X-ray Imaging of Cavitation Dynamics with Free-Electron Laser Pulses

by Hannes Paul Hoeppe
Cumulative thesis
Date of Examination:2024-07-02
Date of issue:2025-06-25
Advisor:Prof. Dr. Tim Salditt
Referee:Prof. Dr. Tim Salditt
Referee:Dr. Robert Mettin
Persistent Address: http://dx.doi.org/10.53846/goediss-11355

 

 

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Abstract

English

Cavitation dynamics are fast non-equilibrium hydrodynamic processes involving matter at extreme states. A first relevant mechanism is the light-matter interaction during (ultrafast) laser-induced optical breakdown and plasma formation. The following bubble formation, and similarly, the collapse of cavitation bubbles involves violent dynamics and the generation of shock waves. Finally, the compression of matter in a collapsing bubble can lead to the emission of light, i.e. sonoluminescence. Open questions in today’s research, however, relate to temporal and spatial scales which often exceed the capabilities of common experimental techniques. In the present work, we have explored time-resolved phase-contrast holography with single hard X-ray Free-Electron Laser pulses as a novel and versatile method to study cavitation dynamics. Enabled by highly brilliant XFEL radiation, this technique allows a unique view on fast (hydrodynamic) processes. Outstanding features are a high spatial resolution, no distortion by curved phase-boundaries and a quantitative contrast, based on the electron density of the sample. This thesis focuses on three experiments carried out at the European XFEL, investigating the scenarios of (i) femtosecond laser-induced optical breakdown and filamentation, (ii) the spherical collapse of a single sonoluminescent cavitation bubble and (iii) collapsing cavitation bubbles close to a solid boundary. Respectively, the results obtained in this work enable a detailed view on the complex breakdown process within a focused laser beam, with sub-micron resolution. We explore the capability to retrieve quantitative information on the bubble size, shape and density distribution of a sonoluminescent bubble during the instant of collapse. Finally, we report the intricate interior shape of collapsing bubbles, revealing an oblique liquid jet with respect to the adjacent solid surface.
Keywords: X-ray imaging; Free-Electron Laser; optical breakdown; shockwave; phase-contrast imaging; sonoluminescence; bubble collapse; cavitation
 
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