‘Active matter’ is matter that is intrinsically out of equilibrium. In particular, an ‘active suspension’ is made up of self-propelled particles or droplets dispersed in a liquid. Active matter is not in thermal equilibrium even in the absence of external driving, and display fascinating properties. Thus, e.g., a so- lution of the filament-forming protein actin and the ‘molecular motor’ protein myosin can ‘burn’ ATP as fuel to produce a gel that flows in the absence of any external pressure gradient; while a suspension of swimming bacteria can have a viscosity that is lower than that of the suspending liquid. There is yet no gener- ally accepted statistical mechanics of active matter, where the absence of detailed balance means that small differences in microscopic dynamics can in principle lead to very different macroscopic behaviour. Moreover, there is no a priori reason to believe that a reduced description in terms of just a few macroscopic parameters (such as effective temperature and density) is possible. I propose a systematic pro- gramme of experiments to discover when and how microscopic dynamics affect the macroscopic behaviour of active suspensions, whether any of their behaviour has analogues in suspensions of passive particles and droplets, and how activity can be described using coarse-grained variables. To ensure that the experiments can be tightly coupled to theory and simulations, I will use well-characterised, model systems of active particles. Developing model systems is therefore a subsidiary, but crucial, goal of my programme. Some of these systems will be designed to be as similar as possible in their passive properties, but quite distinct in terms of their microscopic dynamics – a ‘luxury’ that is typically only available to theo- rists and simulators. Experimenting with such model systems should reveal what phenomena are generic to activity, and what phenomena are specific to particular kinds of microscopic dynamics.
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