Shear-induced jamming

From breakfast cereals to sand on a beach, granular materials are all around us. Under different conditions, these materials can exhibit liquid-like behavior (flowing) as well as solid-like behavior. The transition between solid and liquid phases has been known as the jamming transition.

The basic concept of jamming is pretty intuitive. A simple example of what can induce jamming is the following: compacting loose sand inside a container increases its density. When the container is removed, the sand can form a self-supporting pile, hence becoming jammed. Jamming has been studied extensively in numerical simulations of systems composed of idealized grains without frictional forces.  These studies find a critical density at which jamming occurs. Since these idealized granular materials are non-cohesive (no attractive forces between them)  they can become solids only through externally imposed pressure, such as through compaction, and therefore a critical density makes sense.  Real granular materials, however, have friction, and how this affects jamming is not well understood.

An experimental image of typical Shear Jammed state in a 2-D frictional granular material. The shear strain is applied in the horizontal direction. Red colored grains form the backbone of the system, which provides rigidity with respect to external shear

Newly published in Nature, are results of a collaboration between Prof. Bulbul Chakraborty’s group at Brandeis and Prof. Behringer’s group at Duke University, which show a new class of jammed states in frictional granular materials. This new class of “Shear-Jammed” states exhibits a richer phenomenology than previously seen. An initially unjammed or loose granular material can become jammed not just by increasing its density, but by applying shear strain on it while holding the density fixed. Shear-Jammed states are inherently anisotropic in their stress and grain-to-grain contact network (see photo above). The transition from an unjammed to shear-jammed state is clearly marked by a percolation of the strong force chains in all directions (see video below). The phenomenon of shear-jamming does not currently have a fundamental theoretical description. Ongoing work in Prof. Chakraborty’s group attempts to construct a theoretical framework for this non-equilibrium phase transition using a generalization of equilibrium statistical ensembles.

This video shows the evolution of the strong force cluster and transition from unjammed to fragile and eventually to SJ. The video shows experimental states created under pure shear. Green colored grains form the strong force cluster defined in the paper. Initially, the system is unjammed. As the fraction of force bearing grains increases with increasing strain, the strong force cluster percolates in the compressive (vertical) direction and we call the state fragile.  Eventually the system becomes percolated in all directions with sufficient number of force bearing grains. We call these states Shear Jammed.

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Complex Fluids Workshop on Sep 23

On Friday, Sep 23 2011, Brandeis will play host to the 48th New England Complex Fluids Meeting, run by the New England Complex Fluids Workgroup, of which the Brandeis Complex Fluids group is a charter participant. These quarterly meetings foster collaboration among researchers from industry and academia in the New England area studying Soft Condensed Matter, offer the opportunity to exchange ideas, and help introduce students and post-docs to the local academic and industrial research community.

The workshop, to be held in the Shapiro Campus Center, will have four talks by invited speakers, each about 30 minutes long with ample time for questions. In addition, everyone who attends is encouraged to give a five minute update (soundbite) of their current work.


9:30 AM – Krystyn Van Vliet (Materials Science and Engineering, MIT), Chemomechanics of responsive gels
10:15 AM – Jeremy England (Physics, MIT), Shape Shifting: the statistical physics of protein conformational change

Soundbites: 11:30 – 12:30 PM Five minute updates of current research

1:30 PM – Francis Starr (Physics, Wesleyan), DNA-linked Nanoparticle Assemblies
2:15 PM – Jennifer Ross (Physics, UMass Amherst), Controlling Microtubules Through Severing

More Soundbites: 3:30 PM – 4:30 PM

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