The dynamics of ultrafast cold cavitation in liquids

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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.