Turning water into glass at FXE
Scientists have prepared Amorphous Solid Water (ASW). By heating the ASW layer, they could study structural changes in the amorphous ice layer on the nanosecond timescale, hence enabling studies of the subsequent cooling process.
Looking at the transition between liquid water and its icy glass state
Glass plays a big role in our everyday lives. We find it in windowpanes, mirrors, cups to drink from, or even decorations. When scientists refer to ‘glasses’, however, they are referring to a specific chemical state which has its own special properties and behaviours. Most of the commercially used glasses we find in our homes are known as silicate glasses and are made from silicon dioxide molecules. But many other different molecules also form glasses, including water.
“A glass is a solid where molecules are not regularly arranged in a pattern like in a crystal. Instead, they are arranged in an irregular configuration that we refer to as ‘amorphous’, like you would find in a liquid but not moving around,” says Tobias Eklund, (PhD student at Johannes Gutenberg University, Mainz and EuXFEL). “You can create this state in water by cooling it down very fast until it freezes.”
When one freezes water into ice at home, it creates a crystal, with a regular pattern of molecules. This is because the freezing process takes a relatively long time. To create a ‘glassy ice’ in water, scientists must cool water down extremely quickly, in a millionth of a second. In this case, the ice molecules don’t have time to form a regular pattern, instead turning into a glass. The transition between liquid and glass in water, both freezing and melting, is still an active area of research today.
Amorphous Solid Water (ASW)
Researchers at European XFEL’s Femtosecond X-ray Experiments (FXE) instrument are now looking at this transition between liquid water and its icy glass state. To do this, researchers first form the glass state by putting water vapour in contact with a very cold layer of material. Water prepared in this specific manner is referred to as “amorphous solid water”, or ASW for short. In the experiment, they then use a short flash from an infrared laser to heat the water glass very quickly, melting it. They synchronise this flash with the ultrashort and intense X-ray flashes from the European XFEL which act as a camera, imaging the transition between the states.
“In order to form or dissolve the glass, you need to be very fast. Glass formation happens in a time shorter than a millionth of a second, so we have to be able to measure faster than that,” says Prof. Katrin Amann-Winkel, (Max Planck Institute for Polymer research & Johannes Gutenberg University, Mainz). “The European XFEL is the perfect instrument to study such processes.”
The researchers are focused on water at very low temperatures, minus 150 degrees Celsius or lower, and the way in which the structure of water changes in this temperature region. The research has two aspects that could be important for humanity and society.
“Firstly, we want to understand the fundamental properties of water,” continues Amann-Winkel. “It’s also important for astrophysics; glassy water forms very commonly in outer space. If water vapour hits a cold dust grain in space, it doesn’t have time to crystallise. That’s what we do here: put water vapour on a cold substrate to mimic what happens in outer space. Understanding crystallization processes, on the other hand, is important for producing better climate models, and furthering knowledge of what is happening in our atmosphere and above it.”