Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B57: Physics of Liquids IIFocus
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Sponsoring Units: GSNP GSOFT Chair: Yang Zhang, University of Illinois at Urbana-Champaign Room: BCEC 256 |
Monday, March 4, 2019 11:15AM - 11:51AM |
B57.00001: Colloidal physics with DNA particles Invited Speaker: Francesco Sciortino DNA oligomers can nowadays be assembled to produce a large variety of nanometric constructs, via a cascade of self-assembly processes, each one guided by the length of complementary sequences of distinct DNA strands. In the talk I will show that it is possible to build bulk quantities of DNA-made nanoparticles that closely match idealized colloids, transferring modern in-paper and in-silico intuitions into experimental realizations[1-4]. I will show how unconventional collective behaviors, recently explored theoretically, can indeed be reproduced in the lab. Among others I will discuss how the selectivity of the DNA interactions can be exploited to create gels that form both on cooling and on heating [5-6], how to create networks with swappable bonds[7], how to create systems residing on the verge of a percolation transition. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B57.00002: Contrasting Dielectric Properties of Electrolyte Solutions with Polar and Polarizable Solvents Douglas Grzetic, Kris T Delaney, Glenn Fredrickson We examine the dielectric constant of electrolyte solutions with a polar and/or polarizable small-molecule solvent using a classical field-theoretic approach. We compute corrections to the dielectric constant and screening length due to intra- and inter-molecular correlations via a self-consistent one-loop approximation, accounting for the excluded volume of both solvent and electrolyte. The theory predicts either a non-linear dielectric decrement or increment with increasing salt, depending on whether the fluid correlations are dominated by the dipolar or polarizable nature of the solvent. These contrasting regimes of non-linear dielectric behavior are consistent with those seen experimentally in high- and low-dielectric-constant electrolyte solutions. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B57.00003: Ion Solvation at Thermodynamic Extremes from First Principles Viktor Rozsa, Giulia Galli The properties of aqueous salt solutions at high pressure and temperature are crucial to understanding geochemically relevant fluids. We report on the properties of aqueous monovalent ions at high pressure and temperature (1 GPa/1000 K, 11 GPa/1000 K), as obtained from first-principles molecular dynamics simulations [1]. In particular, we discuss the effect of ions on the structure and diffusion of water and the effect of water dissociation occurring under pressure, on ion solvation. We further analyze vibrational signatures of solvated ions, by comparing computed infrared and Raman spectra of salty and pure water [2] at the same conditions. Finally we present results for ionic conductivity and dielectric constants. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B57.00004: A Universal Equation of State for Hard Polyhedra Thi Vo, Sharon Glotzer The empirical formulation of the ideal gas law marked a major turning point in the development of theoretical models aimed at describing fluid behaviors. Its extension to real systems, however, often result in deviations upon comparison with experimental observations due to the presence of both excluded volume and attractive interactions in non-ideal particles. Consideration of particle shape further complicates the development of a universal theoretical framework, although there have been several attempts with varying degrees of accuracy for convex shapes. Here, we introduce a new equation of state for hard polyhedra. The derived expression not only shows excellent agreement with simulations, but also suggests a “corresponding state”-like behavior, allowing for a reduction across shape space onto a single master curve as well as prediction of the order-disorder transition density for polyhedra. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B57.00005: Dynamical Criticality in Higher-Dimensional Equilibrium Glasses Obtained by Swap Monte Carlo Patrick Charbonneau, Ludovic Berthier, Silvio Franz, Joyjit Kundu Recent implementation of the swap Monte Carlo algorithm to suitably optimized continuously polydisperse mixtures has been remarkably successful in bypassing the sluggishness associated with glass formation in dimensions, d=2 to 8. This advance has renewed the interest in exploring the finite-dimensional echo of the dynamical transition, which leads to a power-law diverging relaxation time in mean-field treatments and in the mode-coupling theory of glasses. Despite competing activated processes, such as hopping and glass nucleation, traces of the dynamics criticality can be observed, especially on the glass-side of the transition. The mean-field-like features of caging and of the dynamical susceptibility are here specifically examined. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B57.00006: All-atom Metadynamics Simulations of the Hierarchical Folding Dynamics of Proteins on the Timescale of Seconds Nathan Walter, Yang Zhang Molecular dynamics simulations have proven instrumental to the understanding of molecular biophysics. However, the temporal constraints of molecular dynamics simulations have limited attempts to capture the protein folding process at the atomic scale. Herein, we circumvent this limitation by using all-atom metadynamics algorithm to directly sample the potential-energy landscape of numerous proteins. Previous applications of the original metadynamics method to proteins penalized select collective variables of the protein assumed to be principle to the folding process. Rather, our method penalizes the full coordinate space of the protein resulting in a 3N-dimensional sampled energy-landscape, unbiased by a priori assumptions. With all-atom sampling, over collective variable sampling, a single simulation captures the folding and unfolding process multiple times, enabling the simulation of protein dynamics on scales of seconds, orders of magnitude longer than recorded molecular dynamics simulations. Further, we predict the folded state of the protein, and the activation barrier and the timescale associated with the folding process of the proteins. We will present these findings for several well-studied proteins, to validate our results, and several new proteins, as a novel extension. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B57.00007: When Dynamics Matters: Structure-Matching Alone is Insufficient in the Development of Realistic Force Fields for Nonaqueous Electrolytes Zhixia Li, Lily Robertson, Ilya A Shkrob, Joerg Neuefeind, Kyle Smith, Lu Zhang, Jeffrey S Moore, Yang Zhang Molecular dynamics are promising methods to compute numerous properties of nonaqueous electrolytes, but quantitative predictions depend critically on the prescribed force fields. Here, we show that several quantum-mechanically refined force fields for the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) - acetonitrile electrolyte yield structures that are in good agreement with the experimental neutron pair distribution function (PDF), while dynamics is dramatically different and inconsistent with NMR measurements. Such glaring discrepancies indicate that inadequate representation of long-range interactions leads to excessive frustration in the free energy landscape. Better agreement is achieved by proportionally scaling down the atomic charges of the ions. This simplification enabled the simulation of concentration dependences of ionic diffusion for 0.2-2 M LiTFSI solutions without sacrificing fit quality of the PDFs. We argue that not only structures but also dynamics constitute important checkpoints on the road to computationally design functional electrolytes. |
Monday, March 4, 2019 1:03PM - 1:15PM |
B57.00008: Stabilization of Trp-cage within Confinement by Optimizing the Interface Hydrophobicity Yanqin Zhai, Zhikun Cai, Yang Zhang Understanding the interaction between proteins and confining interfaces is a critical step towards revealing the folding behaviors of protein in cellular context. In particular, the hydrophobicity of the confinement is an important factor that may affect the hydrophobic collapsing of the proteins. Hence, to study the hydrophobic effects of the confinement on the protein behaviors, we performed molecular dynamics simulations of Trp-cage miniprotein sandwiched between two graphene sheets with varying hydrophobicity by tuning the interaction potential. The structural variations of the protein was characterized by the average root-mean-square-displacement, the radius of gyration of the backbone, and the mean contact area between individual residues and the confining interfaces. The results revealed that the confinement tends to stabilize the native state of Trp-cage by reducing its configurational entropy. Specifically, two stablization mechanisms were identified. The protein may be stabilized by moderately hydrophobic confinement through gentle adsorption and yet without destroying the hydrophobic core of the Trp-cage, or, by highly hydrophilic confinement via volume exclusion caused by dense water layers. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B57.00009: Two-step crystallization pathways of hard particle systems via a metastable fluid-fluid phase transition Sangmin Lee, Erin Teich, Michael Engel, Sharon Glotzer Multi-step crystallization pathways are often found in biomineralization, protein crystallization and tetrahedrally coordinated systems, where a single or multiple intermediate state appear during the crystallization process. Despite the importance of those materials, universal characteristics shared by the multi-step crystallization process are not well understood. Here, we report three hard particle systems (truncated tetrahedra, pentagonal bipyramids and triangular bipyramids) exhibiting two-step crystallization via a metastable fluid-fluid phase transition. Monte Carlo simulations reveal that these systems first form a high-density fluid with prenucleation motifs in the form of clusters, fibers, and networks from a low-density fluid. Subsequently, complex crystal structures (cF432, oF244 and cP92) nucleate from the high-density fluid. Our observations demonstrate the existence of diverse crystallization pathways in entropic systems and reveal various dimensionalities of prenucleation motifs. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B57.00010: Weak Secondary Relaxation in Molecular Glass Formers Sudipta Gupta The physics of glass formers and the glassy dynamics is one of the great mysteries in science. For example, the structural α relaxation, which characterizes the freezing of molecular motions on approaching the glass transition, is well known. However, in recent years in addition to the α relaxation additional secondary dynamic processes were found. Like the so-called excess wing relaxation so far mostly been investigated by dielectric spectroscopy, a method that mainly couples to only one aspect of molecular mobility, namely reorientational motions of molecules carrying a dipolar moment. We present neutron-scattering results, sensitive to density-density fluctuations, on glycerols, in presence of salts and nanoparticles, to identify the nature of this secondary relaxation. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B57.00011: Cluster connectivity and medium-range order in metallic glass Xiaoya Wei, Si Lan, Xun-Li Wang Bulk metallic glasses were discovered more than 30 years ago. It has been established that, for multicomponent metallic glass alloys, the short-range order or the fundamental building block is characterized by solute-centered clusters. Over longer length scales, the clusters are packed on a fractal network forming the medium-range order. Still, there remain many questions on the structure of metallic glasses, particularly with regard to the development of short- and medium-range orders during structure evolution induced by temperature or mechanical deformation. The experimental results seem to show that during crystallization, mechanical deformation, and liquid-to-liquid phase transformation, the short-range order is enhanced but there are no substantial changes in the structure order. Rather, it is the connectivity between clusters at medium-range scale, that drives the dynamic response. In this paper, we explore the meaning of cluster connectivity and apply the concept to explain our experimental observations. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B57.00012: Revisiting the Fragile-to-Strong Crossover in Metallic Glass-Forming Liquids: The Case of the CuxZrxAl100-2x Alloy René Alberto Alvarez-Donado, Samuel Cajahuaringa, Alex Antonelli A reliable description of the dynamic properties of metallic glass-forming liquids is highly important for the understanding of the glass forming ability of these materials. To this end, we investigate the behavior of the dffusion coefficient, vibrational spectra, and shear viscosity of the CuxZrxAl100-2x alloy (for x= 50, 49, 46) through molecular dynamics simulations. The glass transition temperatures (Tg) we obtained for different compositions are in good agreement with both experimental and computational findings, indicating an increase of Tg with the amount of aluminum in the alloy. The inverse of the diffusion coefficient as a function of temperature shows a fragile-to-strong crossover at temperatures (Tfs) in the vicinity of Tg (730 K ≤ Tfs ≤ 797 K), which are significantly lower than previous estimates. These results are corroborated by the development of an excess of vibrational modes at temperatures just below Tfs. Shear viscosity as a function of Tg/T also displays the expected Arrhenius behavior below Tfs. Moreover, our results indicate another dynamic crossover related to the onset of dynamical heterogeneities at a temperature about 1200 K, which much higher than Tfs. |
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