“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.
When they repeated the experiment by shooting a jet of water about 20 millionths of a metre wide with an XFEL pulse, they found a series of interesting phenomena play out in the jet. When the pulse first strikes the jet, the jet splits in two and the water beats a retreat to either side. This is driven by the water directly in the path of the pulse becoming rapidly boiled and vaporised. As the vapour cloud expands and dissipates, it pushes back against the jet-fronts on either side of it, forming an umbrella-like film of water (in the video below, watch the first five seconds at 0.25x).
The force of the first strike also sends a shockwave through the water at seven-times the speed of sound, as well as pressure fluctuations ringing through the jet. Over a few tens of nanoseconds, the fluctuations chop up the shockwave into six smaller ones travelling at close to the speed of sound in water (1.5 km/s). In time, the umbrella-like film slowly folds up and comes closer to the jet on either side of the pulse. This is because the shape of the film tends to the form of a sphere, the shape that has the least amount of surface tension for a given volume.
This is a richer chronicle of what happens in an experiment like this: filament, explosively boil, vaporise, films, shockwaves, pressure waves, fold up – evidently not just water being splashed around. Understanding how water might behave in these circumstances down to the smallest detail could, for example, help doctors improve the performance of laser-induced thermal therapy (LiTT). LiTT is an increasingly popular, and more cost-effective, minimally invasive surgical procedure used to treat people with high-grade gliomas and drug-resistant epilepsy. It involves firing a carefully controlled laser pulse at parts of the brain to damage or remove problematic neurological structures. Since brain tissue – or most tissue for that matter – is thinly coated in a layer of water, understanding how the water reacts to the laser could help control the effects of LiTT and minimise unwanted damage.
But before we go as far as discussing the applications, let’s also stop for a moment and marvel at the granularity with which we’re able to probe the natural response of simple substances to unnatural stimuli. Lasers can’t be found in nature, as Goldfinger helpfully explained, which means that the way the water droplets and jets have been observed to behave aren’t to be found in nature either, at least not yet. What we’ve been able to see the water do is knowledge of how the laws of physics respond to conditions that they might not have experienced before, conditions that provide insights into how the laws do or don’t come together to respond to lots of energy pulsing through a droplet. And to think that it will only become better: the first moments of the experiment couldn’t be studied because, following the pump shot, the XFEL at the LCLS can fire the probe shot no earlier than five nanoseconds later.
The study was published in the journal Nature Physics online in May and in print in October, 2016.