Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D45: Understanding Glasses and Disordered Systems Through Computational Models IIIFocus
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Sponsoring Units: DCOMP DSOFT GSNP DPOLY Chair: Pengfei Guan, Beijing Computational Science Res Ctr Room: 706 |
Monday, March 2, 2020 2:30PM - 3:06PM |
D45.00001: Understanding the boson peak in glasses and glassy polymers Invited Speaker: Alessio Zaccone The density of states of the vibrational modes (VDOS) plays a central role in determining macroscopic physical quantities of solids, such as specific heat, thermal conductivity and the critical temperature towards a superconducting phase. Also, the VDOS is directly linked to the elastic and viscoelastic moduli of the solid. For glasses, the VDOS at low frequency is not simply behaving quadratically in frequency as predicted long ago by Debye, but presents an excess of modes above some frequency where the quadratic Debye law breaks down. Upon normalizing the VDOS by the Debye law a peak is visible, known as the boson peak (BP). Consensus is emerging to explain the origin of the BP in terms of a (Ioffe-Regel type) crossover between ballistic phonon propagation and diffusive-like propagation dominated by randomness and scattering. The diffusive propagation shows up in a damping coefficient which is quadratic in the wavevector [1]. A theoretical description leads to the possibility of studying the VDOS in semi-analytical form as a function of physical parameters [2]. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D45.00002: Revealing the low-temperature fast relaxation peak in a model metallic glass Pengfei Guan The relaxation dynamics and its underlying atomistic mechanism are one of the most important and challenging issues in metallic glasses. Here, we first report a pronounced low-temperature relaxation peak in a model metallic glass, which is located at a much lower temperature than the typical temperature for the conventional β relaxation reported in prior works. Through extensive molecular dynamics simulations, we unravel its intrinsic link with reversible atomic motions, which bridges the vibrational modes and the structural rearrangements governing a normal β relaxation process. Moreover, through the study of the local geometrical anisotropy, we reveal the microscopic mechanism of the fast relaxation process and clearly demonstrate that the fast relaxation dynamics is a trigger to a cascade of critical relaxation modes (such as β and α relaxation). Our findings are of fundamental importance, and further our understanding of the heterogeneous dynamic response of metallic glasses and the dynamic origin of their physical properties. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D45.00003: Accuracy of Classical Potentials for Polyethylene Structures Away from Equilibrium Keara G Frawley, Lihua Chen, Huan D Tran, Naresh N Thadhani, Ramamurthy Ramprasad Realistic simulations of polymers require a large number of atoms due to the anisotropic nature of their morphology. Although density functional theory (DFT) is considered highly accurate, it is too computationally expensive to handle large systems. Classical potentials used for molecular dynamics studies are trained on experimental or quantum mechanical equilibrium structures, and their accuracy away from equilibrium is not well known. This work is a comparative study of two widely used classical potentials—Optimized Potentials for Liquid Simulations (OPLS) and the reactive force-field (ReaxFF)—with respect to DFT. Their performance for polyethylene (PE), a simple model polymer, is benchmarked by comparing the classical energies, forces, and stresses against DFT. Additionally, a pressure-temperature phase diagram for PE is computed using OPLS and ReaxFF. Although the classical potentials do not perfectly capture the DFT and experimental behavior, the results are close enough to justify their use when large system sizes are required. Consequently, these results will be used for shock simulations involving far from equilibrium structures. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D45.00004: Disentangling glassy polymer dynamics: combining simulations and machine learning Anna Lappala Glass-forming liquids are an example of non-equilibrium systems: upon cooling, a glass undergoes a transition from a liquid into an amorphous solid without apparent long-range order. The problem of the glass transition combines the concepts of self-organization, collective and heterogenous dynamics and poses an unsolved fundamental problem in condensed matter physics. We investigate the role of connectivity and topology in the glass transition of polymers, combining coarse grained molecular dynamics simulations with machine learning. ML allows us to quantify the relation between local structural properties of polymers and their topology using clustering of correlated motions of polymer chain segments as an example descriptor to create a training set, leading to the development of a data-based model. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D45.00005: Amorphous polymers in their glass transition regime : Comparison of local and macroscopic small non-linearities Aude Belguise, François Lequeux, Sabine Cantournet, Hélène Montes The mechanical behavior of amorphous polymers in their glass transition regime is linked to dynamical heterogeneities at a nanometric scale. To describe them, we developed a finite elements method where a sample is described as a set of domains each characterized by its own relaxation time τi. The relaxation times are randomly distributed over the domains. We showed that this approach quantitatively describes linear relaxation measured on glassy polymers. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D45.00006: Atomic-scale understanding of highly conducting polymer electrolyte Sungyeb Jung, Yeonhwa Song, U Hyeok Choi, Jae Hyun Park, Jaekwang Lee Electrolyte is one of the key and most important components determining the performance of lithium-ion batteries. Until now, solid polymer electrolytes have attracted considerable attention due to the rapid development of the need for more safety and powerful lithium ion batteries. Despite their high stabilities and endurance, extremely low conductivity has hindered their applications. Here, we find that novel poly acrylic acid polymer-based electrolyte exhibits at least two orders of magnitude higher ionic conductivity compared with conventional solid polymer electrolytes. By combining molecular dynamic simulations with temperature dependent conductivity measurements, underlying mechanism will be addressed at the atomic scale in detail. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D45.00007: Effects of Coarse-Graining on Molecular Simulations of Mechanical Properties of Polymers Ting Ge, Mark Robbins Conventional coarse-graining methods for polymers are typically based on matching equilibrium thermodynamic properties in the melt state, such as the aveage structural correlations or forces. It is not clear whether the resulting coarse-grained potentials can be used to simulate non-equilibrium properties of melts at high rates or the mechanical properties of low-temperature cyrstals or glasses. As a case study, we simulate polystrene using models with different levels of coarse-graining but the same strucutal correlations at thermal equilibrium. The elastic modulus, yield stress, and strain hardening of polystyrene in the glassy state show a steady decrease with increasing degree of coarse-graining. This reflects that the configurational average of fine-grained structure leads to a smoother coarse-grained potentail with lower energy barriers between local configurations. We find that the stress-strain curves with different degrees of coarse-graining can be collapsed with a simple rescaling factor in some cases, but too much coarse-graining leads to qualitiative changes of the curves. We develop a multi-scale method that uses properly selcted coarse-grained models to accelerate the simulation of mechanical response while fine-grained models to capture the accurate stress value. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D45.00008: Ubiquity of Entropy-Driven Local Organization Andrei A. Klishin, Greg Van Anders Soft-matter and biological systems of crowded objects exhibit local organization into preferred motifs. Locally organized motifs in soft systems can, paradoxically, arise from a drive to maximize overall system entropy. The emergence of entropy-driven local order has been directly confirmed in model, synthetic colloidal systems, however similar patterns of organization occur in crowded biological systems ranging from the contents of a cell to collections of cells. In biological settings it is unclear whether entropy acts to promote or inhibit local organization. Answering this is difficult because entropic effects are intrinsically collective, complicating efforts to isolate them. Here, we employ minimal models that artificially restrict system entropy to show that entropy drives systems toward local organization, even when the model system entropy is below reasonable physical bounds. By establishing this bound, our results suggest that entropy drives local organization in all crowded soft and biological systems of rigid objects. |
Monday, March 2, 2020 4:30PM - 4:42PM |
D45.00009: Measuring configurational entropy of glasses using population annealing Chris Amey, Jonathan Lee Machta In this talk we present two methods to calculate the vibrational (Svib) and configurational (Sc) entropies of glassy fluids in the context of population annealing Monte Carlo. The first method for obtaining Svib integrates the pressure of individual configurations from the fluid phase to the glass phase, and the second method introduces a hard shell constraining potential to individual particles at fixed packing fraction in the glassy regime and integrates the entropy to the highly constrained, ideal gas limit. From population annealing, we also obtain the total entropy, Stot, and from it the configurational entropy, Sc = Stot – Svib. We present numerical data for large-scale population annealing simulations of binary hard sphere fluids with 30, 60, and 100 particles. Our results show that the two methods agree and that for even modest-sized systems, finite-size effects appear to be small. |
Monday, March 2, 2020 4:42PM - 4:54PM |
D45.00010: Statistical structural and dynamic heterogeneity as signature of glass transition Yunjiang Wang, Dong Han Glass transition is a ubiquitous phenomenon in nature yet it remains highly elusive. The puzzle is that there lacks intuitive rationalization for the dynamic arrest upon liquid-to-glass transition and a structural rationale is highly anticipated. Here I introduce two parameters based on the Shannon Information Entropy concept which can be served as intuitive signatures of glass transition. The first one is the Shannon entropy on structures, which characterize the diversity of atomic-level structures that undergoes a striking variation across the glass transition, and explains the change found in the excess configurational entropy. The other dynamic Shannon entropy is associated with the variation in the activation barriers to local structural excitations on the underlying potential energy landscape. It is also found to change dramatically at the glass-to-liquid transition and therefore can be treated as another signature of the glass transition. The two parameters can be evaluated based on the recent advanced atomistic simulations techniques, which moves a minor step towards the eventual understanding of physics of glass transition. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D45.00011: RFOT theory explains fragile-to-strong crossover in Wigner glasses Hyun Woo Cho, Mauro Lorenzo Mugnai, Theodore Kirkpatrick, Dave Thirumalai Colloidal suspension with various degrees of softness exhibits rich glass forming behavior, normally observed in molecular glasses. For over a decade, many experiments have shown that glass transition of colloidal suspensions can be either strong or fragile depending on the softness of potential, thus making them ideal testing ground for the structural glass transition (SGT) theories. However, it is still debated whether the softness alone is responsible for such a drastic change in fragility and if the glass transition of the soft colloid with the broad range of fragility can be described using a unified theory. We carried out Brownian dynamics simulations for a binary mixture of micron-sized charged colloidal suspensions and elucidate that they exhibit continuous transition from fragile to strong behavior due to the variation of the softness of potential that is achievable by changing the monovalent salt concentration. We found that the prediction of the random first order transition (RFOT) theory quantitatively describes the universal feature of the glass transitions of the charged colloids, such as growing length scales associated with heterogeneous dynamics. Our predictions are amenable to experimental tests. |
Monday, March 2, 2020 5:06PM - 5:18PM |
D45.00012: Cluster Dynamic Mean Field Theory of Glass Transition chen liu, Giulio Biroli, David Reichman, Grzegorz Szamel Recently, the infinite dimensional theory of the dynamics of super-cooled liquids was established in [1,2,3], and showed to be related to the mode coupling transition (MCT) theory [4]. One of the main mean field predictions is the existence of a sharp dynamical transition, akin to the MCT transition, at a temperature T_d, i.e. as the temperature approaches T_d from above, the relaxation time diverges. However, in three dimensional glass-formers this transition is smeared out and a smooth crossover takes place. We extend the infinite dimensional analysis by developing a cluster dynamic mean field theory (CDMFT) where clusters of particles interact in a mean field way, while correlations involving several particles are taken into account inside clusters. This allows us to progressively take into account correlations between particles and activated dynamical processes. In this talk we will present CDMFT and present results for three dimensional super-cooled liquids. |
Monday, March 2, 2020 5:18PM - 5:30PM |
D45.00013: Understanding TA from dynamic heterogeneity in metallic glass-forming liquids Nannan Ren, Pengfei Guan, Lijin Wang, Lina Hu How to generally describe the structural relaxation upon cooling is still mysterious for metallic glass-forming liquids with various dynamic fragilities. Here a scaling collapse of the temperature dependence of structural relaxation time, τα(T), is achieved by introducing the characteristic temperature T*(α2,max) and characteristic time scale τ*(α2,max) with any identical dynamic heterogeneity α2,max in 12 metallic glass-forming liquids, which is consistent with our previous reports[1]. The scaled master curve τα/τ*(α2,max) ~ T*(α2,max)/T experiencing a crossover behavior from Arrhenius to non-Arrhenius implies that TA is one of T*(α2,max). It is further confirmed by the almost constant α2,max(TA) for all the systems studied here and achieving a general description of relaxation time, together with τA (the structural relaxation time at TA). Our findings provide a new insight of TA from the perspective of dynamic heterogeneity and shed light on the glass forming of metallic liquids. |
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