Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H48: Thermal versus Athermal PlasticityFocus

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Sponsoring Units: GSNP GSOFT Chair: Robert Hoy, University of South Florida Room: LACC 510 
Tuesday, March 6, 2018 2:30PM  3:06PM 
H48.00001: Strain Bursts and Dislocation Avalanches in ObstacleHardened Materials Invited Speaker: Nasr Ghoniem We develop a hybrid continuumdiscrete model for the collective dynamics of dislocations in dense obstacle fields. The density of obstacles is described by continuum conservation equations, and solved numerically. With this hybrid model, we unravel the mystery of how and why irradiationinduced defects enhance or inhibit strain bursts in submicron single crystals. It is shown that smaller strain burst amplitudes in irradiated nano and micropillars are obtained under stress control conditions. However, under strain control conditions, bursts are found not to be sensitive to irradiation, despite the arresting effect of radiation defects. This feature is a result of rapid stress relaxation truncating the strain burst, compared with the influence of irradiationinduced defects. The influence of dislocationdefect interaction mechanisms, cross slip, irradiation dose, as well as loading mode on strain bursts is systematically investigated, and the results compared with experimental observations. We also present results showing the relationship to spatial localization of plastic flow. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H48.00002: Precursor Dislocation Avalanches in Small Crystals: The Irreversibility Transition Xiaoyue Ni, Haolu Zhang, Danilo Liarte, Louis McFaul, Karin Dahmen, James Sethna, Julia Greer The transition from elastic to plastic deformation in crystalline metals shares both history dependence and scaleinvariant avalanche behaviors with other nonequilibrium systems under external loading. Many of these other systems, however, typically exhibit purely elastic behavior only after training through repeated cyclic loading; recent studies in these other systems show power laws and scaling of the hysteresis magnitude and training time as the peak load approaches a reversible—irreversible transition (RIT). We discover here that deformation of small crystals shares these key features. Yielding and hysteresis in uniaxial compression experiments of singlecrystal Cu nano and micropillars decay under repeated cyclic loading; the amplitude and decay time diverge as the peak stress approaches the failure stress, with power laws and scaling as seen in RITs in other nonequilibrium systems. We observe that these effects become smaller as the pillars become larger, perhaps explaining why scaleinvariant training effects have not been observed in macroscopic samples. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H48.00003: Critical scaling of temporal and spatial correlations with strain rate in athermal, disordered solids Joel Clemmer, K. Michael Salerno, Mark Robbins Athermal, disordered solids deform plastically via avalanches in the quasistatic limit of shear. These avalanches display critical behavior including powerlaw distributions. With increasing strain rate, the system moves away from the critical point causing a decrease in the correlation length of the system. We explore 2D and 3D simulations of overdamped binary LennardJones solids in the limit of finite strain rate and characterize the temporal and spatial correlations that govern avalanche plasticity. With increasing strain rate, we identify a decreasing length scale in the power spectrum of the nonaffine displacement of particles and a decreasing time scale in the power spectrum of temporal fluctuations in the average kinetic energy and stress. Using finitesize scaling, we find the critical exponents governing the scaling of correlations with rate. We will also relate these results to the scaling of other system properties with strain rate such as a sizedependent diffusion constant in 2D and the average shear stress. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H48.00004: Dynamics of point dislocations along glide lines in twodimensional colloidal crystal An Pham, Benjamin Yellen We have developed an experimental model to study the growth and healing of crystalline domains in a monolayer of paramagnetic particles exposed to a highfrequency rotating magnetic field. Here, our goal is to measure the stress/strain relationships of point dislocations that move along glide lines during the crystal healing process. These measurements are based on a combination of image processing to determine the local strain and force calculations to determine the local stress based on our prior calibration of the pairwise interaction potential. We will present measurements on both the elastic modulus and the yield stress as the point dislocation moves from the interior of the crystal towards a domain boundary. These measurements are conducted in different applied magnetic fields strengths in order to model the crystal healing properties at different temperatures. We also quantify the net torque and rotation of the domain as the point dislocation moves along a glide line, which allows the domain boundaries between neighboring crystals to heal. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H48.00005: Ability of Local Structure to Predict Particle Rearrangements in Varying Spatial Dimension Sean Ridout, François Landes, Eric Corwin, Andrea Liu In glassy systems, it is difficult to predict which particles will rearrange under thermal fluctuations or applied load using only local structural information. The most successful and least computationally intensive of the many approaches that have been developed to address this problem uses machine learning to construct a scalar function of local structure, “softness.” This particlebased quantity has been shown to be highly predictive of rearrangements in several model disordered systems in two and three dimensions. One might expect that the ability of local structure to predict rearrangements in a given model will decrease with increasing spatial dimension, e.g. due to greater homogeneity in particles' local environments. We use softsphere systems under athermal shear to study systematically the ability of softness to predict rearrangements in different spatial dimensions and test this hypothesis. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H48.00006: Thermal energy as structural indicator in glasses: From universal anomalous statistics to predicting plastic rearrangements Jacques Zylberg, Edan Lerner, Yohai BarSinai, Eran Bouchbinder Identifying heterogeneous structures in glasses  such as localized soft spots  and understanding structuredynamics relations in these systems remain major scientific challenges. Here we derive an exact expression for the local thermal energy of interacting particles in glassy systems by a systematic lowtemperature expansion. We show that the local thermal energy can attain anomalously large values, inversely related to the degree of softness of localized structures in a glass, determined by a coupling between internal stresses, anharmonicity and lowfrequency vibrational modes. These anomalously large values follow a fattailed distribution, with a universal exponent related to the recently established universal ω^{4} density of states of quasilocalized lowfrequency vibrational modes. This powerlaw tail manifests itself by highly localized spots which are significantly softer than their surroundings. These soft spots are shown to be susceptible to plastic rearrangements under external driving forces, having predictive powers that surpass those of the normalmodesbased approach. These results offer a general, system/modelindependent, physicalobservablebased approach to identify structural properties of quiescent glasses and to relate them to glassy dynamics. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H48.00007: Thermalized formulation of soft glassy rheology Robert Hoy We present a version of soft glassy rheology that includes thermalized strain degrees of freedom. It fully specifies systems' strainhistorydependent positions on their energy landscapes and therefore allows for quantitative analysis of their heterogeneous yielding dynamics and nonequilibrium deformation thermodynamics. As a demonstration of the method, we illustrate the very different characteristics of fullythermal and nearlyathermal plasticity by contrasting systems’ evolution under thermalized vs. nonthermalized plastic flow rules. 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H48.00008: Average dynamics, fluctuating relaxation rates, and dynamic susceptibility in glassforming liquids Horacio Castillo We systematically examine the consequences of the assumption that dynamic heterogeneity in glassforming liquids can be described in terms of locally fluctuating relaxation rates. We find that under mild assumptions, the time dependence of the dynamic susceptibility χ_{4}(t) for times of the order of the αrelaxation time τ_{α} is completely determined by the average dynamics, except for an overall scale factor. In particular, the shape and location (but not the overall scale) of the peak in χ_{4}(t) can be predicted from knowledge of the average relaxation function F_{0}(t). 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H48.00009: Fine and CoarseGrained Modeling of Yielding and Strain Hardening in Glassy Polymers Ronald Larson, Weizhong Zou, Robert Hoy We present both molecular dynamics (MD) simulations and coarsegrained Brownian dynamics modeling of glassy polymers under uniaxial extensional flow. The MD simulations show that individual KremerGrest chains collapse into folded states at Hencky strains of order 2, which then undergo unraveling upon further straining, analogous to that in extensional flows of dilute polymer chains. This motivates a coarsegrained picture that divides the stress into a segmental mode governing monomer friction, and a “polymer” mode contributed by the configuration of polymer chains. The Brownian dynamics model uses a schematic model of yielding of the segmental mode, analogous to that of smallmolecule glasses (Fielding, et al. Phys. Rev. Lett., 108.048301, 2012), while the polymer mode is represented by finitely extensible beadspring chains whose bead drag coefficient is proportional to the viscosity of the segmental mode. This produces behavior consistent with experimental work of the Ediger group (Lee et al., Science, 323, 231324, 2009), and produces folded states similar to those observed in the MD simulations. The modeling provides strong evidence that strain hardening arises from the high tensions in the folded polymer strands, consistent with earlier suggestions of Fielding et al. 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H48.00010: Local fluctuations in the relaxation rate in glassy systems: probing dynamical heterogeneity and its exchange time Rajib Pandit, Elijah Flenner, Horacio Castillo We numerically study the equilibrium dynamics of a glassforming binary hardsphere mixture in three dimensions, for different packing fractions. We provide a quantitative description of dynamical heterogeneity in terms of fluctuations in the local relaxation rates. We extract a newly defined twopoint correlator, χ_{2}^{Φ}(t), that probes those fluctuations. The time decay of χ_{2}^{Φ}(t) is characterized by a certain time scale τ_{ex}, which we identify as the exchange time of the slow and fast regions. We calculate the exchange time τ_{ex} as a function of packing fraction, and we find that in the vicinity of the glass transition, τ_{ex} is an order of magnitude longer then the αrelaxation time, τ_{α}. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H48.00011: Granular Plasticity in Triaxial Compression Experiments: Nonuniversal Stress Fluctuations and Particle Shape Dependence Kieran Murphy, Heinrich Jaeger A disordered network of particles in a granular material, when compressed past yielding, can quietly rearrange or violently erupt into restructuring cascades. By varying particle shape we explore a wide range of plastic deformation behavior in triaxial compression experiments on 3Dprinted and laser cut particles. Sudden stress drops are shown via Xray imaging to be localized particle slip events. We calculate from stressstrain data each particle shape’s friction angle, a dimensionless measure of shear strength, and volatility, a method for quantifying fluctuations in financial mathematics. Qualitative regions emerge in the parameter space suggesting distinct microstructural origins of particle rearrangement. For all shapes the magnitude of relaxation events appears to follow a truncated power law distribution, with particle shape driving both the power law exponent and the location of the exponential roll off. We find a nonuniversal range of exponents which are correlated with the particle shape’s ratio of the radii of circumscribed and inscribed spheres. Finally, we discuss extensions to interface depinning and other popular frameworks which better explain our results in the broader context of amorphous plasticity. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H48.00012: Active Microrheology in an Emulsion Glass Nesrin Senbil, Chi Zhang, Frank Scheffold Active microrheology can probe the nonlinear properties of a colloidals on the microscale and thus provide insight into the microscopic origins of macroscopic viscoelasticity and yielding. Here we apply this method to uniform oilinwater emulsions that show nearly hardsphere behavior with an experimentally confirmed glass and a jamming transition at volume fractions of 59% and 64%, respectively. In our experiments, we use a laser tweezer to apply a constant force on a 2 micrometersize polystyrene probe, with the aim to pull it through the emulsion host composed of oil droplets with the same diameter. We observed the trajectory of the probe, released from one cage and moving to the next, within the glassy phase, provided the applied force is high enough to break the cage and delocalize the particle. The force required for this transition can be defined as a critical force . We compare our results with theoretical work based on modecoupling theory. We find agreement with MCTresults at packing fractions close to glass transition. However, as the volume fraction is increased beyond 61% our experimental results indicate much more pronounced increase of the critical force. Our results will provide critical input for the nonlinear viscoelastic response of dense colloidal systems. 
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