"SPECIAL BIOPHYSICS SEMINAR: Self-Organization, Fluctuation Forces and Transport of Active Matter On Disordered Landscapes"

 

 

ABSTRACT:

 

There are many examples of nonequilibrium systems which exhibit collective transport when the system is externally driven over a rough landscape.  Examples of such systems include driven vortices in type-II superconductors, Wigner crystal transport, sliding charge density waves and moving domain walls.  There is another class of systems in which the individual units composing the system are self driven rather than externally driven.  Such systems fall into the class of what is termed active matter [1] and include swimming bacteria, schools of fish, animal herds, and pedestrian and vehicle transport.  Recently, a number of different types of artificial active matter particle systems have been created, such as by using photoactivated colloidal particles that can form what are called living crystals [2]. 

 

Here we examine a simple model of run and tumble particles with repulsive interactions and show that this system can organize into a living crystal state as a function of run length or density. When objects or walls are immersed in a bath of such active matter particles, they experience forces that are much larger than thermal fluctuations, including entropic attractive forces.  We also set up an active matter Casimir type geometry to show that two plates in an active matter bath experience a strong attraction to each other.  For random obstacles we find non-monotonic behavior for the transport as a function of run length, suggesting that biological systems such as run and tumble bacteria may select their running lengths to optimise transport in disordered or crowded environments.

[1] S. Ramaswarmy, "The mechanics and statistics of active matter," Ann. Rev. Conden. Mat. Phys. 1, 323 (2011)
[2] J. Palacci et al, "Living crystals of light-actived colloidal surfers," Science 22, 936 (2013).