Jamming is the physical process by which the viscosity of some mesoscopic materials, such as granular materials, glasses, foams, polymers, emulsions, and other complex fluids, increases with increasing particle density. The jamming transition has been proposed as a new type of phase transition, with similarities to a glass transition but very different from the formation of crystalline solids.
While a glass transition occurs when the liquid state is cooled, the jamming transition happens when density is increased. This crowding of the constituent particles prevents them from exploring phase space, making the aggregate material behave as a solid. The system may be able to unjam if volume fraction is decreased, or external stresses are applied such that they exceed the yield stress. This transition is interesting because it is highly nonlinear with respect to volume fraction.
The jamming phase diagram relates the jamming transition to inverse density, stress and temperature.
The density at which systems jam is determined by many factors, including the shape of their components, the deformability of the particles, frictional interparticle forces, and the degree of dispersity of the system. The overall shape of the jamming manifold may depend on the particular system. For example, a particularly interesting feature of the jamming transition is the difference between attractive and repulsive systems. Whether the jamming surface diverges for high enough densities or low temperatures is uncertain.
Simulations of jammed systems study particle configurations leading to jamming in both static systems and systems under shear. Under shear stress, average cluster size may diverge after a finite amount of strain, leading to a jammed state. A particle configuration may exist in a jammed state with a stress required to “break” the force chains causing the jam.
A static sand pile is jammed under the force of gravity and no energy is being dissipated. Systems which are consuming energy are also sometimes described as being jammed. An example is traffic jams, where due to jamming the average velocity of cars on a road may drop sharply. Here the cars on a road may be thought of as a like a granular material or a non-newtonian fluid that is being pumped through a tube. There under certain conditions the effective viscosity may rapidly increase, dramatically increasing the granular material or fluids's resistance to flowing and so causing the velocity to drop or even come to a complete stop. In this analogy the cars are like the grains in a granular material and if they are dense enough (i.e., closely enough spaced along the road) then interactions between the cars (as they must avoid each other to avoid crashing) cause jamming. A simple model of this behavior is the Nagel-Schreckenberg model.