– POSTECH-Stockholm University joint research team succeeds in experimental observation of supercooled water with the 4th generation accelerator.
– The team succeeds in observing the phase transition from heavy water to light water using
– 70℃ water. Also notable for undergraduate students’ participation in the study.

Paramount in the creation and prolongation of life and essential to life on Earth, water is still shrouded in mystery. Just as the surface of the river freezes in -20℃ weather but the underlying current does not, water has several anomalous properties that distinguish it from other liquids. Despite long-standing research, there is still a debate in academia about the secret to water.

Among them, the liquid-liquid critical point (LLCP) hypothesis that water consists of heavy water (HDL) and light water (LDL) can explain the anomalous properties of water, but has remained in hypothesis since producing unfreezing water below -43°C was impossible.

Recently, a joint research team led by Professor Kyung Hwan Kim of the Department of Chemistry at POSTECH and Professor Anders Nilsson’s team at the Stockholm University in Sweden have together succeeded in creating heavy water at -70°C, and observed as it turned into light water using X-rays of less than 100 femtoseconds.

Published in the latest issue of Science, these research findings garnered extraordinary interest from academic circles as it is a direct evidence for the theory that water is originally composed of heavy water and light water. The study drew more attention as Sun-joo Yoo and Sang-min Jeong, who were still undergraduate students at the time, also participated in this study.

Among the various hypotheses about the properties of water, the LLCP hypothesis states that under extremely cooled conditions, water is divided into heavy water and light water, and the state changes between the two waters. To prove this hypothesis, it is necessary to make unfreezing water below -43℃, but that has long been considered impossible.

The research team, that had published the results of a study measuring unfreezing water at -46°C three years ago in the same journal, further developed the study and tackled an experiment to create water that stays unfrozen briefly at -70°C.

First, the research team produced high-density-amorphous (HDA) ice at -160°C, then instantaneously heated it with a powerful laser to produce heavy water at -70°C. This water exists only for an instant and needs extremely bright and fast light to observe it. To measure this phenomenon, the research team used X-rays from Pohang fourth-generation accelerator (PAL-XFEL), coined the “light of dreams,” to momentarily measure the unfreezing water at -70°C. Then, the team observed the process of this heavy water’s phase transition to light water.

The results of this study are direct evidence supporting the hypothesis that water is originally composed of two liquid phases – heavy water and light water – and is anticipated to clarify the causes of various properties of water related to this. Above all, academic circles evaluate this as an important clue to fundamentally understanding why water is essential and suitable for life.

“This research finding will settle the long-standing debate among researchers who have questioned the secret of water and will serve as a starting point for solving the mystery of water and life by understanding its anomalous properties,” explained Professor Kyung Hwan Kim from POSTECH. “It is also noteworthy that undergraduate students contributed to a great extent in this important research which will have notable significance in basic science.”

The research was conducted with the support from the Young Researcher Program of the National Research Foundation of Korea funded by the Ministry of Science and ICT, and utilized the 4th generation accelerator PAL-XFEL of the Pohang Accelerator Laboratory in Korea.