Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L32: Chemical PhysicsFocus
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Sponsoring Units: DCP Chair: Der-you Kao, NASA Goddard Space Flight Center Room: BCEC 204A |
Wednesday, March 6, 2019 11:15AM - 11:27AM |
L32.00001: Verification of acoustic mismatching model for sub-THz acoustics in glass Fan Jun Wei, Richard A. Mole, Sunil Karna, Jin-Wei Shi, Jinn-Kong Sheu, Kung-Hsuan Lin Because of high acoustic attenuation of glass in sub-THz regime, it is difficult to measure the highly damped acoustic waves passing through the media. In theory, the acoustic properties of the media could be experimentally studied by reflection of acoustic waves which do not propagate into the media. Complex acoustic impedance, which includes the information of dispersion relation and frequency-dependence of attenuation, is a parameter in the acoustic mismatching model to describe the phenomena when acoustic waves encounter the interface of media. The mismatch of acoustic impedance results in reflection and transmission of acoustic waves at the interfaces. We report the verification experiments for this model in sub-THz regime. We generated coherent THz acoustic phonons and measured the amplitude and phase of the reflected acoustic phonons from the interface of GaN and silica. Based on the acoustic mismatching model, the frequency dependence of sound speed and the attenuation were obtained up to 0.3 THz. These values were compared with the results from typical method. This verified technique enables the measurement of sub-THz acoustic properties of thick and highly damped materials. |
Wednesday, March 6, 2019 11:27AM - 11:39AM |
L32.00002: Modulated order and unconventional phase coexistence in a model for lattice mismatched solids Layne Frechette, Christoph Dellago, Phillip L Geissler Lattice mismatch is a common occurrence in modern materials, but just how it influences the arrangement of atoms in materials is incompletely understood. Here we consider a simple microscopic model for lattice mismatch, in which the difference in natural bond lengths between atoms produces interactions mediated by elastic strain. Monte Carlo simulations reveal that the model exhibits rich phase behavior, supporting structures with modulated order and unusual coexistence scenarios. To explain this phase behavior, we derive an effective pair potential between atoms, revealing preferred spatial arrangements of atoms driven by spatial variations in the interaction energy. Based on this effective interaction, we then develop a mean field theory, which captures the modulated structures observed in our simulations. Finally, we explain the observed coexistence scenarios using a modified Maxwell construction, which is based on the realization that the free energy cost of phase separation in elastic systems is extensive. These results clarify the equilibrium effects of lattice mismatch in macroscopic solids and suggest a role for lattice mismatch in creating spatially heterogeneous compositions in nanoscale materials. |
Wednesday, March 6, 2019 11:39AM - 11:51AM |
L32.00003: Hybrid Nanopatterning Techniques Utilizing Chemical Self-Assembly Daniel Santavicca, Alexandra M. Patron, Alisha M Bramer, Thomas J. Mullen Patterning strategies that combine conventional top-down lithographic techniques with molecular self-assembly show significant promise. These hybrid strategies are of particular interest because they couple a key aspect afforded by conventional lithography, the ability to create complex architectures over large areas, to the flexibility and resolution afforded by molecular self-assembly. We described two such hybrid techniques based on mercaptohexadecanoic acid (MHDA) self-assembled monolayers (SAMs). The first technique combines the molecular ruler process with local nanoshaving using an AFM tip to create nanowires that span well-defined nanogaps. The second technique seeks to extend nanoshaving to Si substrates by using an organosilane SAM to begin the multilayer growth. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L32.00004: Fundamental Link between β Relaxation, Excess Wing, and Cage-Breaking in Metallic Glasses Menghao Yang, Haibin Yu, Yang Sun, Feng Zhang, Jianbo Liu, Cai-Zhuang Wang, Kai-Ming Ho, Ranko Richert, Konrad Samwer In glassy materials, the Johari-Goldstein secondary (β) relaxation is crucial to many properties, as it is directly related to local atomic motions. However, a long-standing puzzle remains elusive: why some glasses exhibit β relaxations as pronounced peaks while others as unobvious excess wings? Using microseconds atomistic simulation of two model metallic glasses (MGs), we demonstrate such a difference is associated with the amount of string-like collective atomic jumps. Relative to that of excess wings, we find that MGs having pronounced β relaxations contain larger numbers of such jumps. Structurally, they are promoted by the higher tendency of cages-breaking events of their neighbors. Our results provide atomistic insights for different signatures of the β relaxation that could be helpful for understanding the low-temperature dynamics and properties of MGs. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L32.00005: Ultrafast Conformer Relaxation of Provitamin D Analogs in Lipid Bilayers Danielle Sofferman, Roseanne J Sension Ultrafast photochemical transformations of 7-Dehydrocholesterol (DHC, Provitamin D3), DHC-Acetate and Ergosterol (Ergo, Provitamin D2) to previtamin (D3, D3-acetate and D2) occurs upon a ring-opening reaction in the excited state where a cyclohexadiene (CHD) chromophore embedded within the molecules opens to form a hexatriene previtamin D species. Conformer relaxation back to the ground state of isolated CHD happens on the time scale of picoseconds, therefore it is necessary to use ultrafast techniques such as transient absorption to capture the relaxation. Here we are studying conformer relaxation of DHC and its analogs in liposomes as a simple model for biologically relevant skin membranes. DHC has been studied extensively in isotropic solvents where the molecule is free to relax down to the ground state to form a stable previtamin D3 species. However in the cell membrane the molecule is hypothesized to be locked in an unstable previtamin D3 conformation that thermally relaxes down to form Vitamin D3. To understand the conformer relaxation that takes place in biological membranes we extend our studies to DHC analogs, DHC-Acetate and Ergo in isotropic solvents and in simple lipid bilayers. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L32.00006: Probing the Molecular Interactions of Small Molecules with Sapphire Nityanshu Kumar, Sukhmanjot Kaur, Rajat Kumar, Selemon Bekele, Mesfin Tsige, Ali N Dhinojwala Numerous challenges in the field of adhesion, biocompatibility, and purification thrives a persistent need to better understand the interactions of small molecules at any interface. The interaction energy of small molecules at a solid/liquid interface is governed by the surface structure of the solid substrate, the interfacial molecular orientation of the small molecules and their chemical nature. Sometimes, the interaction could be driven by the presence of hydrogen bonding leading to a specific molecular orientation. Here, we seek to understand how small molecules compete for interaction sites, orient and interact at the solid/liquid interface. Using MD simulations, we contemplate and present a model to visualize the interaction of acetone and chloroform molecules on the sapphire. Further, combining the simulation results with experimental data obtained using sum frequency generation (SFG) spectroscopy, we attempt to understand the spectroscopic features and their hidden correlations. This work paves the way for understanding the interaction of molecules at the solid/liquid interface with potential applications in the field of adhesives, coatings, medicines, and water purification. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L32.00007: Shock Compression Microscopy for Tabletop Detonations in PETN-based Explosives Will Bassett, Dana Dlott, Belinda Pacheco, Lawrence Salvati We have developed a microscope system for performing shock compression experiments with km s-1 impactors and nanosecond-duration shock waves. In the present work, this system is used to study polymer-bound explosive charges (PBXs) 1 mm in diameter and tens to hundreds of microns thick. The microscope diagnostics include particle velocity (Photon Doppler velocimetry) measurements, fast imaging, and optical pyrometry. The PBX under study is a pentaerythritol tetranitrate (PETN)-based PBX (80% PETN, 20% elastomer binder) and has been characterized using x-ray tomography and scanning electron microscopy. At lower shock-pressures we observed sub-detonative behavior and hot spots have structures which evolved over 20 ns and cooled relatively slowly. At higher shock-pressures, the shock wave was supported by fast chemical reactions and the particle velocity remained constant. In this regime the shock wave is detonation-like and the hot spot cooling rate increased dramatically. |
Wednesday, March 6, 2019 12:39PM - 1:15PM |
L32.00008: Quantum Confinement of Helium, Molecular Nitrogen and Molecular Deuterium Clusters in Carbon Nanotubes Invited Speaker: Maria Pilar de Lara-Castells NA |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L32.00009: COARSE-GRAINED SIMULATIONS OF DENDRITIC CARBOHYDRATE NANOPARTICLES Eduardo Mendez, Mohammad Hassan Khatami, Hendrick W de Haan PhytoSpherix are naturally ocurring carbohydrate-polymer dendritic nanoparticles extracted from sweet corn, with branching points at regular intervals. We investigate the structural details of model Px particles via coarse grained strategies (MARTINI forcefield and GROMACS molecular dynamics simulations), and also employ knowledge gathered from small atomistic models to build large structures many thousands of carbohydrate units in size, beyond the typical scales reached by standard atomistic simulations. The scheme used here may be employed to build structures with branched motifs that may not be regular. These structures are produced by other plant species and have a wide degree of applicability due to their sponge-like properties. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L32.00010: Designing Interdisciplinary Undergraduate Research Experiences to Increase Diversity in STEM Solomon Bililign The recruitment of undergraduate students, especially minorities, into STEM career paths and in particular into majors in physics continues to be a challenge. One approach for addressing this issue provides an interdisciplinary research opportunity that has societal relevance and uses basic concepts in physics. The program design involves year-round research combined with summer or short visits to major university, national lab, or private research facilities. Other program components include (1) cohort-building activities and effective mentoring; (2) short-course professional development modules focused on research ethics, literature survey, writing, and data management; and (3) student engagement in the creation of personalized professional development plans |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L32.00011: QPython as mobile computational physics laboratory anywhere, anytime and anyhow Godfrey Akpojotor, Godwin Ibeh, Famous Akpojotor, Sharon Asegheleme Egboro Computational physics (CP) has become an integral part of physics. However, an important choice in deciding the computational approach is accessibility to both the computing device and the programming language. The continuous growth of smartphones in Africa and the availability of the free and open source Python programming capabilities in QPython (QP) meet the accessibility requirement to make them mobile computational physics laboratory (MCPL). We present here one of our current projects under the Python African Computational Science and Engineering Tour Project (PACSETPro) of using QP as MCPL for teaching CP and development of physics Apps to students, new beginners as well as expert programmers anywhere, anytime and anyhow. We examine the third party libraries especially the future of matplotlib in QP. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L32.00012: Triplet State of a Quantum Dot in a Magnetic Field: A 'Quantal-Newtonian' First Law Study Marlina Slamet, Viraht Sahni The triplet 23S state of a two-electron quantum dot in a |
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