One of the breakthroughs in the understanding of water has been possible through a combination of studies using X-rays at Argonne National Laboratory near Chicago, where the two different structures were evidenced and at the large X-ray laboratory DESY in Hamburg where the dynamics could be investigated and demonstrated that the two phases indeed both were liquid phases. Water can thus exist as two different liquids. We have in particular been able to follow the transformation of the sample at low temperatures between the two phases and demonstrated that there is diffusion as is typical for liquids. When we think of ice it is most often as an ordered, crystalline phase that you get out of the ice box, but the most common form of ice in our planetary system is amorphous, that is disordered, and there are two forms of amorphous ice with low and high density. The two forms can interconvert and there have been speculations that they can be related to low- and high-density forms of liquid water. To experimentally investigate this hypothesis has been a great challenge that has now been overcome. The new results give very strong support to a picture where water at room temperature can’t decide in which of the two forms it should be, high or low density, which results in local fluctuations between the two. In a nutshell: Water is not a complicated liquid, but two simple liquids with a complicated relationship.
Another recent discovery has to do with when water is being cold. If you chill water below 4 degrees, it starts to expand again. If you continue to cool pure water (where the rate of crystallization is low) to below 0, it continues to expand – the expansion even speeds up when it gets colder. Many more properties such as compressibility and heat capacity become increasingly strange as water is cooled. With the help of ultra-short x-ray pulses at x-ray lasers in Japan and South Korea, we have succeeded in determining that water reaches the peak of its strange behaviour at -44°C. This is the temperature when the fluctuations of the two local form of water reaches its maximum. This gives an indication that there may even exist a critical point somewhere close. The next stage is to find the location of the critical point in terms of pressure and temperature. A big challenge in the next few years.
