Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K56: Jamming and Glassy Behavior 
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Sponsoring Units: GSNP Chair: Andrea Liu, University of Pennsylvania Room: BCEC 255 
Wednesday, March 6, 2019 8:00AM  8:12AM 
K56.00001: Isostatic jamming of twodimensional monodisperse system under quenched disorder Wen Zhang, Shiyun Zhang, Ning Xu The existence of amorphous packings in twodimensional monodisperse system is a classical unsolved problem. By minimizing the enthalpy of packings of frictionless particles, we obtain isostatic jammed packings under quenched disorder at desired pressures. Quenched disorder is introduced by pinning a fraction of particles N_{f}=n_{f}N. The result shows that there is a unique critical pinning density, n_{fc}, at which packings become completely disorder. The static correlation length ξ_{6} diverges at the same n_{fc} for different pressure p, which suggests new ways to investigate the jamming transition in twodimensional monodisperse system. Finally, we confirm the existence of isostatic jammed packings with packing fraction Φ_{J∞}=0.845, and jamming scaling is still satisfied in twodimensional monodisperse system. 
Wednesday, March 6, 2019 8:12AM  8:24AM 
K56.00002: Spatiotemporal correlations of local relaxation rates in glassy systems Rajib Pandit, Elijah Flenner, Horacio Castillo

Wednesday, March 6, 2019 8:24AM  8:36AM 
K56.00003: Machinelearned structure/dynamics relation in sheared jammed packings Sean Ridout, Jason W Rocks, Andrea Liu In disordered systems, using local structure to identify which particles are likely to rearrange under thermal fluctuations or applied load has been a longstanding challenge. Recently, machine learning has been used to construct a local structural variable, "softness", that is highly predictive of rearrangements in several disordered systems. Here we describe modifications made in the analysis that simplify interpretation and raise training accuracy for athermal packings of soft spheres under quasistatic shear. We obtain a "softness" that is highly predictive of the rearrangements at the onset of instabilities. Furthermore, we show that for jammed Hertzian packings, softness can be represented simply in terms of gaps and contacts between neighboring particles. We show how this picture depends on pressure above jamming and spatial dimension. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K56.00004: Contact numbers and radial distributions in suspensions of smooth and rough colloids Shravan Pradeep, Lilian Hsiao The contact number distribution for hard sphere suspensions is a measure of nearest neighbor interactions between colloidal particles. Many experimental studies in the literature uses the primary minima of the radial distribution function, g(r), to estimate the contact number distribution. Our experimental data show that this method overestimates the mean contact number, <z>. Instead, we investigate a different method to estimate the contact distance by extrapolating contact values of different volume fractions (f) to jamming point at random close packing (RCP). For a 3D suspension of smooth spheres, f_{RCP} ~ 0.64 with <z> ~ 6. For rough colloids, the added friction is expected to generate a reduced f_{RCP} and <z>. We hypothesize that these values of <z> at RCP could be extrapolated to better understand particle contacts at lower values of f. To test this hypothesis, we synthesized sterically stabilized, fluorescent poly(methyl methacrylate) colloids with smooth and rough surface morphologies. Confocal microscopy imaging and processing shows that the g(r) of the suspensions agree well with the OrsteinZernicke equation for hard spheres. Depending on the stabilizer length and roughness, we found that the contact distance ranges between 1 to 1.1 times the particle diameter. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K56.00005: A realspace renormalization group for jamming Abe Clark Jamming occurs in grains, emulsions, dense suspensions, and other amorphous, particlebased systems. As the packing fraction φ, defined as the ratio of particle volume to system volume, is increased past a critical value φ_{c}, a liquidsolid phase transition occurs, and grains are no longer able to rearrange. Previous studies have shown evidence of spatial correlations, corresponding to collective rearrangements, that diverge near φ ≈ φ_{c}, but there has been no explicit spatial renormalization group scheme that has captured this transition. Here, I present a candidate for such a scheme, using a simple blockspinlike transformation of a randomly vacated lattice of grains. I define a realspace renormalization transformation that coarsegrains the lattice based on local mechanical stability. This model displays a critical packing fraction φ_{c} and estimates of critical exponents in 2D and 3D that are consistent with previous work. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K56.00006: Marginality gap in MariKurchan meanfield model for jammed packings Yue Li, Eric Corwin, Andrea Liu A marginal system is one for which there exists an infinitesimal deformation that will lead to instability. Frictionless sphere packings at jamming, as well as infinite packings above jamming, seem to meet this definition. Further, the scaling of the excess contact number with pressure is consistent with the meanfield expectation of marginal stability. However, the prefactor in threedimensional systems is slightly larger than predicted. This discrepancy in the prefactor, termed the “marginality gap”, is expected to vanish in the meanfield limit. We investigate numerically the MariKurchan model of jammed packings, in which a Gaussian random shift is added to each separation of pairs of particles in the pair potential. By tuning the amplitude of the random shift, we study the range from jammed packings to the meanfield limit to see how the marginality gap evolves. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K56.00007: Jamming as a Multicritical Point Danilo Liarte, Xiaoming Mao, Olaf Stenull, Tom Carl Lubensky The discontinuous jump in the bulk modulus B at the jamming transition is a consequence of the formation of a critical contact network of spheres that resists compression. We introduce lattice models with underlying undercoordinated compressionresistant spring lattices to which nextnearestneighbor springs can be added. In these models, the jamming transition emerges as a kind of multicritical point terminating a line of rigiditypercolation transitions. Tuning the undercoordinated lattice to the jamming critical point yields a faithful description of jamming and its relation to rigidity percolation. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K56.00008: Can a large jammed packing be assembled from smaller ones ? Daniel Hexner, Pierfrancesco Urbani, Francesco Zamponi The principle of equivalence of ensembles asserts that, in the thermodynamic limit, a system with periodic boundary conditions behaves identically to a subsystem of the same size, cut out from a larger system. We compare these two ensembles on finite length scales in the case of amorphous jammed packings of soft spheres at zero temperature. Focusing on the statistics of the contact fluctuations, we find that systems with periodic boundary conditions have significantly smaller fluctuations compared to the subsystems. This difference is largest near the jamming transition. Moreover, these two ensembles converge only at a surprisingly large system size. The crossover to the thermodynamic limit defines a length scale for each ensemble. Surprisingly, these diverge, as a function of the distance to the transition, with two different exponents. We argue that this disparity is the result of the system being above the upper critical dimension and that, based on the values of the exponents, the upper critical dimension can be measured. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K56.00009: Response to controlled perturbations in frictional granular jamming Mahesh Bandi Jamming in frictional granular media results in metastable configurations due to stability imparted by frictional contacts against sliding. We experimentally study the frictional energy differene (ΔE) between an unperturbed and a perturbed configuration subject to uniaxial compression under identical conditions in a twodimensional system comprised of a bidispersed set of disks whose friction we tune through contact roughness d. The homogeneous systemwide acoustic perturbations are independently tuned with amplitude A and frequency f. We find the frictional stress σ released from perturbation follows a stretched exponential form σ = σ_{0 }exp[(ΔE/T_{eff})^{β}], where σ_{0} is the unperturbed stress, T_{eff} = (1/2)M (A_{RMS} f_{RMS})^{2}, M is total mass of disks in the configuration, and A_{RMS} and f_{RMS} are the respective RMS perturbation amplitude and frequency; the stretched exponent β is the only fit parameter. At low T_{eff}, we obtain a best fit around β ~ 1/3. As T_{eff} increases and more frictional stress is relieved, the stretched exponent β transitions smoothly and approaches an asymptotic value of β = 1 with an Activated or Arrheniuslike relaxation behavior. Eventually when A_{RMS} ≥ d, all frictional stresses in the system are relieved and frictionless jamming behavior is recovered. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K56.00010: Aging is a (log)Poisson Process, not a Renewal Process^{1} Stefan Boettcher, Dominic M Robe, Paolo Sibani Aging is a ubiquitous relaxation dynamic in disordered materials. It ensues after a rapid quench from an equilibrium ``fluid'' state into a nonequilibrium, historydependent jammed state. We propose a physically motivated description that contrasts sharply with the trap model^{2} or a continuoustime random walk (CTRW) with broadly distributed trapping times commonly used to fit aging data.^{3} A renewal process like CTRW proves irreconcilable with the logPoisson statistic exhibited, for example, by jammed colloids as well as by disordered magnets. A logPoisson process is characteristic of the intermittent and decelerating dynamics of jammed matter usually activated by recordbreaking fluctuations (``quakes''). We show that such a record dynamics (RD) provides a universal model for aging, physically grounded in generic features of freeenergy landscapes of disordered systems.^{4} 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K56.00011: Direct Measurement of Force Configuration Entropy in Jamming James Sartor, Eric Corwin Thermodynamics connects the microscopic details of a system’s entropy to bulk measurements of the system’s properties. In granular systems, for which the thermal energy scale is so small as to be irrelevant, this has been proposed using temperature analogues such as compactivity and angoricity. We present a method of linking the measurements of such quantities to the entropy of the force network by measuring the multiplicity directly. For systems at the critical jamming point there is only one mechanically equilibrium force network compatible with the spring network representation of the system, so the force configurational entropy of a jammed system is zero. For each new contact formed, the dimensionality of the space of allowed force configurations increases by one. Within this space lies a subspace of positivedefinite forces, which is compatible with a granular packing. We propose that the volume of this subspace is proportional to the multiplicity of the packing’s force network configuration. To determine the constant of proportionality, we measure the angoricity over 6 decades of pressure using the method of overlapping histograms. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K56.00012: Void Percolation Threshold and Critical Proporties of the Random Lorentz Gas Patrick Charbonneau, Eric Corwin, Yi Hu Percolation and glass formation share interesting dynamical features, in which they both exhibit caging of tracers/particles. Although first noticed long ago, this analogy has grown in physical relevance since a simplified model of structural glasses (MariKurchan) was found to display corrections to caging that are analogous to those observed in percolating systems. Interestingly, our recent study has shown that the glasslike caging transition is absent in lattice percolation for all dimensions. In order to better understand the origin of caging, we consider transport in an offlattice percolation model. Specifically, we study the caging and critical scaling of transport in the random Lorentz gas, which can be mapped onto the MariKurchan model and to void percolation. We first develop numerical strategies to determine precisely the void percolation threshold in high dimensions, and then study dynamical criticality around that threshold. Our results provide key insights into the dynamics of glass formers and transport in heterogenous media, more generally. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K56.00013: Fluctuation Distributions of Energy Minima in Complex Landscapes HorstHolger Boltz, Andrea Liu, Jorge Kurchan We discuss the properties of the distributions of energies of minima obtained by gradient descent in complex energy landscapes.Specifically, we study the distribution of energies of minima in the spherical pspin model and the distribution of jamming threshold packing fractions in jammed particle configurations as archetypal manifestations of disorderinduced complexity. We numerically find universal distributions that resemble the TracyWidom distributions often found in problems of random correlated variables, and nontrivial finitesize scaling. Deeper insight into this problem is achieved by realizing the importance of a firstpassage process in the eigenvalues of the Hessian to the termination of the steepest descent process, which also manifests the link to problems where the TracyWidom distribution is established. This firstpassage view of steepest descent dynamics is generic and therefore we expect similar phenomenology in many problems. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K56.00014: Shear response of jammed disk and sphere packings Kyle VanderWerf, Mark Shattuck, Corey Shane O'Hern The response of purely repulsive disk and sphere packings to athermal, quasistatic simple shear near jamming onset is highly nonlinear. Previous studies have shown that the ensembleaveraged static shear modulus G is nearly constant at small pressure p, and at a characteristic pressure p*, G begins to increase as a powerlaw: G ~ p^{α}, where α=0.5. Also, p* decreases with increasing system size N, such that p* ~ N^{β}, where β=1. Although scaling arguments have rationalized the scaling behavior of p* and G, there is currently no quantitative theoretical framework that can predict the values of α and β. Here, we carry out numerical simulations of 2D bidisperse disk packings near jamming onset undergoing athermal, quasistatic simple shear at fixed pressure to explain these exponents. We show that α and β can be understood by examining the "geometrical families" of jammed packings, which are intervals of shear or pressure where the packings maintain the same network of interparticle contacts without rearrangements. We present a statistical model based on random switching of the packings from one geometrical family to another to predict the values of the exponents α and β. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K56.00015: Densest vs. jammed packings of 2D bentcore trimers Austin Griffith, Robert Hoy We identify the maximally dense lattice packings of tangentdisk trimers with fixed bond angles (θ = θ_{0}) and contrast them to both their nonmaximallydensebutstrictlyjammed lattice packings as well as the disordered jammed states they form for a range of compression protocols. While only θ_{0} = 0, 60°, and 120° trimers can form the triangular lattice, maximallydense maximallysymmetric packings for all θ_{0} fall into just two categories distinguished by their bond topologies: halfelongatedtriangular for 0 < θ_{0} < 60° and elongatedsnubsquare for 60° < θ_{0} < 120°. The presence of degenerate, lowersymmetry versions of these densest packings combined with several incommensurable families of lessdensebutstrictlyjammed lattice packings act in concert to promote jamming. Systems jam via a twostage, twolengthscale process. First, randomlyoriented crystalline grains form and grow to a size that increases with decreasing compression rate and depends strongly on θ_{0}. Since these grains cannot be compressed further, they effectively behave as single nearlyrigid particles as compression continues. Jamming occurs when they can no longer rotate/translate away from one another upon colliding. 
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