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Physicists Make Water Explode with X-Ray Laser Pulses

A whole new way to get matter to do really weird stuff.

An explosion is the release of a high energy density within some material. Ignite a wad of gunpowder, for example, and the result is a chemical reaction generating heat, which represents a sudden accumulation of energy. The heat causes gas to expand quickly enough that a pressure differential is created between the rapidly expanding gas and the atmosphere around it. This high-pressure front is what goes on to destroy things—maybe a whole city, or a galaxy, or a nanoscale bit of human tissue.

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That last example is what occurs in laser surgery: A bit of tissue is heated and then vaporized in an ultratiny explosion. Indeed, lasers are a great way to make things explode—as in laser ablation, generally—as they enable the delivery of large amounts of energy very suddenly into the guts of some material. This is most often envisaged with optical wavelength laser beams, but the electromagnetic spectrum is of course much broader than this.

A large team of researchers hailing from Stanford University and the Max Planck Institute, among others, has been hard at work blowing things up using X-ray free electron lasers (XFELs), which they describe in a paper published Monday in Nature Physics. Generating X-ray pulses at femtosecond scales is a new capability, the physicists explain, and such pulses offer the very desirable benefit of creating dynamically simple explosions, at least relative to optical lasers.

"Explosions are spectacular and intriguing phenomena that expose the dynamics of matter under extreme conditions," the paper begins. "We investigated, using time-resolved imaging, explosions induced by ultraintense X-ray laser pulses in water drops and jets. Our observations revealed an explosive vaporization followed by high-velocity interacting flows of liquid and vapour, and by the generation of shock trains in the liquid jets."

"These flows are different from those previously observed in laser ablation, owing to a simpler spatial pattern of X-ray absorption."

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Image: Stan et al

The basic idea is that the incoming beam of X-rays results in the creation of positive ions within the target liquid, which then go on to trap electrons via the electrostatic force (positive charges heart negative charges, etc.) These electrons, meanwhile, are absorbing photons from the pulse, and soon enough the result is an abrupt, extreme heating in a long, narrow region through the liquid droplet following the path of the beam.

The extreme energy density together with the long filamentary explosive region are something never before seen in more typical laser ablation experiments. Firing an optical laser beam into a water droplet instead has the effect of focusing the beam into a single point, or hotspot. With an X-ray pulse the beam cuts right through, leaving instead a long explosive thread of energized matter.

As an X-ray explosion preceded in the group's experiments, the eventual result was the fragmentation and vaporization of the water droplet being targeted, leaving a disc-shaped cloud surrounding the original path of the pulse, rather than the sphere you might expect from an explosion bursting from a single focused point.

So, weird X-ray explosions are all well and good, but what's the actual use? For one thing, it's a way of creating relatively controlled explosions that don't suffer from the abovementioned point-focusing problem. And this offers the potential for using the beams to create and study usual new dynamics in matter.

As the authors explain, "such experiments may reveal, with unprecedented spatial and temporal resolution, the structure and dynamics of pure liquids and of chemical or biological samples in solution."