Researchers at European XFEL in Schenefeld near Hamburg have taken a closer look at the formation of the first crystallisation of nuclei in supercooled liquids. They found: The formation starts much later than previously assumed. The findings could help to better understand the creation of ice in clouds in the future and to describe some processes inside the Earth more precisely.
Every child knows that water freezes into ice when it gets icy cold. For water, this normally happens below zero degrees Celsius, the melting temperature of water. This is a fixed point on the Celsius temperature scale that we use.
However, the transition from the liquid to the solid phase is a very complex process and is difficult to study experimentally at the atomic level. One reason for this is that crystals are formed randomly: You don’t know exactly when and where it will happen. Furthermore, a liquid can remain in a metastable state for a long time: It remains liquid even though it should actually freeze and become solid. This makes it extraordinarily difficult to pinpoint the right moment for a crystal to form and watch its growth.
However, these effects are highly relevant in nature. For example, they play a decisive role in the formation of ice in clouds or in processes inside Earth.
Using the intense X-ray flashes of the European XFEL’s X-ray free-electron laser, an international team of researchers at the European XFEL in Schenefeld near Hamburg has now succeeded in precisely measuring the nucleation of supercooled liquids. The experiments took place in a vacuum so that the X-ray light does not interact with the molecules in the air, which would interfere with the experiments. Because of its complexity, however, water is one of the most difficult liquids to model. For that reason, the researchers used instead argon and krypton in liquid form in their experiments. In fact, supercooled noble-gas liquids are the only systems for which reliable theoretical predictions can be presently made.
The researchers explicitly investigated the so-called crystal nucleation rate J(T). This is a measure of the probability that a crystal will form in a certain volume within a certain time. The rate at which this happens is an important parameter, for example in order to be able to mathematically describe real processes in models – in weather forecasting, for example, or in climate models.
Read more on XFEL website
Image: X-ray of a crystal. The diffraction pattern results from 34,000 single-pulse x-ray exposures of a krypton jet shortly after the onset of crystal nucleation. The rings indicate x-ray scattering from specific molecular planes within the small crystals.
Credit: European XFEL