Active Jammed Systems
May 26-28 2012, New York City, USA

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Conference Organizers:
Jasna Brujic
Paul Chaikin
Matthieu Wyart

Photo Courtesy of Spirit Buffalo Pictures ©2012

The aim of this workshop is to bring together a diverse set of scientists who are doing cutting edge work on crowded dynamical systems. These include hard or deformable particles that interact at high volume fraction, such that the effects of friction and viscosity can be hard to separate and both the hydrodynamics and particle flow are potentially important. The non-equilibrium driving of these systems is either externally applied or internally generated by active living or artificial sources. Such complex behavior is relevant to a diverse set of problems of fundamental, biological and industrial interest, including flow of granular materials and suspensions, plastic deformation of amorphous solids, elasticity of gels of semi-flexible polymers and tissues, cellular motility and morphogenesis.

Photo Taken By Paul Chaikin

Over the past decade there has been considerable progress in characterizing some aspects of these systems. Granular and emulsion flow were shown to be governed by constitutive relations and a characteristic length scale valid in the infinitely rigid limit. On the other hand, when particles are deformable it was found that the elastic load is released by local plastic events that coarse grain into larger and larger avalanches as the stress decreases towards its yield stress value. Furthermore, in elastic networks where interactions with different energy scales are competing, such as the bending and stretching of semi-flexible polymers, the elastic response can be highly heterogeneous, non-linear and correlated on large scales. In all these examples the material behavior appears to be controlled by the jamming transition, when fluids cease flowing and when solids under stress yield to movement, or when stretching bonds form a rigid cluster. Near this point the motion of the particles becomes increasingly heterogeneous, its correlation length diverges, and the rheology (or the elasticity) displays scaling. The effects associated with jamming are expected to play an important role in internally active systems as well, such as dense bacterial and artificial swimmers, actin filament networks in the cell or the packing of cells and their migration in tissue morphogenesis. The aim of this workshop is to bring together seemingly disparate communities to compare physical and biological systems that exhibit aspects of jamming. This interdisciplinary approach will unravel which aspects of their resemblance are fortuitous and which aspects can be unified in a common framework.