Dynamically available volume: A novel order parameter for dense and nearly arrested systems

Many common materials ‘solidify’ upon a small change in their surroundings (such as temperature or pH), and the underlying phenomenon is the collective slowing of the motion of the constituent molecules. Examples include gels, polymers, proteins, and other soft matter systems. It has been known for some time that ‘arrest’ is related to some concept of ‘free volume’. However, it has emerged more recently that the important parameter is the volume usable in the long-ranged reorganisation of the system (the dynamically available volume, DAV) as this determines mobility in these systems. The DAV diminishes as systems arrest, and this is the canonical order parameter for understanding the arrest of complex systems. This idea has been taken a step further here, wherein it is clearly indicated that thermal fluctuation effects can be accommodated within the DAV scenario.