From its unique properties to the physics of phase transitions, Limei Xu explains how studying water can reveal fundamental laws of nature.

Water might look simple at first glance—just two hydrogen atoms bound to a single oxygen atom. Yet its complex behavior continues to puzzle scientists around the globe. Understanding this seemingly simple molecule could hold the key to explaining the behavior of many other materials and physical phenomena. 

Limei Xu, Boya Distinguished Professor at Peking University’s School of Physics, began studying water and its phase transitions—from solid to liquid to gas—during her PhD at Boston University. After postdoctoral positions at the University of Utah and at Tohoku University in Japan, Xu joined the International Center for Quantum Materials (ICQM) at Peking University in 2011, shortly after its foundation. Since then, her research group has become a world leader in investigating the behavior of water at the atomic level.

In recognition of her impactful contributions and dedication to innovation, Xu was named one of the winners of the 2026 Wiley Women in Science Awards. Through collaborations within the ICQM, Xu combines her background in classical physics with quantum physics, enabling her group to gain a deeper understanding of the unusual properties of water and the fundamental physics behind them. 

Since I was young, physics helped me understand the universe. It might look like the world is a mess, but studying natural phenomena made me think about the laws behind everything that exists. I believe that once you learn physics, you can apply it to all kinds of fields, like biology, engineering, materials science, computer science, economics… In high school, I had a physics teacher who taught us to fully grasp the concepts before solving a problem. This gave me the confidence to continue studying physics after high school and into college.  

You may know that water shows an anomaly where it reaches maximum density at 4°C, but there are many other similar anomalies. It’s a real headache to understand the phase diagram of water; there are more than 20 different known phases of ice alone. During my PhD under Professor Gene Stanley, I wanted to see whether we could build a simple model to understand these anomalies. At that time, I studied them from a macroscopic point of view. Since joining the ICQM, we have been able to study them at the atomic level to understand why water exhibits so many anomalies. 

We have developed a technique based on atomic force microscopy (AFM) that detects interactions between the tip of a probe and water molecules. When the structure you are studying is very small, it is relatively easy to predict it from these signals. However, when it is a large area and the structure is completely disordered—as in the case of water—it is not possible to recover the full atomic structure. 

Our team built an AI-assisted AFM analysis method that can accurately predict the complete two-dimensional structure of amorphous materials. We can also apply this approach to scanning tunneling microscopy (STM) to recover three-dimensional structures. For instance, this allows us to observe the ice nucleation process at the atomic level and establish a link between the real-space structure of water and its macroscopic properties. For this work, we recently received the first prize of China’s National Natural Science Award. 

By studying ice nucleation, we found a new way of growing two-dimensional quantum materials. To achieve a particular property, you have to grow these materials with a particular structure, but growing a material without defects is still a big challenge. Understanding the mechanism behind this can guide the growth; some groups are using our theoretical method and predictions to grow these quantum materials. 

We can also predict water properties, like superlubricity, from both a theoretical and an experimental point of view. This can help us design materials with these properties.

I think one of the big challenges is solving disordered structures in two dimensions. For ordered structures, like crystals, we already have a good theory to describe them. But for disordered materials, like glass, the structure is far from equilibrium. These materials can also show aging over time, with a non-linear evolution. For these non-equilibrium disordered materials, we still don’t have a theory to describe their behavior. We are trying to develop a method or theory to understand their phase transitions. 

Non-equilibrium phase transitions are still a kind of no man’s land. Anything you do contributes to the field. It’s difficult because there is no established theory yet and, while progress is slow, we are making steady advances in this area. 

We are studying a simple but really important concept called supercritical phenomena. For instance, when water is in its gaseous form and you keep increasing the temperature and pressure, it reaches a point where the difference between liquid and gas disappears—we call this a supercritical point. We are asking, what happens if you go to even higher pressure and higher temperature? We found extremely important properties in this region and discovered that this is a general phenomenon not only of water but to all kinds of materials. 

For instance, the supercritical point is a type of second-order phase transition similar to superconductivity. I think we can find a way to unify all these observations into a much simpler mode of this type of phase transitions. This is a long-term goal; I hope to have achieved a good understanding of that by the time I retire. 

The Wiley Women in Science Awards 2026, held in March at the Institute of Physics, Chinese Academy of Sciences, brought together leading scientists and scholars to honor the outstanding contributions of nine exceptional women researchers for their impactful contributions and dedication to innovation. The establishment of the Wiley Women in Science Awards, supported by the journal Advanced Physics Research, aims to inspire the next generation of researchers and support a more inclusive and diverse research community.

Photo courtesy of Limei Xu.