Active and driven systems are all around us. Active systems include flocks of animals, the cytoskeleton inside each of our cells, swarms of bacteria and an increasing number of synthetic systems in which colloidal (micron-scale) particles suspended in a fluid are caused to self-propel. In each of these systems, local motion is sustained by the continuous use of an energy source. Driven systems differ from active ones, in that the motion of the constituent particles is not caused by local effect, but by sustained forcing at the boundaries; an example is pumping fluid down a pipe. In each case the system's behaviour, even when time-independent, can differ strongly from systems in thermal equilibrium, such as a suspension of colloidal particles that is neither self-propelled nor driven. ADSNeSP’s agenda focused on the following questions:
• When does local irreversibility of the dynamics have global consequences, and when does it not?
• When it doesn’t, can we find an effective equilibrium description at large scales?
• When it does, is the active system equivalent to some globally driven one, and in what sense?
• What can we learn by perturbatively connecting active to passive or globally driven models?
This work is important for society, because active systems underly the viability of almost all living objects. They also promise new ways of making functional materials where very precise microstructure is required. Driven systems such as sheared colloidal suspensions underpin large areas of technology such as the production of foodstuffs and homecare products.
The project has led to a number of important conclusions. Firstly, it has led to a completely new understanding of how to quantify the irreversibility of active systems, by developing new tools to identify not only the physical origin of irreversibility but also the precise spatial locations of its causes. This new understanding has been deployed across a wide range of systems relevant to active matter. Second, the work has led to a new understanding of improbable processes in active matter and other irreversible systems. These improbably processes represent tipping points whereby a system which has been stable a long time suddenly makes a transition into a new state (such as nucleation of liquid from its vapour). Also, by understanding the unlikely process whereby a rare behaviour is achieved, it is possible to design a new process for which this behaviour is the normal one, creating an important route to material design. Along similar lines, we have developed new models for driven systems (especially dense suspensions of frictional particles) that allow precise connections to be made between flow behaviour and microscopic interactions. This has created a new tool for designing the flow properties of materials ranging from molten chocolate to ceramics.
