Water is considered as the most important liquid on earth and, despite the apparent simplicity of the water molecule, many properties of water remain surprising. In particular, at supercooled temperatures, the thermodynamic response functions such as heat capacity or compressibility do not follow a linear trend as observed in other liquids but rather shows a drastic divergence. 30 years ago, a hypothesis about the origin of the anomalous behavior of water has been put forward. It describes water at ambient conditions as two fluctuating structural configurations of high- and low-density above a hidden critical point. Current advances in theoretical and experimental techniques increasingly converge to support this hypothesis.
An important role in the ongoing debate is assigned to the amorphous forms of water (high- and low-density amorphous ice, HDA and LDA), which are proposed to be the glassy counterparts of two distinct liquid phases (HDL and LDL).
The process of crystallization in the deep supercooled regime happens on very fast timescales of tens of ns, as only cooling rates exceeding 107 K/s prevent the water to crystallize. Using pump-probe experiments the amorphous-to-liquid as well as liquid-liquid transition can be probed (see Science 2020, Nat.Comm. 2023, Science 2026 ). We make use of short and highly intense X-ray pulses of X-ray free electron lasers (XFEL). You can find out more about our experiments in this video.
Water at different interfaces. Hypothesis on HDL and LDL.
In many processes related to soft matter, water acts as solvent. The structure of water, however, changes with temperature and in contact with solutes or interfaces. Largely unexplored is the question, how the structural formation in soft matter is influenced by local structural changes in the water network and vice versa. Within the collaborative research training group GRK2516
“Structure formation of soft matter at interfaces”, we investigate how the interfacial water structure changes upon supercooling or compression, which is related to a change in water´s phase diagram and a shift of the liquid-liquid critical point (LLCP). We aim to answer the following questions: “How does a change in the water hydrogen bond network eventually change the properties of the polymer?” and “How do tailored soft interfaces influence heterogenous ice nucleation?”
n order to investigate deeply supercooled water and different amorphous ices, cryogenic temperatures and elevated pressure beyond 1 GPa are required. We refer to this as extreme conditions. Our lab consists of several cryostat environments as well as high-pressure equipment such as a piston cylinder and a diamond anvil cell.
We aim to understand the effect of pressure on phase transitions in water and aqueous solutions at cryogenic temperatures, in order to map the related phase diagram. The diffusive dynamics in amorphous ice was further investigated by X-ray photon correlation spectroscopy, both at ambient as well as elevated pressure. We have investigated the pressure dependence of the glass transition in HDA by combining XPCS with a diamond anvil cell and a customized cryostat. The figure shows the experimental setup with two detectors and the X-ray beam passing through the diamond anvil cell (see Karina et al. 2025).
We use dynamic light scattering (DLS) as well as its complementary X-ray based method, X-ray photon correlation spectroscopy (XPCS), to study both the intrinsic dynamic of glass forming liquids, hydrogels, hydrated proteins as well as using nanoparticles as tracers (see publications Eklund et al. 2026).
In our project related to the collaborative research centre SFB1552 we are investigating the dynamics of supramolecular hydrogels using XPCS at ambient pressure. An interesting property of this group of materials is their ability to self-heal mechanical damage. Studying the microscopic dynamics and relaxation within such polymer networks is key to understanding the underlying mechanism behind such re-connectivity processes.).