Jamming & Yielding

jamming diagram

 

Shaving cream and mayonnaise flow easily when smeared, but carry their own weight like a solid when sticking to chin or sandwich. How does this mixture of solid and fluid-like behavior come about? Recall that a foam or emulsion is a dispersion of easily deformable gas bubbles or liquid droplets. When packed above the close-packing density, the bubbles or droplets deform and adjust their positions and shapes in order to minimize the total interfacial area; the resulting packing configuration lies in some local energy minimum. The thermal energy is many orders smaller than the characteristic energy barrier height, so that bubbles cannot spontaneously rearrange and explore the phase space: the system is athermal and jammed. Under small external stresses, the bubbles deform, increasing their surface energy and providing a restoring force, while under stronger shear, the barriers are overcome and the material yields: this is an example of a shear induced unjamming transition. Another transition can be expected to occur when the density of the dispersed phase is decreased below closed packing so that the bubbles are able to flow past each other unhindered. The fact that systems as diverse as emulsions and foams but also sand piles and supercooled liquids all “jam” for sufficiently high packing density, small shear and low temperature, led to the suggestion of a so-called jamming diagram (Fig. 1a) [1], inspiring an explosion of activity. Central questions are: Is the jamming transition sharp, well-defined, critical? Do different systems that jam along various trajectories, exhibit universal properties?

 

References [1] Jamming is not just cool anymore.