Details of Grant 

EPSRC Reference:	EP/H027254/1
Title:	Fluctuations in Active Matter: From Micro to Macro
Principal Investigator:	Tailleur, Dr J
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Department:	Sch of Physics and Astronomy
Organisation:	University of Edinburgh
Scheme:	Postdoc Research Fellowship
Starts:	01 April 2010	Ends:	30 April 2011	Value (£):	253,183
EPSRC Research Topic Classifications:	Complex fluids & soft solids Condensed Matter Physics
EPSRC Industrial Sector Classifications:	No relevance to Underpinning Sectors
Related Grants:
Panel History:	Panel Date Panel Name Outcome 26 Jan 2010 PDRF Physics Interview Panel Announced 21 Dec 2009 PDRF PHYSICS Sift Panel Excluded
Summary on Grant Application Form
The last decade has seen an upsurge of studies by physicists of biological systems.In many cases, the expertise of experimental physicists has successfully answeredbiological questions. In others, biological systems have inspired questions neglectedby biologists, but of great importance for the physics community. Among these standsthe study of what has became known as 'Active Matter': systems where non-thermalenergy is converted in the presence of dissipation into an effective motion. The classof systems considered is very large, including for instance bacterial suspensions,bird flocks, vibrated rods and - even further from physics and biology - pedestriansand traffic flows. These non-equilibrium systems comprise a large number of interactingparticles, that offer an experimentally controlled benchmark for non-equilibrium statisticalphysicists willing to confront new theories with the physical world.Currently there is no generic method to analytically study these systems, whosedescription has thus attracted lots of effort over the past few years. One of the majorgoals of this proposal is to address this important problem. One of the apparently mostsuccessful approaches has been to extend to these non-equilibrium systems thephenomenological theories developed for systems close to thermodynamic equilibrium.The underlying idea is to identify mesoscopic quantities (e.g. a density field) and topostulate coarse-grained hydrodynamic equations for these fields which respectsymmetries and conserved quantities of the system. The validity of this approach is howeverquestionable and significant results in this direction have been scarce. In particular,fluctuations out-of-equilibrium are not constrained by the detailed-balance symmetryand can generate much richer phenomenology. The first axis of my proposal deals preciselywith this point. Using methods established in other area of statistical mechanics, I willstart from a class of microscopic models and construct their fluctuating hydrodynamics,which will allow for a direct comparison with the phenomenological equations.Besides the phenomenological derivation of continuum equations, other ideas have emergedover the last ten years for applications to out-of-equilibrium systems. These rely on the useof large deviation theory to study fluctuations in macroscopic systems. This line of researchhas proved to be very fruitful. For instance, among the few general results valid out-of-equilibriumstands the fluctuation theorems, which can be read as symmetries of large deviation functionsand have given birth to practical tools such as the Jarzynski equality.As far as applications are concerned, large deviation theory has first been probed in idealized1d out-of-equilibrium lattice gases. These models have served as framework to test new ideas,in particular for dynamical phase transitions, which have indeed been observed in certaincases but are still poorly understood. These ideas have recently started to be exported to thestudy of the glass transition, with promising preliminary results. Biological systems and ActiveMatter should provide a perfect field to apply these concepts due to their inherent out-of - and indeedoften far-from - equilibrium nature. However, large deviation theory has mostly been limited to thestudy of single molecule experiments where Jarzynski equality is used to evaluate free energydifferences. For collective behaviour of biological agents, theoretical results are scarce mainly due to thedifficulty of analytic solution as soon as the complexity of the model increases, and to the lack,until recently, of efficient numerical methods. The second part of my proposal aims at exploring thepotential of large deviation theory in the study of Active Matter, which could open the way to moresystematic use of the large deviation theory in biophysics and bridge the gap between the two communities.
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