Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B49: Understanding Amorphous Matter Through Modeling and Simulation IIFocus Recordings Available

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Sponsoring Units: DCOMP DSOFT GSNP Chair: Jeorg Rottler, DCOMP Room: McCormick Place W471B 
Monday, March 14, 2022 11:30AM  12:06PM 
B49.00001: A microscopic view of the response of disordered polymer solids to deformation Invited Speaker: Robert Riggleman Disordered polymers with solid properties come in a variety of shapes and sizes, from amorphous polymer glasses to porous membranes to crosslinked polymer networks. While homogeneous on large length scales, on the molecular scale the mechanical response is known to be strongly inhomogeneous. That is, upon deformation, some regions deform more than others and can even rupture before others. In this talk, I will describe two vignettes from my group's work to understand how inhomogeneous polymer solids response to deformation. In polymer glasses under shear stress, it is commonly observed that the dynamics become more homogeneous under load, and the distribution of relaxation times narrows. We use a machine learning approach to show how the energy barriers of the slower regions respond more strongly to stress and as a result have their energy barriers reduced more than the faster domains. In crosslinked polymer networks, there has been some recent debate about the amount of energy that is dissipated when strained to the point of chain scission. We have performed a series of simulations using model polymers with breakable bonds and calculated the energy dissipated as a function of the polymer chain length and volume fraction, providing new clues on the energy required to fracture a polymer gel. 
Monday, March 14, 2022 12:06PM  12:18PM 
B49.00002: Recovering dynamics and material properties in chemically specific coarsegrain models of polymer melts using a dissipative potential Lilian C Johnson, Frederick R Phelan Coarsegrain (CG) representations of polymeric materials aim to reduce computational effort, enabling simulations to reach pertinent length and time scales, while preserving some features, typically, either of the allatom (AA) representation ("bottomup" methods) or material properties ("topdown" methods). Here, we combine a bottomup CG model with a dissipative potential to design a chemically specific and dynamically correct model. First, we utilize iterative Boltzmann inversion (IBI) to parameterize a conservative potential which recovers AA structure. Second, we introduce Langevin dynamics and parameterize the dissipative potential, the Langevin friction coefficient, to correct for the spedup dynamics of the IBIgenerated forcefield. We recently showed that the friction coefficient of the CG representation can be parameterized to recover AA dynamics for various measures of translational and rotational diffusion [J. Chem. Phys. 154, 084114 (2021)]. Here, we additionally compare to a friction coefficient derived from a material property, viscosity. We show that the viscositybased friction may be predicted by a simpler dynamicsbased friction. We point to possible CG model modifications that yield consistent dynamics and material properties. 
Monday, March 14, 2022 12:18PM  12:30PM 
B49.00003: Atomistic Simulations of Polynorbornenes: the Effect of Stereochemistry Nobahar Shahidi, Jeffrey A Laub, Konstantinos D Vogiatzis, Manolis Doxastakis Vinyladdition polynorbornenes are candidates for designing high performance polymers due to their unique characteristics, such as a highly rigid backbone associated with a high glass transition temperature. Recent studies have established that processability and properties of these polymers can be finely tuned by using targeted substitutions. At the same time, different catalysts can result in materials with distinct properties, potentially due to the presence of various stereoisomers that are difficult to quantify. Herein we develop forcefield descriptions of polynorbornene oligomers based on classical forcefields and density functional theory calculations. In order to establish the relationship between microstructure and viscoelastic properties, we characterize polynorbornenes and their derivatives by performing detailed molecular dynamics simulations. 
Monday, March 14, 2022 12:30PM  12:42PM 
B49.00004: Probing the Equilibrium Adsorption Morphologies of Surfactants at MetalWater Interfaces Using a Novel Free Energy Sampling Methodology in Molecular Simulations Sumit Sharma, Himanshu Singh Adsorption of surfactants is a facile way of adjusting interfacial properties of metals, which has applications in electrochemistry, corrosion inhibition, heterogeneous catalysis and synthesis of anisotropic metal nanoparticles. Surfactant molecules are understood to adsorb in densely packed organized morphologies at the metalwater interfaces. Traditional molecular simulation methods as well as existing free energy sampling methodologies are inefficient in studying the formation of these morphologies due to the long timescales and large free energy barriers involved in the process. In this work, I will discuss a new umbrella samplingbased simulation methodology that our research group has developed that allows the study of these highdensity morphologies. Using this methodology, we have investigated the adsorption behavior of different cationic, anionic and chargeneutral surfactants and surfactant mixtures. The role of molecular properties is governing the equilibrium adsorption morphologies will be discussed. 
Monday, March 14, 2022 12:42PM  12:54PM 
B49.00005: Machine learning potentials for amorphous oxides modeling and simulations Jun Jiang, Alec S Mishkin, Rui Zhang, HaiPing Cheng Machine learning (ML) offers a powerful approach to generate interatomic potentials that are superior to the conventional classical functions used in simulations. With an accuracy comparable to firstprinciples calculations and a much lower calculational cost, potentials generated by ML enable simulations of large number of atoms and better predictions of structures and energy landscape properties for amorphous materials, especially for the doping systems where current classical pair potentials are not available or fail to reproduce the atomic structure features from experiments. In this work, we demonstrate a ML potential for zirconiadoped amorphous tantala based on spectral neighbor analysis potential (SNAP), which allows us to model the amorphous LIGO mirror coatings. Compared to firstprinciples calculations, the SNAP ML potential has very low energy mean absolute error (MAE) (~10^{4} eV/atom) and force MAE (~10^{1} eV/A), and is over 1000 times faster. Structure models from the SNAP potential also show better agreement with the experimental metalmetal pair distribution functions compared to existing wellbuilt MorseBKS potentials. 
Monday, March 14, 2022 12:54PM  1:06PM 
B49.00006: Sublattice Melting in Amorphous Tantala Rui Zhang, Jun Jiang, Alec S Mishkin, HaiPing Cheng We use molecular dynamics to investigate amorphous tantala and doped tantala. We found an interesting sublattice melting phase during annealing process. Our simulation shows that in this particular phase only oxygen is diffusive while tantalum is not. The calculated selfdiffusion coefficient of oxygen is about 100 times larger than that of tantalum, or is in the order of 10^{5} cm^{2}/s at around 2000K. Simulations are also performed for Zr and Tidoped tantalum oxides, and the dopant effects are analyzed. 
Monday, March 14, 2022 1:06PM  1:18PM 
B49.00007: Atomistic simulations of mechanical loss in amorphous silicon Daniel Wong, Joerg Rottler Brownian noise from the mirror coatings in current gravitational wave detectors serves as one of the predominant limiting factors in detecting gravitational wave events [1]. Projected reductions in Brownian noise through the lowering of operational temperatures prove to be insufficient for future sensitivity targets and necessitates the search for alternative candidates. To gain insight into the atomistic mechanisms of internal friction, we explore the potential energy landscape of amorphous silicon, a paradigmatic amorphous coating material, with molecular simulations. Structural transitions in the form of twolevel systems (TLS) are identified, and the distributions of relevant TLS parameters (barriers, asymmetry, attempt frequencies) are obtained and compared with other amorphous coating materials. A calculation of mechanical dissipation in the 101000 Hz regime is performed via harmonic transition state theory and discussed in the experimental context. 
Monday, March 14, 2022 1:18PM  1:30PM 
B49.00008: Transient Nature of Fast Relaxation in Metallic Glass Leo Zella, Takeshi Egami, Jaeyun Moon, David Keffer Understanding dynamic relaxation in glass is of importance to elucidating phenomena such as deformation, glass transition, diffusion and aging. Unlike other relaxation processes, nearly constant loss (NCL) relaxation is subtle with no welldefined characteristic time and length scales. Prior works attributed possible rattling and caged dynamics as the microscopic mechanism of this structural relaxation, but the origin of the mechanism is still unclear. Through molecular dynamics simulations of a model metallic glass, Cu_{64.5} Zr_{35.5}, we implement dynamic mechanical analysis with atomic level stresses to determine the group of atoms responsible for NCL. Our work demonstrates that NCL is due to transient and varying groups of atoms that participate in the relaxation rather than a specific, single group of atoms causing NCL throughout the simulation. 
Monday, March 14, 2022 1:30PM  1:42PM 
B49.00009: SpaceTime Cluster Analysis of an Aging Athermal System Katharina VollmayrLee, Rituparno Mandal, Peter K Sollich We analyze the spacetime structure of athermal aging. Specifically, we consider a 50:50 binary mixture of a purely repulsive harmonic binary system of size ratio 1:1.4 at packing fraction 0.9. We simulate the dynamics of the particles immersed in a solvent for the case of overdamped dynamics. Starting with random positions corresponding to a quench from infinite temperature, we observe the system as it approaches the final jammed state. We focus on rearrangement events by identifying particles whose velocity is large compared to the average velocity of their neighbors normalized by the average velocity of all particles. Particles undergoing rearrangement events form spacetime clusters. We find that these spacetime clusters are compact in space and time and that with increasing waiting time these clusters increase both in spatial size as well as in time duration following nontrivial power laws. The distribution of quiescent times between successive rearrangement events are waiting time independent and follow an initial power law with exponential tail. 
Monday, March 14, 2022 1:42PM  1:54PM 
B49.00010: Estimation of the equilibrium free energy for glasses using the Jarzynski equality H. A Vinutha, Daan Frenkel The free energy of glasses cannot be estimated using thermodynamic integration as glasses are intrinsically not in equilibrium. We present numerical simulations showing that, in contrast, plausible freeenergy estimates of a Kob–Andersen glass can be obtained using the Jarzynski relation. Using the Jarzynski relation, we also compute the chemical potential difference of the two components of this system and find that, in the glassy regime, the Jarzynski estimate matches well with the extrapolated value of the supercooled liquid. Our findings are of broader interest as they show that the Jarzynski method can be used under conditions where the thermodynamic integration approach, which is normally more accurate, breaks down completely. Systems where such an approach might be useful are gels and jammed glassy structures formed by compression. 
Monday, March 14, 2022 1:54PM  2:06PM 
B49.00011: Counting and Characterizing the Catchment Regions of Jammed Packings Valerie Beale, Eric I Corwin The potential energy landscapes of granular and amorphous systems are complex, high dimensional, and vast. A direct characterization of these complex energy landscapes would reveal the configurational entropy as well as provide an understanding for the origin of the commonalities that link all amorphous configurations. Because the landscape is far too large to exhaustively sample we instead characterize it by a direct measurement of the catchment regions found within randomly chosen small volumes of the landscape. By exhaustively searching these volumes, we are able to report on the size, shape, and structure of the catchment regions contained therein. From these measurements we estimate the configurational entropy (and the statistical complexity) of the energy minima and discuss its relation to the Edward's conjecture. 
Monday, March 14, 2022 2:06PM  2:18PM 
B49.00012: Hyperuniformity of Wigner Glasses Erdal C Oğuz, Haim Diamant Hyperuniform systems possess density fluctuations that are strongly suppressed at large length scales as characterized by a vanishing structure factor, commonly of the form S(k) ~ k^{α} with α > 0, as wavenumber k goes to 0. In contrast, typical disordered systems such as ideal gases and classical liquids are nonhyperuniform. 
Monday, March 14, 2022 2:18PM  2:30PM 
B49.00013: Understanding degeneracy of twopoint correlation functions via Debye random media Murray Skolnick, Salvatore Torquato It is wellknown that the degeneracy of twophase microstructures with the same volume fraction and twopoint correlation function is generally infinite. To elucidate the degeneracy problem explicitly, we examine Debye random media (DRM), which are entirely defined by a purely exponentially decaying twopoint correlation function. In this work, we consider three different classes of DRM. First, we generate the "most probable" class using the YeongTorquato construction algorithm. A second class of DRM is obtained by demonstrating that the corresponding twopoint correlation functions are effectively realized by certain models of overlapping, polydisperse spheres. A third class is obtained by using the YeongTorquato algorithm to construct DRM that are constrained to have an unusual prescribed poresize probability density function. We structurally discriminate these three classes of DRM from one another by comparing and contrasting their other statistical descriptors, percolation thresholds, as well as their diffusion and fluid transport properties. We find that these three classes of DRM are distinguished to varying degrees by the aforementioned descriptors and have visually distinct microstructures. We also discuss applications of our work to the design of materials with a spectrum of physical properties. 
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