Characterising the ordering in liquid and amorphous materials on different length scales

What is it about?

Liquid and glassy materials do not have random structures. Instead, ordering does appear on different length scales, depending on the nature of the interatomic forces and the structural units they create. Here, we consider this ordering for a variety of glass-forming materials, ranging from densely packed amorphous metals and hard-sphere glassy colloids, to simple tetrahedral systems that include amorphous silicon and patchy colloids, to decorated tetrahedral systems that include amorphous ice and network-forming glasses such as silica. The ordering manifests itself as distinct peaks in the total structure factor S(k) measured in diffraction experiments. The geometrical origin of the peaks in S(k) and corresponding partial structure factors is discussed, and equations are given for predicting the peak positions. The change in the system fragility with the emergence of ordering on different length scales is also considered, along with the effect of pressure. The fragility describes the rate-of-change of the liquid viscosity with temperature as the glass transition is approached.

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Photo by Brett Jordan on Unsplash

Photo by Brett Jordan on Unsplash

Why is it important?

The signatures of the different structural motifs in common glass-forming materials are written directly into the patterns measured in diffraction experiments. The features in these patterns also give insight into the nature of the liquid dynamics as the glass transition is approached.

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