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.

see also:

• story at BrandeisNOW
• Bi D, Zhang J, Chakraborty B, Behringer RP. Jamming by shear. Nature. 2011;480(7377):355-8.