- Scientists developed a technique that cools down water to minimum 228 Kelvin without making it freeze.
- The tinier the drop, the colder it could get while being in liquid phase.
- They used laser pulses to monitor the size of drops and to measure the positions of resonance peaks in scattered light.
We all know the freezing point of water is 0°C, but that is not the coldest (in liquid form) it could be. Recently, researchers developed a new methodology that measured the lowest temperature ever recorded for liquid water.
One team of scientists managed to cool down the liquid water to 228 Kelvin (-45.15°C), and another down to 230 Kelvin (-43.15°C). This is cool for many science reasons and the fact that water is weird and extremely important for understanding the planet we live on. Also, it doesn’t always obey the physics rules you learned in school.
Water has odd properties that get even weirder as temperatures reaches to minimum. It doesn’t always freeze below 0°C under certain conditions. Researchers have demonstrated some of these scenarios in their latest findings.
Temperature depends on the average kinetic energy of a set of molecules. The less they move around the lower their temperature. Actually, it depends on a lot of other factors whether molecules begin to form crystal (or freeze). It requires a nucleus to form and start attracting other neighbor molecules. Below 0°C, water prefers to be a crystal.
However, researchers have developed a new technique that prevents water crystallization. This includes measuring temperature of water drops through their diameters.
Both teams made their own “super-cold water” by shooting jets of very small water drops into a vacuum. Smaller water drops are less likely to crystallize from other particles. The tinier the drop, the colder it could get while being in liquid phase.
Splitting the drops into further small pieces exposes more of them to a vacuum, causing an unusual type of vacuum cooling. Lowering the pressure allows the particles (on the surface) to rapidly evaporate, which immediately eliminates the heats from the water drops, reducing their temperature steeply. When measured, these water drops record astonishingly low temperatures.
Reference: Journals.aps | doi:10.1103/PhysRevLett.120.015501
Setting Up Experiments
Setting up experiments was not an easy task. Both teams squirted a jet of tiny (micrometer-sized) water drops into a vacuum chamber. Some of the drops’ liquid evaporated as they traveled, cooling the left portion and making the drop to shrink. Therefore, the amount of cooling could be measured by observing the diameter of drops.
To monitor the size of drops to a precision of 10 nanometers and to measure the positions of resonance peaks in scattered light, they used laser pulses. Analyzing how these drops change in size (diameter) as they leave the nozzle helped scientists to accurately measure their temperatures.
The Two Liquid Phases
The team who got their liquid water down to -45.15°C, noticed a weird thing – the supercooled water could exist in two different density phases at the same time. This depends on the clusters of some connected water molecules stacked up in the drops.
By examining the pure liquid drops’ diffraction patterns, the scientists calculated how their compressibility varied with temperature. The compression rate was maximum at -44° C.
When a fluid is in fluctuating equilibrium between low-density and high-density phases, it becomes squishier. This happens because some substance transforming from the low- to high-density phase can accommodate increase in pressure. According to the researchers, the compressibility mostly occurs near the crossover point, where liquid water contains almost equal proportions of low and high-density local structures.
Maximum compressibility as a function of pressure difference compared to “liquid-liquid critical point”
Reference: Sciencemag | doi:10.1126/science.aap8269
Despite being the important liquid on Earth, water is a puzzling substance with properties that make it different from other liquids. We still don’t know exactly how cold liquid water can get under certain scenarios.
The supercooled water drops occur naturally in the upper atmosphere of our planet. Knowing the nature of water could improve our understanding of atmospheric ice formation and build more reliable cloud models. For now, the next big challenge is to find the location of the critical in terms of temperature and pressure.