When a molten phase is cooled very slowly, the atoms have time to organise themselves into an ordered arrangement, a crystal phase. Each atom is in a position of mechanical stability with symmetrically arranged nearest-neighbours; the system attains a new rigidity as the material undergoes a phase transition to a solid. But if you cool the melt down very quickly, the atoms don’t have that opportunity to get into these equilibrium positions. The material still solidifies, but retains the disordered structure of the liquid. This is how glass is formed. And it’s a mystery to science.
Glass poses so many challenges. At a fundamental level, physicists don’t understand the nature of the glass transition. Phase transitions, where the property of a system changes abruptly, are usually signalled by a change in the order, or a “broken” symmetry as physicists would say. Identifying these “broken symmetries” drives a lot of physics research, including into the structure of the early universe and exotic phases of matter like superfluids and superconductors. This research has largely been successful, with the behaviour of many systems fitting into this unifying concept. However, identifying the order parameter that marks the glass transition has remained elusive. It is a strange fact that one of the most ubiquitous and mundane materials is a deep, unsolved problem in physics.
Glasses have fantastic properties that make them an ideal material in many applications. For example, their lack of crystal grain boundaries means they are transparent and will transmit light. Their lack of crystal surface facets also gives them a smooth surface. Glasses are generally quite hard, and good for structural applications. They have one well-known failing, which is that they are brittle. So when they are strained beyond their failure point, they will tend to fail catastrophically, with a crack propagating through the whole material.
This is how glass is formed. And it’s a mystery to science.
Crystalline metal alloys like aluminium or titanium aren’t brittle like glasses. Yet glasses can also be made of metals if you can cool the melt down quickly enough. These metallic glasses have improved ductility like crystalline metallic alloys. They are ideal in niche applications, such as MEMS (microelectromechanical) devices and blades on surgical tools. There are also some very specialised applications for metallic glasses being investigated at NASA’s Jet Propulsion Laboratory for the extreme environment of space. Metallic glasses are difficult to make in large volumes, as the cooling rates to avoid crystallisation and make a metallic glass need to be so high (a million degrees a second). They are also prone to brittle failure.
Glassy phenomena also occur at larger length scales and in different systems. For example, microspheres and colloids form glasses, and the behaviour of granular systems like sand share many features with glasses. Indeed, the mechanical failure of glasses seems quite similar to avalanching in granular assemblies. Other disordered systems where flow can suddenly turn into arrest may be analogous to glasses in some respects – for example, the behaviour of crowds dispersing from a venue, like a sell-out match at the MCG.
I find these phenomena fascinating. At some level, all the challenges of glasses – like their fundamental solid nature, finding good glass-forming systems, and improving their mechanical properties – are related to their structure. The science of crystallography that determines the structure of crystals, largely through x-ray or electron diffraction, has really underpinned our understanding and development of crystalline materials. But currently there is no routine experimental method that can determine the structure of a glass.
This is where I have chosen to do my research, trying to develop new methods to understand the structure of glasses. My work is done in small teams of collaborators at local facilities like the Monash Centre for Electron Microscopy and the Australian Synchrotron (ANSTO). These teams are diverse and include experimental and theoretical physicists, chemists, materials scientists and synchrotron scientists. We examine metallic and colloidal glasses with electron and x-ray diffraction. A major advance we have made is to shrink the electron or x-ray beam down to the size of a nearest-neighbour structural unit in a glass. These small-beam diffraction patterns have subtle angular symmetries that reflect the local structures in glasses. From these, we can extract a wealth of new and rich information about the structure of glasses. This new information promises to reveal some of the secrets of glass, but there are many more challenges to overcome.
While grand scientific challenges can be found in the smallest particles and the largest structures in the universe, deep mysteries also reside in very commonplace materials, like glass.
I am fortunate to have had the professional opportunities and access to internationally leading research infrastructure to develop my research into glasses. I gratefully acknowledge fellowship support from the Science Faculty, Monash University and the Australian Research Council (FT180100594) and the experimental facilities and expert scientific staff at the Australian Synchrotron and the Monash Centre for Electron Microscopy. This research has been enabled and enriched by many wonderful collaborations.
As told to Graem Sims.