Using laser pulses to crystallise supercooled water into ice may seem counter-intuitive, but that's exactly what researchers in Germany and the UK have achieved. Because the pulses can be focused to a specific point in the liquid, the researchers believe that their technique will be valuable for future material and crystal growth studies. (Physical Review Letters 99 045701)
"We wanted to highlight the mechanisms behind sonocrystallization, where ultrasound triggers the nucleation of a supercooled liquid," researcher Robert Mettin of the University of Goettingen told optics.org. "We replaced the sound wave with a focused laser pulse to generate a single, well-controlled and localized bubble in the liquid."
A supercooled liquid is one that has been chilled to below its normal freezing point without crystallization occurring. Relying on a process called acoustic cavitation, sonocrystallization uses ultrasound to trigger nucleation (crystal formation). The sound waves generate bubbles in the supercooled liquid and the collapse of these bubbles leads to crystallization.
Mettin and colleagues, including one from Unilever Corporate Research in the UK focused single 8 ns pulses from an Nd:YAG laser emitting at 1064 nm into a cuvette containing the supercooled water. Each pulse, which had an energy between 1 and 2 mJ, was focused to a spot in the range of 50 microns in diameter. The team used a high-speed CCD camera to capture the response in the liquid after each pulse had been fired.
"The focused laser pulse locally evaporates the water causing a strong pressure wave and a bubble of water vapor and other gases to grow," explained Mettin. "The bubble subsequently collapses, which causes a second strong pressure wave. We believe that the strong transient increase of pressure in the supercooled liquid close to the bubble causes nucleation."
Mettin adds that the water shifts into a "pressurised regime" for a short time, which leads to a much larger supercooling effect and increases the likelihood that an ice crystal will form.
The team tried to link the laser energy, maximum bubble radius and bubble fragmentation to ice nucleation, but found "no significant correlation". However, there was a relationship between temperature and nucleation.
"We could achieve supercooling down to -8degC otherwise the liquid froze spontaneously," said Mettin. "The water volume was about 40 cubic centimetres and once a crystal was initiated, the whole volume froze. On the other hand, the higher the temperature, the smaller the supercooling and the less likely a nucleation event becomes. We did not observe crystallization by the laser shots between 0 and -2degC."
The team is now devising a set of experiments to find out more about the nucleation trigger. "We are interested, for example, in direct measurement of the high pressure waves and other bubble conditions with (or without) nucleation," concluded Mettin.
Author
Jacqueline Hewett is editor of Optics & Laser Europe magazine.
