Research
IRG I
IRG 1. Jamming and Slow Relaxation in Materials Far from Equilibrium
Faculty (* = coordinators): P. Constantin, A. Dinner*, M. Gardel, E. Isaacs, H. Jaeger*, B. Lin, S. Nagel, S. Rice, T. Rosenbaum, N. Scherer, T. Witten
Large classes of materials obtain their characteristic properties from being driven or trapped far from equilibrium. This includes materials as industrially relevant and diverse as macroscopic granular media, dense colloidal suspensions, and liquids or polymers quenched into a solid-like phase. Metallic alloy glasses, for example, are among the strongest materials available; yet, unable to relax into a crystalline state, their amorphous structure still resembles the original liquid.
Similarly, granular material flowing through a pipe or out of an orifice can easily jam into a state that is rigid and load-bearing, even though the material is unable to reach an optimized, regular structure and becomes quenched into a disordered, heterogeneous configuration with a large fraction of voids. Predicting and controlling the properties of these amorphous materials involves new terrain for materials science. Indeed, the recent NAS CMMP2010 report identified progress in understanding far-from-equilibrium behavior, and the associated phenomenon of jamming, as one of six grand challenges in condensed matter and materials physics for the next decade. This IRG addresses this challenge by investigating the fundamental mechanisms that control when and how amorphous systems cease to flow or develop rigidity far from equilibrium.
The Chicago MRSEC has been one of the pioneers in developing a generally applicable approach to this problem, based on the concept of a jamming phase diagram. At the jamming transition, the relevant constituent units of a material, which can be particles like grains of sand or pharmaceutical powders but also colloids, magnetic domains or molecules, become jammed in the sense that they collectively prevent each other from moving and can no longer explore phase space efficiently. Concomitant with this rigidity onset, other manifestations of jamming include anomalously slow relaxation and memory effects, like those seen in spin glasses. However, because jamming dynamically self-generates disordered, amorphous configurations it does not require an underlying free-energy landscape with fixed, static disorder as found in electronic glassy systems such as spin, vortex or magnetic glasses.
Over the last few years the jamming concept has been applied with much success to explain experiments on the rheology of dense colloidal suspensions where the type and range of the interactions can be tuned and thus the boundaries of the jammed region can be varied. On the theory side, connections to established models known to produce glassy behavior, including kinetically-constrained lattice models and single-particle barrier-hopping theory, are now emerging. Concepts based on jamming are also beginning to help with the understanding of mechanical behavior seen in biological cells.
The concept of jamming can thus be used as a unifying umbrella to cross-pollinate ideas about behavior of systems far from equilibrium at the macro-, meso-, and microscopic levels. Success in this endeavor will mean that advanced notions introduced to describe jamming in granular materials or colloids can be transferred to molecular glasses and vice versa, and that information about rigidity onset at all scales will be used to design and control complex materials.
Beyond the obvious theoretical hurdles, tracking the local evolution of jammed or glassy systems with high spatial and temporal resolution remains a major experimental challenge. This is true not only for molecular glasses but also for three-dimensional (3D) granular assemblies since they are optically opaque. This IRG applies cutting-edge techniques to probe time-dependent local dynamics across a wide range of systems. X-ray photon correlation spectroscopy will track relaxation of individual magnetic domains, fluorescence resonant energy transfer will signal folding events in large bio-polymers, high-speed confocal microscopy will reconstruct local configurations inside dense emulsions, and time-resolved X-ray tomography will track each “monomer” in a macroscopic granular “polymer melt”.
Our goal in this IRG is to elucidate and exploit common structural and dynamic signatures of jamming. We plan to (i) identify structural features associated with jamming, (ii) characterize anomalously slow dynamics arising either at the onset of rigidity or deep within the jammed state, and (iii) explore novel design principles to create materials with unique properties based on jamming characteristics. An interdisciplinary team is essential for this ambitious program. This IRG thus brings together experimentalists with expertise in biophysics (Gardel, Scherer), colloids (Lin, Rice), granular materials (Jaeger, Nagel), low-temperature systems (Rosenbaum), and X-ray scattering (Isaacs) who closely collaborate with theorists from mathematics (Constantin), biophysical chemistry (Dinner), and soft-matter physics (Witten).
Summary: Materials driven or trapped in the vicinity of a jamming or glass transition are in a far-from-equilibrium state that is perched at the threshold between a liquid and a solid: very subtle structural changes control striking dynamic changes, turning systems that flow like liquids into materials that are as rigid as a solid. Understanding and controlling this behavior is a grand challenge for the next decade. With the jamming phase diagram, the Chicago MRSEC has introduced a new concept connecting the dynamics of glassy and slowly-relaxing microscopic systems with that of meso- and macroscopic systems, such as colloids and granular materials. This IRG explores a wide range of systems where the signatures of jamming can be tracked experimentally with precision to relate them to local changes. It breaks new ground in advancing the fundamental understanding of the interplay of structure and dynamics at the onset of rigidity and by developing the tools for the design of unique material properties based on this interplay.