Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T17: Focus Session: Jamming and the Glass Transition |
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Sponsoring Units: GSNP Chair: Corey O'Hern, Yale University Room: 402 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T17.00001: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T17.00002: Understanding glass transition from structural and vibrational properties of zero-temperature glasses Lijin Wang, Ning Xu We claim that the dynamical differences between the supercooled attractive Lennard-Jonesian (LJ) and purely repulsive Lennard-Jonesian (WCA) systems and the density dependence of their glass transition temperatures are understandable from properties of the $T=0$ glasses. Below a crossover density $\rho_s$, the $T=0$ LJ and WCA glasses show distinct structures, resulting in differences in their vibrational properties such as the boson peak frequency and quasi-localization of low frequency modes. These differences make LJ glasses more stable and thus have higher glass transition temperatures than WCA ones. Above $\rho_s$, the $T=0$ LJ and WCA glasses are isomorphic, showing scaling collapse of the pair distribution function, density of vibrational states, and mode participation ratio spectrum. The scaling collapse helps us predict the density scaling of the glass transition temperature from dimension analysis, which is in excellent agreement with simulation results. Interestingly, the dimension analysis suggests a possibly general expression of the glass transition temperature in terms of the structural and vibrational quantities of the $T=0$ glasses, which can fit simulation results very well over a wide range of densities for both LJ and WCA systems. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 12:15PM |
T17.00003: The relationship between efficient packing and glass-forming ability in hard-sphere systems Invited Speaker: Kai Zhang When supercooled liquids are rapidly quenched at rates R exceeding a critical value Rc, they avoid crystallization and form amorphous solids, such as bulk metallic glasses (BMGs). However, engineering applications of BMGs are often limited by the high cost of the constituent elements and their small casting thickness. Thus, we seek to design particular alloys with controllable stoichiometry and maximal critical cooling rate Rc. We perform numerical simulations to compress binary hard-sphere mixtures into glasses as a function of the particle size ratio and stoichiometry. We measure the packing fraction and local structural order for each glass to determine the critical compression rate. We find that large packing fraction differences between the crystalline and amorphous states implies poor glass forming ability, whereas small packing fraction differences yield better glass-formers. In addition, we show that an abundance of icosahedral order in amorphous packings enhances the glass forming ability of the mixtures. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T17.00004: When do jammed sphere packings have a valid linear regime? Carl Goodrich, Andrea Liu, Sidney Nagel The physics of jamming can be studied in its purest form in packings of soft spheres at zero temperature. One of the successes of this approach is that bulk material properties, such as the elastic moduli or density of normal modes, can be predicted solely from the distance of the system to the jamming transition. Such properties are both defined and measured in the linear-response regime. It is thus tacitly assumed that the harmonic approximation to the local energy landscape can capture the meaningful physics, and it is therefore essential to delineate when this assumption is valid. We will examine the regime of validity of the harmonic approximation in jammed sphere packings as a function of system size and density. We will also discuss the crossover from linear response of the zero-temperature jammed solid to thermal behavior at nonzero temperatures. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T17.00005: Asymmetric crystallization upon heating and cooling in model glass-forming systems Minglei Wang, Kai Zhang, Yanhui Liu, Jan Schroers, Mark Shattuck, Corey O'Hern We perform molecular dynamics simulations of binary Lennard-Jones (LJ) and hard-sphere (HS) systems to understand the asymmetry in the critical cooling and heating rates for crystallization observed in experiments on bulk metallic glasses, where much faster heating rates are required to prevent crystallization. For the LJ systems, we cool the systems at different rates (above the critical cooling rate $R_c$) to temperatures below the glass transition, and subsequently begin heating the samples at different rates to measure the critical heating rate $R_h$ below which the system crystallizes. We perform companion studies of HS systems, except we measure the asymmetry in the critical compression and dilation rates to enhance the asymmetry. We show that the asymmetry increases with the glass-formability of the binary mixtures and explain this result by characterizing the structural order of the systems. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T17.00006: Defect Dynamics in the Network Glass SiO$_2$ Katharina Vollmayr-Lee, Annette Zippelius We study the dynamics of the strong glass former SiO$_2$ via molecular dynamics simulations below the glass transition temperature. To focus on microscopic processes, we average single particle trajectories over time windows of about 100 particle oscillations. The structure on this coarse-grained time scale is very well defined in terms of coordination numbers, allowing us to identify ill-coordinated atoms, called defects in the following. The most numerous defects are O-O neighbors, whose lifetimes are comparable to the equilibration time at low temperature. On the other hand SiO and OSi defects are very rare and short lived. The lifetime of defects is found to be strongly temperature dependent, consistent with activated processes. Single-particle jumps give rise to local structural rearrangements. We show that in SiO$_2$ these structural rearrangements are coupled to the creation or annihilation of defects, giving rise to very strong correlations of jumping atoms and defects. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T17.00007: T1 Process and Dynamics in Hard-Sphere Glasses Yuxing Zhou, Scott Milner The relationship between dynamics and structure in a glass-forming liquid is elusive. Inspired by studies in foam topology, we propose a criterion for T1-active particles in a dense hard-sphere fluid: namely, those that can have a T1 process by moving within their free volume given all other particles fixed. From newly devised hybrid Monte Carlo simulations that effectively suppress crystal without altering the dynamics, we obtain the geometrical and dynamical properties for monodisperse hard-spheres along the whole metastable branch. We find that the fraction of T1-active particles vanishes at random close packing, and the percolation property of T1-active clusters changes dramatically at the glass transition density $\phi_g \approx 0.58$. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T17.00008: Packing fraction of continuous distributions Jos Brouwers This study addresses the packing and void fraction of polydisperse particles with geometric and lognormal size distribution. It is demonstrated that a bimodal discrete particle distribution can be transformed into said continuous particle-size distributions. Furthermore, original and exact expressions are presented that predict the packing fraction of these particle assemblies. For a number of particle shapes and their packing modes (close, loose) the applicable parameters are given. The closed-form analytical expression governing the packing fractions are thoroughly compared with empirical and computational data reported in the literature, and good agreement is found. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T17.00009: Soft spots correlate with rearrangements in sheared glasses Samuel Schoenholz, Andrea Liu, Robert Riggleman, Joerg Rottler Solids flow under shear via localized rearrangements. In crystals it is known that these rearrangements occur at topological defects, particularly dislocations. In disordered solids, Manning and Liu showed that discrete ``soft spots'' - analogous to defects in crystalline solids and constructed from the low-frequency vibrational modes of the material - exist in athermal suspensions of soft finite repulsive disks under quasi-static shear. These soft spots were shown to predict where rearrangements would occur, to be long lived with respect to the time between individual rearrangements, and to be distinct from the rest of the sample in terms of commonly-used structural quantities such as free volume and bond orientational order (although such quantities alone could not \textit{a priori} identify the soft spot population.) In this work we show that soft spots remain a valid description of plastic flow in sheared Lennard-Jones glasses over a range of strain rates at temperatures extending up to the glass transition and beyond. We further discuss soft spot lifetimes and conclude that the $\alpha$-relaxation time sets the lifetime of the soft spot population. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T17.00010: The emergence of elasticity in glass-forming fluids from the spatial correlations of particle displacements Elijah Flenner, Grzegorz Szamel We study the emergence of elasticity in supercooled fluids by examining the spatial correlations of particle displacements. To this end we calculate a four-point structure factor $S_4(\Delta x,q;t)$ that measures the correlations of particle displacements $\Delta x$ after a time $t$. We focus on correlations of displacements perpendicular to the initial separation of the particles, i.e. transverse displacement correlations. We examine the time and temperature dependence of these correlations for a model supercooled fluid. We find that the long-range correlations of displacements are related to the plateau height of the stress-stress correlation function of the supercooled fluid and thus provide insight into its emerging elastic properties. [Preview Abstract] |
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