Pump-probe X-ray holographic imaging of laser-induced cavitation bubbles with femtosecond FEL pulses

Abstract

Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water. The fast dynamics are associated with extreme states of gas and liquid, especially in the nascent state. This offers a unique setting to probe water and water vapor far-from equilibrium. However, current optical techniques cannot quantify these early states due to contrast and resolution limitations. X-ray holography with single X-ray free-electron laser pulses has now enabled a quasi-instantaneous high resolution structural probe with contrast proportional to the electron density of the object. In this work, we demonstrate cone-beam holographic flash imaging of laser-induced cavitation bubbles in water with nanofocused X-ray free-electron laser pulses. We quantify the spatial and temporal pressure distribution of the shockwave surrounding the expanding cavitation bubble at time delays shortly after seeding and compare the results to numerical simulations.

Cavitation-induced shock wave behaviour in different liquids

“…Shock waves from collapsing cavitation bubbles have been the subject of interest to academics and industrialists due to their extraordinary potential of generating severe response dynamics in a variety of fields, ranging from biomedical sciences, nanomaterials, food processing, microelectronics, to liquid metal processing and additive manufacturing [1] , [2] , [3] , [4] , [5] , [6] . Also recently, fundamental studies investigated the spatial shock wave pressure in the cavitation centre and the threshold of shock waves upon solid–liquid impact respectively [7] , [8] further advancing the existing knowledge in the field. Hence, understanding the acoustically-induced cavitation shock waves, and specifically the detection and elucidation of acoustic signals (that can be used as a powerful tool to control cavitation activity during ultrasonic processing as in [9] , [10] ) emanated from bubble activity is vital for the development and further improvement of various applications and techniques allied to these fields.…”

Structural dynamics of water in a supersonic shockwave

“…By comparing our data to data from the literature measured at static pressures, 14,15 we report a peak pressure of 10 GPa at Dt ¼ 6 ns in agreement with our previous study. 22 Importantly, we can attribute the change in the structure factor S(q) after dielectric breakdown to a transition of the structure from a tetrahedral arrangement of water molecules to the molecular structure of a simple liquid. 7,15 Future extensions of this work will be directed at a wider q-range in order to compute the pair distribution function g(r) using the AGIPD-detector 40 and a higher photon energy.…”

“…To spatially control the location where the structural dynamics of water is probed in an inhomogeneous and transient perturbation such as a shockwave and to provide complementary density information, we here combine the pump-probe diffraction in the wide-angle x-ray scattering (WAXS) regime with our recently developed x-ray near-field holography (NFH) scheme. 21,22 By using advanced phase retrieval and a generalized empty-beam division, 21 we showed earlier that the local densities inferred from phase retrieval can reveal the pressure profile of a shockwave 22 in the regime of several GPa. We now use this NFH scheme in combination with WAXS to investigate the structural dynamics of water following dielectric breakdown induced by an infrared (IR) laser pulse and during the subsequent formation and growth of a cavitation bubble.…”

A matter of brightness: table-top X-ray generation inside waveguides and X-ray holography with single free-electron laser pulses

“…While this property of the refractive index benefits radiography, it simultaneously imposes a difficulty on the fabrication of high-quality refractive, diffractive or reflective X-ray optics 2 . Reflective optics, for example, can only be operated in grazing incidence below the critical angle, which is on the order of a few milliradians.…”

“…We define hard X-rays to be roughly in the range of E ph = 5 keV to 300 keV. In the following X-rays will be used synonymously with hard X-rays, if not stated otherwise 2. See[Pag06] for an overview on X-ray optics.…”