20.02.2018 | Spectacular low temperature: Research team measures liquid water with a temperature of -42.6 degrees Celsius
It is a spectacular low temperature: A research team headed by Robert Grisenti from GSI Helmholtzzentrum für Schwerionenforschung has successfully detected liquid water at a temperature far below the freezing point: -42.6 degrees Celsius. This discovery is the result of development work on experiments for the future accelerator center FAIR, but it could also enable us to make great progress in our understanding of the earth’s climate.
Although it has long been known that water can remain liquid far below 0 degrees Celsius without freezing, the result depends on the volume of the sample used. The fast evaporative cooling of tiny water droplets in a vacuum gives scientists a good means of analyzing supercooled water, i.e. water that is still liquid even though its temperature is below freezing. However, it’s difficult to get a reliable value for the droplet temperature under such extreme experimental conditions. Because this value is of crucial importance for further studies, the reliable and exact measurement of the temperature of supercooled water is a great challenge.
In their research, the scientists demonstrated a new technology that achieves an unparalleled level of precision when measuring the temperature of extremely small droplets of water. The system does this by determining the temperature of a droplet on the basis of its diameter. In this process, uniform droplets of warm ultrapure water that are only a few thousandths of a millimeter wide are sprayed in a targeted jet of liquid into a vacuum chamber. The upper layers of the droplets evaporate and the inner layers cool off greatly, so the droplets shrink. This shrinkage can be precisely measured with optical methods, and the result is used to determine the droplets’ temperature. A key element for such high-precision measurements is the unique instrumentation available at GSI for Raman spectroscopy, in which the droplets are illuminated with a laser beam. The spectrum and form of the scattered light enable scientists to determine the diameter of the droplets.
In its research, the team can build on the expertise gathered over many years at GSI and at the international FAIR accelerator center, which is currently under construction. This expertise especially benefits the development of targets for atomic and nuclear physics experiments. Inside the accelerator facility, the particle beams are guided to the experimental stations, where they hit the targets made of the material samples. Targets that consist of tiny jets of liquids also have to be specially developed for such studies in the target hall and in the experimental storage rings at GSI and FAIR. FAIR will be a unique accelerator center with great knowledge potential. As a result, the current research with supercooled water for the development of targets for FAIR is also an example of the innovation potential at FAIR.
Droplets of supercooled water can also be found in nature — in the upper layers of the earth’s atmosphere, where they exist under conditions that are similar to those created experimentally by the research team. That’s why the work being conducted by the scientists under the direction of Robert Grisenti can also improve our understanding of ice formation in the atmosphere. It is thus an important step on the path toward the development of reliable climate models. (BP)