Getting into a Proper Jam

The study of jammed systems began as a culinary curiosity in 1727, when the Reverend Stephen Hales studied how peas pack when compressed in an iron pot [1]. Fill a pot with peas and you can run your hand through them, because they can flow out of the way much like a liquid would. But as pressure, and thus the density, is increased, you will find that there is a critical point, above which the peas “jam” into a stable amorphous solid. This behavior is very general. Pretty much everything composed of discrete chunks large enough that thermal fluctuations can be ignored can go through a jamming transition: colloids in solution, a pile of sand, a jar full of candies, even cars in a traffic jam. Since the work of Hales, and at an accelerating pace since the late 1950s, jamming has emerged as a fascinating research topic and as a paradigm for studying the formation of amorphous solids, in particular, glasses [2]. Many experimental techniques have been applied to the study of jammed states. However, experimentally, it is extremely difficult to access the complex dynamics of systems very near the jamming transition. Instead, researchers increasingly rely on numerical simulations, which allow them to explore situations extraordinarily close to the jamming threshold. Unfortunately, simulations are limited to a finite (and small) number of particles. One kilogram of beach sand can easily contain 30 billion individual grains, far beyond the capabilities of even the largest computing schemes. As a consequence of such experimental and computational hurdles, a general theory for the description of jammed systems is still lacking. By simulating the jamming behavior of finite-size systems, two papers in Physical Review Letters come to general conclusions on the nature of jamming transitions that may help shape a unified theory of jamming. Simon Dagois-Bohy, at Leiden University in the Netherlands, and co-workers expose the pitfalls of interpreting simulations in finite-size systems, showing that properly jammed systems must be stable not only to compression but also to shear [3]. In an independent but related paper, Carl Goodrich, at the University of Pennsylvania, Philadelphia, and co-workers investigate the scaling behavior of finite-size systems jammed under varying constraints and provide strong evidence that the jamming transition can in fact be considered a phase transition [4].