advanced image analysis and computational fluid dynamics (CFD) simulations will be used to increase the accuracy of pressure measurements at the inception of cavitation. The work will be performed in university labs and at X-ray free-electron laser (XFEL) facilities and will involve a collaboration on CFD simulations with Prof. Dr. Nikolaus Adams at the Technical University of Munich. The project will train graduate and undergraduate students and will expose them to university research and also to large science facilities and international collaborations.The project will investigate cavitation in water and diesel fuel below the boiling point (cold cavitation) at time and length scales that represent the experimental frontier of the field (nanosecond and nanometer, or “ultrafast”), using pulsed X-ray laser ablation and pulsed optical ablation. The techniques to induce cavitation at the nanosecond-nanometer scales have been recently demonstrated, but they must be refined, and high accuracy measurements of cavitation pressures and nucleation rates are not yet possible. Therefore, new techniques will be developed to characterize the dynamics of ultrafast cavitation: (1) extracting directly from high-accuracy optical images of drops and jets detailed hydrodynamic parameters, such as pressure wave kinematics and amplitude, using advanced image analysis in combination with image simulations; (2) combining high-accuracy image data with high-accuracy computational fluid dynamics (CFD) simulations to measure very high cavitation rates as a function of pressure; and (3) imaging rapidly evolving nanobubbles using femtosecond X-ray laser scattering. Additionally, liquid jet ablation experiments will be designed and used to quantify a poorly understood type of cavitation that occurs in pure water.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.