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A sparsity-promoting resolvent analysis for the identification of spatiotemporally-localized amplification mechanisms
No full textThis work introduces a variant of resolvent analysis that identifies forcing and response modes that are sparse in both space and time. This is achieved through the use of a sparse principal component analysis (PCA) algorithm, which formulates the associated optimization problem as a nonlinear eigenproblem that can be solved with an inverse power method. We apply this method to parallel shear flows, both in the case where we assume Fourier modes in time (as in standard resolvent analysis) and obtain spatial localization, and where we allow for temporally-sparse modes through the use of a linearized Navier--Stokes operator discretized in both space and time. Appropriate choice of desired mode sparsity allows for the identification of structures corresponding to high amplification that are localized in both space and time. We report on the similarities and differences between these structures and those from standard methods of analysis. After validating this space-time resolvent analysis on statistically-stationary channel flow, we next implement the methodology on a time-periodic Stokes boundary layer, demonstrating the applicability of the approach to non-statistically-stationary systems
Nonlinear Behavior of IM7 Carbon Fibers in Compression Leads to Bending Nonlinearity of High-Strain Composites
No full textThis paper presents an experimental characterization of carbon fibers under compression and the influence of their nonlinear behavior on the bending nonlinearity of high-strain composites (HSC). The study is focused on HexTow IM7, a pan-based carbon fiber compressed under in-situ scanning electron microscopic (SEM) imaging. The sample preparation and experimental procedure of both single fiber direct compression tests and column bending tests of HSC samples are presented. Nonlinearity at the individual fiber level could not be observed due to the limitations of the load cell in the single fiber direct compression experiment. Using the column bending tests, a shift in the neutral axis due to the nonlinear compression behavior of fibers is observed
Buckling Analysis of a Ladder Deployable Structure Supporting a Prestressed Film
No full textControlling the deflection and distortion of lightweight space structures is crucial for their function in space. In this paper, we consider the distortion of a ladder structure when being connected to a film using prestressed kirigami springs. The ladder structure is made with longitudinal CFRP Triangular Rollable and Collapsible (TRAC) booms braced by battens in the transverse direction due to prestress. This paper shows that the distortion is the result of the global buckling of the ladder structure. Then, the design space of the ladder structure has been explored to study the effects of design variables on the magnitude of shape distortion
Single‐Shot Reconfigurable Femtosecond Imaging of Ultrafast Optical Dynamics
No full textUnderstanding ultrafast dynamics in the femtosecond timescale plays a pivotal role in fundamental research and technology innovation. Spatiotemporal observation of those events in real-time requires imaging speeds greater than 10¹² frames per second (fps), far beyond the fundamental speed limits of the ubiquitous semiconductor sensor technologies. In addition, a majority of femtosecond events are non-repeatable or difficult-to-repeat since they either work in a highly unstable nonlinear regime or require extreme or rare conditions to initiate. Therefore, the traditional pump-probe imaging approach fails since it heavily depends on precise event repetition. Single-shot ultrafast imaging emerges as the only solution; however, existing techniques cannot reach more than 15×10¹² fps, and they only record an insufficient number of frames. Compressed ultrafast spectral photography (CUSP) is proposed to overcome these limitations. Here, CUSP's full design space is explored by manipulating the ultrashort optical pulse in the active illumination. Via parameter optimization, an extraordinarily fast frame rate of 219 × 10¹² fps is achieved. This implementation of CUSP is also highly flexible, allowing various combinations of imaging speeds and numbers of frames (several hundred up to 1000) to be readily deployed in diverse scientific studies, such as laser-induced transient birefringence, self-focusing, and filaments in dielectric media
Single and dual metal atom catalysts for enhanced singlet oxygen generation and oxygen reduction reaction
No full textWe demonstrate rational design of graphene-supported single and dual metal atom catalysts (SACs and DACs) for photocatalytic applications, such as singlet oxygen (¹O₂) sensitization and H₂O₂ production. Here we combine density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations with experimental verifications. We found a synergistic effect between triplet sensitization and triplet–triplet (Dexter) energy transfer; both play a role in the photocatalytic activity through the volcano plot of 3d transition metal SACs. More specifically, FeN₄-SAC exhibits a low ISC energy gap (ΔE_(ISC)) of 0.039 eV, compared with 0.108 eV for FeNiN8-DACs, both possessing a high Bader charge transfer of 0.366 e⁻ and 0.405 e⁻, respectively. Guided by these computational results, we synthesized a series of SACs and a DAC and confirmed their structures with scanning transmission electron microscopy (STEM) along with the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). We then confirm their band structures with low-energy inverse photoemission spectroscopy (LEIPS) and UV-vis-NIR. Subsequently, we synthesized the catalysts for the photooxygenation of anthracene and two-electron oxygen reduction reaction (ORR) to measure their photocatalytic activity. We found that H₂O₂ production through the two-electron ORR competes with the ¹O₂ generation through Dexter energy transfer. FeN₄-SAC demonstrates a high photooxygenation conversion of 86% and a high ¹O₂ quantum yield of 1.04, obtained from electron spin resonance (ESR) spectroscopy, with low H₂O₂ production. In contrast, NiN₄-SAC exhibits a low ¹O₂ generation and a high H₂O₂ production mainly because of the high Gibbs free energy of the OOH* intermediate. This work proposes an effective DFT-guided strategy for designing SACs and DACs for various photocatalytic applications
First Observations of the Brown Dwarf HD 19467 B with JWST
No full textWe observed HD 19467 B with JWST’s NIRCam in six filters spanning 2.5–4.6 μm with the long-wavelength bar coronagraph. The brown dwarf HD 19467 B was initially identified through a long-period trend in the radial velocity of the G3V star HD 19467. HD 19467 B was subsequently detected via coronagraphic imaging and spectroscopy, and characterized as a late-T type brown dwarf with an approximate temperature ∼1000 K. We observed HD 19467 B as a part of the NIRCam GTO science program, demonstrating the first use of the NIRCam Long Wavelength Bar coronagraphic mask. The object was detected in all six filters (contrast levels of 2 × 10⁻⁴ to 2 × 10⁻⁵) at a separation of 1.″6 using angular differential imaging and synthetic reference differential imaging. Due to a guide star failure during the acquisition of a preselected reference star, no reference star data were available for post-processing. However, reference differential imaging was successfully applied using synthetic point-spread functions developed from contemporaneous maps of the telescope’s optical configuration. Additional radial velocity data (from Keck/HIRES) are used to constrain the orbit of HD 19467 B. Photometric data from TESS are used to constrain the properties of the host star, particularly its age. NIRCam photometry, spectra, and photometry from the literature, and improved stellar parameters are used in conjunction with recent spectral and evolutionary substellar models to derive the physical properties of HD 19467 B. Using an age of 9.4 ± 0.9 Gyr inferred from spectroscopy, Gaia astrometry, and TESS asteroseismology, we obtain a model-derived mass of 62 ± 1 M_J, which is consistent within 2σ with the dynamically derived mass of 81₋₁₂⁺¹⁴ M_J
The first occurrence of the carbide anion, C⁴⁻, in an oxide mineral: Mikecoxite, ideally (CHg₄)OCl₂, from the McDermitt open-pit mine, Humboldt County, Nevada, U.S.A.
No full textMikecoxite, ideally (CHg₄)OCl₂, is the first mercury-oxide-chloride-carbide containing a C⁴⁻ anion coordinated by four Hg atoms (a permercurated methane derivative) to be described as a mineral species. It was found at the McDermitt open-pit mine on the eastern margin of the McDermitt Caldera, Humboldt County, Nevada, U.S.A. It is monoclinic, space group P2₁/n, Z = 4; a = 10.164(5), b = 10.490(4), c = 6.547(3) Å, V 698.0(5) ų. Chemical analysis by electron microprobe gave Hg 86.38, Cl 11.58, Br 0.46, C 1.81, sum = 100.23 wt%, and O was detected but the signal was too weak for quantitative chemical analysis. The empirical formula, calculated on the basis of Hg + Cl + Br = 6 apfu, is (C_(1.19)Hg_(3.39))(C_(l2.57)Br_(0.05))_(Σ2.62), and the ideal formula based on the chemical analysis and the crystal structure is (CHg4)OCl2. The seven strongest lines in the X-ray powder diffraction pattern are [d (Å), I, (hkl)]: 2.884, 100, (230); 2.989, 81, (301, 301, 112, 112, 131, 131); 2.673, 79, (122, 122, 212, 212); 1.7443, 40, (060, 432, 432); 5.49, 34, (101, 101); 4.65, 32, (120); 2.300, 30, (312, 312). The Raman spectrum shows three bands at 638, 675, and 704 cm⁻¹, well above the range characteristic of NHg₄ stretching vibrations between 540 and 580 cm⁻¹, that are assigned to CHg₄ stretching vibrations. Mikecoxite forms intergrowths of bladed crystals up to 100 μm long that occur on granular quartz or in vugs associated with kleinite. It is black with a submetallic to metallic luster and strong specular reflections and does not fluoresce under short-or long-wave ultraviolet light. Neither cleavage nor parting were observed, and the calculated density is 8.58 g/cm³. In the crystal structure of mikecoxite, (C⁴⁻Hg₄²⁺) groups link through O²⁻ ions to form three-membered rings that polymerize into corrugated [CHg₄OCl]⁺ layers with near-linear C⁴⁻–Hg²⁺–O and C⁴⁻–Hg²⁺–Cl linkages. The layers link in the third direction directly via weak Hg²⁺–O²⁻ and Hg²⁺–Cl⁻ bonds to adjacent layers and also indirectly via interlayer Cl⁻. A bond-valence parameter has been derived for (Hg²⁺–C⁴⁻) bonds: Rₒ = 2.073 Å, b = 0.37, which gives bond-valence sums at the C⁴⁻ ions in accord with the valence-sum rule. The source of carbon for mikecoxite in the volcanic high-desert environment of the type locality seems to be methane, with the reaction catalyzed by microbiota through full mercuration of carbon atoms, beyond the first stage that produces the volatile and highly mobile methylmercury, [CH₃Hg]⁺, a potent neurotoxin that accumulates in marine food chains. Both the mineral and the mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2021-060). The mineral is named after Michael F. Cox (b. 1958), a founding member of the New Almaden Quicksilver County Park Association (NAQCPA) who was responsible for characterizing and remediating environmental mercury on-site and who recovered the rock containing the new mineral
α-NiO/Ni(OH)₂/AgNP/F-Graphene Composite for Energy Storage Application
No full textThe α-NiO/Ni(OH)₂/AgNP/F-graphene composite, which is silver nanoparticles preanchored on the surface of fluorinated graphene (AgNP/FG) and then added to α-NiO/Ni(OH)₂, is investigated as a potential battery material. The addition of AgNP/FG endows the electrochemical redox reaction of α-NiO/Ni(OH)₂ with a synergistic effect, resulting in enhanced Faradaic efficiency with the redox reactions of silver accompanied by the OER and the ORR. It resulted in enhanced specific capacitance (F g⁻¹) and capacity (mA h g⁻¹). The specific capacitance of α-NiO/Ni(OH)₂ increased from 148 to 356 F g⁻¹ with the addition of AgNP(20)/FG, while it increased to 226 F g⁻¹ with the addition of AgNPs alone without F-graphene. The specific capacitance of α-NiO/Ni(OH)₂/AgNP(20)/FG further increased up to 1153 F g⁻¹ with a change in the voltage scan rate from 20 to 5 mV/s and the Nafion-free α-NiO/Ni(OH)₂/AgNP(20)/FG composite. In a similar trend, the specific capacity of α-NiO/Ni(OH)₂ increased from 266 to 545 mA h g⁻¹ by the addition of AgNP(20)/FG. The performance of hybrid Zn–Ni/Ag/air electrochemical reactions by α-NiO/Ni(OH)₂/AgNP(200)/FG and Zn-coupled electrodes indicates a potential for a secondary battery. It results in a specific capacity of 1200 mA h g⁻¹ and a specific energy of 660 W h kg⁻¹, which is divided into Zn–Ni reactions of ∼95 W h kg⁻¹ and Zn–Ag/air reactions of ∼420 W h kg⁻¹, while undergoing a Zn–air reaction of ∼145 W h kg⁻¹
Dimer model and holomorphic functions on t‐embeddings of planar graphs
No full textWe introduce the framework of discrete holomorphic functions on t-embeddings of weighted bipartite planar graphs; t-embeddings also appeared under the name Coulomb gauges in a recent paper (Kenyon, Lam, Ramassamy, and Russkikh, Dimers and circle patterns, 2018). We argue that this framework is particularly relevant for the analysis of scaling limits of the height fluctuations in the corresponding dimer models. In particular, it unifies both Kenyon's interpretation of dimer observables as derivatives of harmonic functions on T-graphs and the notion of s-holomorphic functions originated in Smirnov's work on the critical Ising model. We develop an a priori regularity theory for such functions and provide a meta-theorem on convergence of the height fluctuations to the Gaussian Free Field. We also discuss how several more standard discretizations of complex analysis fit this general framework
Tracing energy inputs into the seafloor using carbonate sediments
No full textCarbonate rocks provide unique and valuable sedimentary archives for secular changes in Earth’s physical, chemical, and biological processes. However, reading the stratigraphic record produces overlapping, nonunique interpretations that stem from the difficulty in directly comparing competing biological, physical, or chemical mechanisms within a common quantitative framework. We built a mathematical model that decomposes these processes and casts the marine carbonate record in terms of energy fluxes across the sediment–water interface. Results showed that physical, chemical, and biological energy terms across the seafloor are subequal and that the energetic dominance of different processes varies both as a function of environment (e.g., onshore vs. offshore) as well as with time-varying changes in seawater chemistry and with evolutionary changes in animal abundance and behavior. We applied our model to observations from the end-Permian mass extinction—a massive upheaval in ocean chemistry and biology—revealing an energetic equivalence between two hypothesized drivers of changing carbonate environments: a reduction in physical bioturbation increased carbonate saturation states in the oceans. Early Triassic occurrences of ‘anachronistic’ carbonates—facies largely absent from marine environments after the Early Paleozoic—were likely driven more by reduction in animal biomass than by repeated perturbations to seawater chemistry. This analysis highlighted the importance of animals and their evolutionary history in physically shaping patterns in the sedimentary record via their impact on the energetics of marine environments