Workshop proposal for Aspen Center for Physics Summer 2007 Program Title: Jamming Organizers: Andrea J. Liu Narayanan Menon [email protected] [email protected] (215) 573-7374 (office) (413) 545-0852 (office) (215) 898-2010 (fax) (413) 545-0648 (fax) Dept. of Physics & Astronomy Dept. of Physics University of Pennsylvania University of Massachusetts Amherst Eric R. Weeks (contact person) [email protected] 404-727-4479 (office) 404-727-0873 (fax) Dept. of Physics Emory University N. Menon is the organizer responsible for working to ensure diversity in the applicants. Description and justification: All around us things seem to be getting jammed. Before breakfast, coffee grounds and cereal jam as they refuse to flow into our filters and bowls. On the way to work, we are caught in traffic jams. In factories, the raw materials, powders, get jammed as they clog in the conduits that were designed to have them flow smoothly from one side of the factory floor to the other. Our recourse in all these situations is to pound on our containers, dashboards and conduits until the jam miraculously disappears. We are usually so irritated by the jam that we have not really noticed that the approach to jamming and the jammed state, in all of these situations, have common properties and similar behaviors that are quite different from those in systems near the liquid-solid transition. More precisely, jamming occurs when a system with no quenched disorder changes from an ergodic and mobile state to an immobile and solid-like yet disordered state. Jamming transitions can be induced in a variety of systems by tuning different control parameters. Examples include supercooled liquids 1 that become glasses as temperature is lowered below the glass transition tem- perature, flowing granular materials that become static granular packings as the shear stress is decreased below the yield stress, and colloidal suspensions that become colloidal glasses as the density is increased near random close packing. These systems display common behavior near the jamming tran- sition, such as extremely long relaxation times, cooperative dynamics with large spatial correlations, lack of obvious structural changes at the transition, and locally anisotropic and nonlinear responses to perturbations. Despite these similarities, it is still an open question whether a unified description of jamming in these diverse systems is possible. However, even lacking a unified description, the cross-fertilization of these different fields has been extremely productive in the last few years. For example, in the past three years the APS March Meeting has held 13 contributed sessions on jamming and several invited symposia. These sessions draw speakers who study traditional glass-forming systems, granular materials, gelation and aggregation processes, and soft materials such as colloids and foams. The aim of our proposed workshop is to enable in-depth discussion. While there have been conferences on jamming, glasses, and granular materials, there has not been an in-depth working workshop since the very first one at the KITP in Fall 1997, where the field was first defined. Since the 1997 KITP workshop, substantial progress has been made. In that workshop, intriguing but superficial phenomenological similarities be- tween different systems led to the proposal that those systems might be related, and that it might be useful to study all these systems within a broader context, namely jamming. Shortly thereafter a \phase diagram" was proposed to organize on a common footing data from systems that can be unjammed in different ways; see Fig. 1. Now, in 2006, this phase diagram has been refined and tested in a variety of systems, both experimentally and computationally, thus showing that it is indeed a useful way to organize data. More specific similarities between various jamming transitions have been identified in simulations of different systems (hard spheres, foams, Lennard-Jones glasses, etc.) and also in ex- periments (colloids, glasses, gels, etc.) A major theoretical success involves the identification of \Point J" in the phase diagram, where the jamming transition occurs at zero temperature and zero shear stress. Several system quantities scale as power laws as Point J is approached (such as the shear modulus and a diverging length scale) and the exponents are identical for several different systems. There is an analogy with \k-core" or bootstrap percolation, which exhibits the same exponents in the mean-field approximation. Furthermore, the same scaling exponents 2 Figure 1: The axes of the jamming phase diagram are temperature T , inverse density 1/φ, and shear stress Σ. Jammed states lie close to the origin. For example, the T - 1/φ plane represents the molecular glass transition, induced by lowering T or increasing the pressure (thus decreasing 1/φ). (From O'Hern et al., Phys. Rev. E 68 011306 (2003).) are seen in mean-field for the p-spin interaction spin glass and other models that have been proposed for the glass transition. This is the first quantitative sign that jamming may indeed be a universal phenomenon. Despite these successes, there are many open questions our proposed workshop will address: • The similarity of exponents in different jamming systems has so far been seen mainly at the mean-field level. What happens in finite dimensions? • Point J is at zero temperature, yet many experiments are done at finite temperature. Can the understanding of Point J behavior be extended to jamming at finite temperatures? • Can the understanding of Point J behavior be extended to jamming in a dissipative system? What if the dissipation is history-dependent, e.g. static friction? • Is jamming the same for repulsive particles (granular media, colloidal glasses) and attractive particles (network glasses, gelation)? • Is jamming the same for spheres, ellipsoids, and polymers? To what extent does particle shape matter? • What is the role of hysteresis near the jamming transition? • The shear stress axis in the jamming phase diagram is a nonequilibrium axis, because an unjammed system flows at nonzero shear stress. To what extent does shear stress give rise to an effective temperature? • In many systems, when the externally applied shear stress is increased, the strain is localized to a narrow region known as a shear band. Why does this stress-focusing occur and what determines the shear profile chosen by the system? • To what extent is aging similar in the different jamming systems? 3 Schedule: We propose a three-week workshop, with one or more focus themes (based on the questions above) for each week. Each theme will be chosen to span across different systems and to build upon both experimental and theoretical work. Preferred times for the workshop are any three weeks between July 23 and August 24. Our second choice preference is between May 28 and July 6. It is important to avoid July 9-13 as that is when the STATPHYS conference will be held in Italy, and we'd like to avoid the week after as well. Participants: Our email announcement of our proposal has resulted in many enthusiastic responses. The following 58 people have expressed strong interest in attending: Robin Ball (Warwick) Andrea Liu (U Pennsylvania) Bob Behringer (Duke) Wolfgang Losert (U Maryland) Giulio Biroli (CEA Saclay) Tom Lubensky (U Pennsylvania) Raphael Blumenfeld (Cambridge) Satya Majumdar (University-Paris Sud) Horacio Castillo (Ohio University) Craig Maloney (UC Santa Barbara) Bulbul Chakraborty (Brandeis) Narayanan Menon (U Mass. Amherst) Lincoln Chayes (UCLA) Cristian Moukarzel (Unidad M´erida) Luca Cipelletti (Montpellier) Sid Nagel (U Chicago) Itai Cohen (Cornell) Corey O'Hern (Yale) Sue Coppersmith (Wisconsin) Cynthia Olson-Reichhardt (LANL) Leticia Cugliandolo (ENS-Paris) David Pine (NYU) Herman Cummins (CCNY) Wilson Poon (Edinburgh) Karen Daniels (NC State U) Charles Reichhardt (LANL) Kenneth Dawson (U College Dublin) Sri Sastry (JNCASR, Bangalore, India) John de Bruyn (U Western Ontario) Peter Schiffer (Penn State) Michael Dennin (UC Irvine) Jennifer Schwarz (Syracuse) Doug Durian (U Pennsylvania) Francis Starr (Wesleyan) Phil Duxbury (Michigan State) Robin Stinchcombe (Oxford) Deniz Ertas (ExxonMobil) Harry Swinney (U Texas Austin) Michael Falk (U Michigan) Salvatore Torquato (Princeton) Scott Franklin (RIT) Veronique Trappe (Fribourg) Dan Goldman (Georgia Tech) Martin van Hecke (Leiden) Jerry Gollub (Haverford) Wim van Saarloos (Leiden) Harvey Gould (Clark) Eric Weeks (Emory) Piotr Habdas (St. Joseph's U) Matthieu Wyart (Harvard) Randall Kamien (U Pennsylvania) Ryoichi Yamamoto (Kyoto) Laura Kaufman (Columbia) Maria Kilfoil (McGill) Each organizer has read and approved Walter Kob (Montpellier) Arshad Kudrolli (Clark) this proposal: Jorge Kurchan (ESPCI-Paris) Andrea J. Liu, Narayanan Menon, Bob Leheny (Johns Hopkins) Eric R. Weeks 4.