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Colloquium: November 12, 2014

Charles Reichhardt*
Jamming and Clogging Transitions in
Driven Systems in the Presence of Obstacle Arrays
Day Time Place
November 12, 2014 Wednesday 15:40 EE01

Host: Giovanni Volpe and Miguel Navascues

ABSTRACT —  When a flowing assembly of particles goes from a liquid-like state to solid non-flowing state, the system is said to be jammed.  There is growing evidence that the onset of jamming has properties similar to those of a second order phase transition.  One of the hopes of the field is that a wide class of systems that exhibit jamming behavior could all have essentially the same type of transition.  Here we consider a binary assembly of disks in a 2D system where in the absence of obstacles there is a well defined jamming transition at point J.  We apply a uniform drive to the system and define a jammed or clogged state to occur when the system can no longer flow.  For disk densities greater than point J the system can be effectively  pinned by one obstacle. Below point J the system can reach a jammed or clogged state for a critical number of obstacles, where the number of obstacles needed to jam the system increases for lower densities.  Near point J the system is homogeneous when it jams, while for densities well below point J the system forms a highly heterogeneous or phase separated  state in which regions with a density near point J coexist with regions with zero density.  Additionally we find that the system exhibits a memory effect in which, once it jams in one direction, it is less likely to jam in another direction.  Based on these simulations we propose that jamming and clogging can be characterized as having distinct behaviours.  The jammed states are uniform states that have no memory of the previous driving and are controlled by the criticality of point J.  Clogging is the formation of fragile heterogeneous  states which retain a memory of the direction of drive and arise through a coarsening process.   Finally we also consider jamming and clogging of active matter or self-driven particle assemblies moving through random disorder. We show that as a function of increasing activity it is possible to go form the zero activity heterogeneous clogged state, to a flowing unjammed active liquid, to a high activeity partially jammed state. The active matter jammed state is associated with motion occurring in the form of avalanches with a broad size distribution, indicating that the active jammed state is also a critical state.

*Los Alamos National Laboratory, New Mexico, USA

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