University of Bristol
Local structure in liquids out of equilibrium
When a liquid is cooled below its freezing temperature, it can either freeze into a crystal, or ‘choose’ not to, and become a disordered solid, or glass. Common glass, silicon dioxide, is just one example of a generic state of matter. Unlike crystals, glasses are not in equilibrium, in general they ‘want’ to be crystalline, but are ‘frustrated’. Neither crystallisation nor glass formation are understood, although they have been observed for 4000 years. Apart from curiosity, we need to understand glass formation and freezing because:
(1) With more understanding of glass formation, we could design new materials. For example, metallic glasses promise large improvements in mechanical properties. One example of metal failure is the first jet airliner, the Comet whose tragic accidents were caused by metal fatigue. Normal metals fail at the boundaries between the microscopic crystal grains, each grain is a crystal lattice, at the boundary, lattices of different orientations form weak points. Glasses have a disordered structure, so have no grains nor grain boundaries and are less prone to failure.
(2) Understanding crystallisation is key to tackling the protein problem. According to biologists, ‘structure is function’: to find the structure the protein must be crystallised. Of the 60,000 proteins that comprise the human genome, only 20% have been crystallised. Until more proteins can be crystallised their function –and purpose – remains elusive.
We study the role of local structures of molecules in glass formation and crystallisation, using computer simulation and novel experiments based on 3D imaging of individual nanoparticles which form nano-particle crystals and glasses. Our work reveals a much higher level of detail than conventional methods, and can determine the origins of the ‘frustration’ which keeps glass-forming materials from crystallising, and how the local structure of glasses and liquids is different.