Scientists fired an ultra-intense laser into a thin jet of water to gain insights into medical surgery, carbon nanocrystals, superdense aluminium synthesis and materials science.
“Choose your next witticism carefully, Mr Bond, it may be your last.”
These were the words from the James Bond film Goldfinger (1964) that introduced the terrible splendour of the laser to the world barely four years after it had been invented. The words were uttered by the super-villain Auric Goldfinger in an iconic torture scene that gave the laser one of its first places in history and demonstrated its then state-of-the-art abilities (with some helpful commentary from Goldfinger himself).
Lasers have a stupendous variety of applications to show for their abilities – from optical power correction to inertial nuclear fusion to surreal art. Off late, they’re also increasingly being used as probes in very-sensitive microscopes. High-energy laser pulses are shot at a group of particles under study to impart energy to them. As they become excited and start to vibrate, move around or whatever, a second laser pulse is shot in to react to the particles; the reaction is caught by different kinds of detectors. By closely coordinating the timing between the two pulses, called the pump and the probe, scientists can study the progress of chemical reactions over one millionth of a billionth of a second. This has given rise to a branch of chemistry dedicated to studying the reactions that happen over this time-scale: femtochemistry.
Their prowess in probing techniques (as well as probing Bond) owes itself to lasers being able to carry and deliver tractable amounts of energy in tightly focused beams. And sometimes, these beams can impart so much energy that the things they’re targeting might just go boom. This needn’t be a bad thing if you’re keen on understanding how matter behaves when the amount of energy it contains becomes extremely high – the sort of situations common in exploding stars and, as it turns out, the synthesis of superdense aluminium. Then again, it does sound curious when scientists turn a powerful laser and point it at something that we don’t often see going boom at all: water.
These scientists, from Germany, Switzerland and the US, directed an “ultra-intense” X-ray free electron laser (XFEL), a device that can generate powerful X-ray laser pulses that last for a few femtoseconds, towards droplets and streams of water in a vacuum. Using a high-speed camera, they were then able to draw up a femtosecond picture of how the water absorbed, redistributed and released large amounts of energy. Such an experiment could be useful for illuminating the behaviour of any system that involves fluids and high temperatures – like surgery to remove lesions on the liver, creating protective coatings from materials that are otherwise hard to vaporise and to synthesise carbon nanocrystals. However, none of these applications quite capture the wonder of something like water exploding. What does that look like?
An XFEL pulse generated by the Linac Coherent Light Source (LCLS), California, was shot at two configurations of water: droplets and a stream. The pulse itself was about a millionth of a metre wide and carried X-ray photons holding about a billionth of a billionth of a mega-joule (MJ) each. The scientists found that when the energy density crossed about 20 MJ/kg in the water body, it would begin to shed smaller droplets in a bid to rapidly lose energy – a.k.a. an act of explosive boiling, albeit on an extremely small scale. To compare, the energy density required to vaporise liquid water is 2.6 MJ/kg. So how does the water even begin to boil?
When an XFEL pulse is shot through a water droplet about 40 millionths of a metre across, it loses about 5% of its energy to the water. The process involves the high-energy photons knocking out electrons from the water molecules – the photoelectric effect whose discovery (by different means) won Albert Einstein the Nobel Prize for physics in 1921. The electrons then knock on even more electrons, which all then knock on the molecules and energise them. However, because the pulse shoots through the droplet so fast, this ‘heatwave’ doesn’t look like much like a growing sphere as much as a streak of hot water within the droplet just as wide as the pulse itself. And this streak, called a filament, contains enough energy to vaporise the droplet many times over (50-750 MJ/kg).
While the electrons generated by the photoelectric effect are busy exciting other electrons and molecules, the molecules they’re knocked out of become positively charged ions. The scientists found that the water started to boil explosively at the same time the ions recaptured their electrons to become neutral once more.