X-ray flash imaging of laser-induced bubbles and shockwaves in water
Everyone is familiar with tiny gas bubbles gently rising up in sparkling water. But the bubbles that were created by intense focused lasers in this experiment were ten times smaller and contained water vapour at a pressure around a hundred thousand times higher.
Under these conditions, the bubble expands at supersonic speed and pushes a shockwave, consisting of a spherical shell of highly compressed water, ahead of itself.
Now a research team led by the University of Göttingen, together with scientists from DESY and European XFEL, has created such an event and then, with an innovative technique that they developed using holographic flash imaging and nanofocused X-ray laser pulses, captured data and images. The research was published in Nature Communications.
The team first created tiny bubbles with a radius of a few thousandths of a millimetre by focusing an infrared laser pulse in water to create "cavitation" (a phenomenon in which small vapour-filled cavities, i.e. bubbles, form in a liquid). The researchers observed the expanding bubble with synchronised but carefully controlled delayed X-ray pulses from European XFEL.
"In contrast to visible light, where refraction and scattering blur the image, X-ray imaging not only resolves the shape but also the density profile of the interior of both the bubble and the shockwave," explains Malte Vassholz, lead author from the University of Göttingen. "This enabled us to generate X-ray holograms of the tiny bubbles and record a large data stream with thousands of events which we then analyzed by a specially devised 'decoding algorithm' to obtain the density of the gas in the bubble and the shockwave around it."
Thanks to the well-controlled time delay between the seeding laser pulse that created the effect and the X-ray pulse that measured it, the team could then record a 'movie' of the process. "In total, we recorded about 20,000 events and analysed more than 3,000 bubbles," reports DESY lead author Johannes Hagemann. "For each bubble, the energy deposited in the water by the infrared laser was determined and the density and pressure in the resulting shock wave were calculated from the X-ray data. From this, the pressure can be plotted over time and as a function of the energy in the bubble."
The results of his experiment will help other scientists develop better models of the behaviour of water at extreme conditions, as principal investigator Tim Salditt from the University of Göttingen explains: "Even though water is the most important liquid on Earth, there is still much to learn about this mysterious and elusive substance. Thanks to the unique properties of the X-ray laser radiation generated at the European XFEL, and our new single shot holography method, we can now observe what really goes on in vapour and liquid water under extreme conditions."
This research technique provides insights for processes relevant in other applications: "Cavitation can be an undesirable effect in fluids in pumps or propellers for instance, but it can be harnessed for use in laser processing of materials or to modify chemical reactions," explains co-author Robert Mettin, an expert researching cavitation for many years at the University of Göttingen. "In laser surgery, shockwaves and compressed gases in tiny bubbles are created intentionally in tissue, by laser pulses," adds Salditt. "In the future, such processes could be 'filmed' in detail, using the methodology which we have developed, at a microscopic level and at high temporal resolution."