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DESY News: X-ray flash imaging of laser-induced bubbles and shockwaves in water
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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.

Cavitation bubbles 0, 5, 9 and 18 nanoseconds (billionths of a second) after the arrival of the infrared laser pulse. The shock wave of the bubble on the far right has a diameter of about 0.1 millimetre. Credit: University of Göttingen, Malte Vassholz
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.”

An infrared laser pulse (red) generates a plasma (green cloud) in pure water; its expansion subsequently creates a shock wave and a bubble (hemispheres). An acoustic signal recorded with a microphone (top, red) is used to determine the deposited energy; the X-ray beam (purple cone) of the European XFEL is used to generate a hologram that is detected by a detector (circle on the right). Credit: University of Göttingen, Markus Osterhoff
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.”
Reference:
Pump-probe X-ray holographic imaging of laser-induced cavitation bubbles with femtosecond FEL pulses; Malte Vassholz et al.; Nature Communications, 2021; DOI: 10.1038/s41467-021-23664-1
Source: University of Göttingen, with a detailed anmation, explaining the experiment