The newly-discovered state of water that's both solid and liquid

Did you know water can be both a solid and liquid at the same time?

Water has been around for a whopping 4.4 billion years on Earth, but it was only until around two weeks ago when scientists at the University of Tokyo first uncovered a state of water in which it behaves as if it's about to freeze and melt at the same time, known as the "premelting state". This "hybrid" state defies traditional solid or liquid classifications, and occurs when water is confined in extremely small, nanoscale spaces. Due to technological limitations, scientists have had a hard time studying this unique state, until now.

Although it may sound counter-intuitive at first, the premelting state is actually quite simple. In this state, the water molecules' positions were relatively fixed as one would expect of solids, but their rotational motions were more like those of a liquid and were very fast. To observe the premelting state, the scientists used static solid-state deuterium nuclear magnetic resonance (NMR) spectroscopy (just think of it as an advanced way to examine molecular movement). This method differs from other previously used methods, like X-ray analysis, because it allows scientists to observe the picosecond (one-trillionth of a second)-scale rotational movements of hydrogen atoms and water molecules.

To conduct the experiment, the scientists filled crystal nanopores of 1.6 nanometer diameters with heavy water (D2O). The D stands for deuterium, an atom that contains one proton and one neutron in its nucleus compared to hydrogen with only one proton, doubling the mass (hence the name "heavy water"). This allowed the scientists to better track and observe this state using NMR. During the experiment, the scientists observed a clear three-layered structure of confined water: an inner layer where molecules moved freely and liquid-like, a middle layer where with intermediate mobility, and an outer layer that was much more rigid and solid-like. To further examine the premelting state, the scientists began to slowly heat the crystals that contained the confined water to get the water from a frozen state to liquid state. During this process, the NMR confirmed a transition into the premelting state, with some water molecules starting to move like a liquid would while others remained locked in place. This phase was explained by leading research professor Makoto Tadokoro as "a novel phase of water in which frozen H₂O layers and slowly moving H₂O coexist."

Overall, this experiment widened our understanding of the behavior of water under extreme confinement, offering insight into how water moved within tiny biological structures such as cell membranes and proteins. It could also help develop new materials that store gases like methane using special ice-like networks and contribute to the development of new hydrosphere materials -- functional materials designed using water.

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