The experimental set-up

Revealing structural motifs that control ice growth upon spontaneous crystallization at SPB/SFX

Scientists were studying structural motifs that control the spontaneous growth of ice nuclei in micron-sized, evaporation cooled water droplets with the fluctuation x-ray scattering technique using the 200 nm beam from SPB/SFX.

Phase changes in water

Almost everyone in the world is familiar with the transformation of water from state to state: as it freezes, it turns into ice; as it evaporates, it turns into steam. While a seemingly obvious process, the turning of water from state to state—known as a phase change—is still a mysterious and complex process, which remains under investigation today. In particular, scientists using European XFEL’s Single Particles, Clusters, and Biomolecules & Serial Femtosecond Crystallography (SPB/SFX) instrument, are looking at the way water freezes to form ice.

As a liquid freezes, it crystallizes, forming a solid structure. In water, we refer to this crystallized structure as ice. In liquid water, molecules are flying around in a disordered fashion, but as the liquid cools, the speed of the molecules decreases and decreases. At some point, a tiny cluster of ice forms. Other molecules attach themselves to this cluster, and it grows and grows in size until the water is entirely frozen. The shape and properties of this initial cluster determines everything about the way ice forms: whether it grows in ordered sheets, or a more irregular form. These clusters can be used to predict the behaviour of water as it freezes.

Investigating the freezing process

“You can think of this crystal cluster as a barrier,” says Jonas Sellberg, associate professor at KTH Royal Institute of Technology, Sweden, and user at European XFEL’s SPB/SFX instrument. “If we know what the barrier looks like, its shape and structure, we can tell a lot about how ice forms in water. Whether it forms in regular sheets, or whether it is growing as dendrites.”

Sellberg and his team are measuring the structure of these clusters using a method involving freezing a jet of liquid water in a vacuum chamber. As droplets of water fly through the vacuum chamber, they lose heat through evaporation, just like an athlete cools down when sweating. The further they travel through the vacuum, the more likely they are to freeze. The scientists can then use European XFEL’s ultrashort X-ray pulses to make measurements of the water droplets at different points in the freezing process, looking for the moment when a cluster forms. They collect a large amount of data, which can be reverse engineered through a statistical process to reconstruct the structure of the ice.

Fundamental research drives insights

“The scientists at the SPB/SFX instrument have a lot of experience with implementing these kinds of liquid jets in vacuum, which is not always straightforward,”

Sellberg continues. “We also need European XFEL’s high rate of pulse delivery. The probability of hitting the water molecules with the tiny X-ray beam as they freeze is small, so the more shots we can make per second, the better the quality of the structural information we can obtain.”

Learning about the nature of ice formation is useful to physicists since it contributes to their understanding of water’s fundamental properties. This is important since they want to know how things freeze, and how they can be safely and predictably unfrozen, particularly biological and natural systems. Learning about the formation of ice can also drive insights in fields like atmospheric physics, where icy water droplets impact the formation of clouds and weather systems.