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Caltech Authors (170.931 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 2.259

  1. Erosional surfaces in the Upper Cretaceous Castlegate Sandstone (Utah, USA): Sequence boundaries or autogenic scour from backwater hydrodynamics?

    Trower, Elizabeth J.; Ganti, Vamsi; Fischer, Woodward W.; Lamb, Michael P.
    Sequence stratigraphy relies on the identification of unconformity-bound sedimentary packages in order to understand variations in sediment supply, subsidence, and eustasy, which are themselves controlled by external (allogenic) drivers such as climate and tectonics. However, intrinsic (autogenic) river dynamics can also create a rich stratigraphic architecture in the absence of allogenic changes. Here, we outline scaling relationships for the expected depth and length scales of autogenic scour resulting from non-uniform flows in coastal rivers, and apply these relationships to the Upper Cretaceous Castlegate Sandstone—a classic fluvial sandstone unit in the Book Cliffs of Utah (USA). Theoretical and experimental work suggests...

  2. Community seismic network and localized earthquake situational awareness

    Kohler, M. D.; Guy, R.; Bunn, J.; Massari, A.; Clayton, R.; Heaton, T.; Chandy, K. M.; Ebrahimian, H.; Dorn, C.
    Community-hosted seismic networks are a solution to the need for large numbers of sensors to operate over a seismically active region in order to accurately measure the size and location of an earthquake, assess resulting damage, and provide alerts. The Community Seismic Network is one such strong-motion network, currently comprising hundreds of elements located in California. It consists of low-cost, three-component, MEMS accelerometers capable of recording accelerations up to twice the level of gravity. The primary product of the network is to produce measurements of shaking of the ground and multiple locations of every upper floor in buildings, in the...

  3. Freeze-Cast Alumina Pore Networks: Effects of Processing Parameters in Steady-State Solidification Regimes of Aqueous Slurries

    Miller, S. M.; Xiao, X.; Setlock, J. A.; Faber, K. T.
    Aqueous alumina slurries with varying solids loading and particle size were freeze-cast under seven freezing conditions to investigate the influence of these on pore network characteristics including pore size and geometric specific surface area. Slurry temperatures were recorded in situ to determine freezing front position and velocity during solidification, which were then analyzed via regression and modeled using solidification theory. Classic mathematical models for the time dependence of freezing front position and velocity were found to hold for freeze-cast slurries. Building on these, a one-phase Stefan problem was used to describe freezing kinetics. Models for freezing front velocity were combined...

  4. Dense Freeze‐cast Li_7La_3Zr_2O_(12) Solid Electrolytes with Oriented Open Porosity and Contiguous Ceramic Scaffold

    Buannic, Lucienne; Naviroj, Maninpat; Miller, Sarah M.; Zagorski, Jakub; Faber, Katherine T.; Llordés, Anna
    Freeze casting is used for the first time to prepare solid electrolyte scaffolds with oriented porosity and dense ceramic walls made of Li_7La_3Zr_2O_(12) (LLZO), one of the most promising candidates for solid state battery electrolytes. Processing parameters ‐ such as solvent solidification rate, solvent type, and ceramic particle size ‐ are investigated, focusing on their influence on porosity and ceramic wall density. Dendrite‐like porosity is obtained when using cyclohexane and dioxane as solvents. Lamellar porosity is observed in aqueous slurries resulting in a structure with the highest apparent porosity and densest ceramic scaffold but weakest mechanical properties due to the...

  5. Optimizing the structure and movement of a robotic bat with biological kinematic synergies

    Hoff, Jonathan; Ramezani, Alireza; Chung, Soon-Jo; Hutchinson, Seth
    In this paper, we present methods to optimize the design and flight characteristics of a biologically-inspired bat-like robot. In previous, work we have designed the topological structure for the wing kinematics of this robot; here we present methods to optimize the geometry of this structure, and to compute actuator trajectories such that its wingbeat pattern closely matches biological counterparts. Our approach is motivated by recent studies on biological bat flight which have shown that the salient aspects of wing motion can be accurately represented in a low-dimensional space. Although bats have over 40 Degrees of Freedom (DoFs), our robot possesses...

  6. Kinetics and Product Yields of the OH Initiated Oxidation of Hydroxymethyl Hydroperoxide

    Allen, Hannah M.; Crounse, John D.; Bates, Kelvin H.; Teng, Alexander Paichung; Krawiec-Thayer, Mitchell P.; Rivera-Rios, Jean C.; Keutsch, Frank N.; St. Clair, Jason M.; Hanisco, Thomas F.; Møller, Kristian H.; Kjaergaard, Henrik G.; Wennberg, Paul O.
    Hydroxymethyl hydroperoxide (HMHP), formed in the reaction of the C_1 Criegee intermediate with water, is among the most abundant organic peroxides in the atmosphere. Although reaction with OH is thought to represent one of the most important atmospheric removal processes for HMHP, this reaction has been largely unstudied in the laboratory. Here, we present measurements of the kinetics and products formed in the reaction of HMHP with OH. HMHP was oxidized by OH in an environmental chamber; the decay of the hydroperoxide and the formation of formic acid and formaldehyde were monitored over time using CF_3O^- chemical ionization mass spectrometry...

  7. In situ observations of phase changes in shock compressed forsterite

    Newman, M. G.; Kraus, R. G.; Akin, M. C.; Bernier, J. V.; Dillman, A. M.; Homel, M. A.; Lee, S.; Lind, J.; Mosenfelder, J. L.; Pagan, D. C.; Sinclair, N. W.; Asimow, P. D.
    Shockwave data on mineral‐forming compounds such as Mg2SiO4 are essential for understanding the interiors of Earth and other planets, but correct interpretation of these data depend on knowing the phase assemblage being probed at high pressure. Hence direct observations of the phase or phases making up the measured states along the forsterite Hugoniot are essential to assess whether kinetic factors inhibit the achievement of the expected equilibrium, phase‐separated assemblage. Previous shock recovery experiments on forsterite, which has orthorhombic space group Pbnm, show discrepant results as to whether forsterite undergoes segregation into its equilibrium phase assemblage of compositionally distinct structures upon...

  8. Convergent evolution of unusual complex I homologs with increased proton pumping capacity: energetic and ecological implications

    Chadwick, Grayson L.; Hemp, James; Fischer, Woodward W.; Orphan, Victoria J.
    Respiratory complex I is part of a large family of homologous enzymes that carry out the transfer of electrons between soluble cytoplasmic electron carriers and membrane-bound electron carriers. These complexes are vital bioenergetic enzymes that serve as the entry points into electron transport chains for a wide variety of microbial metabolisms, and electron transfer is coupled to proton translocation. The core complex of this enzyme is made up of 11 protein subunits, with three major proton pumping subunits. Here, we document a large number of modified complex I gene cassettes found in genome sequences from diverse cultured bacteria, shotgun metagenomics,...

  9. Locating Si atoms in Si-Doped Boron Carbide: a Route to Understand Amorphization Mitigation Mechanism

    Khan, Atta U.; Etzold, Anthony M.; Yang, Xiaokun; Domnich, Vladislav; Xie, Kelvin Y.; Hwang, Chawon; Behler, Kristopher D.; Chen, Mingwei; An, Qi; Lasalvia, Jerry; Hemker, Kevin J.; Goddard, William A., III; Haber, Richard A.
    The well-documented formation of amorphous bands in boron carbide (B_4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B_(12) icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide is produced through a conventional scalable route. A powder mixture of SiB_6, B_4C, and amorphous boron is reactively sintered, yielding a dense Si-doped boron carbide...

  10. Estimation of random coefficients logit demand models with interactive fixed effects

    Moon, Hyungsik Roger; Shum, Matthew; Weidner, Martin
    We extend the Berry, Levinsohn and Pakes (BLP, 1995) random coefficients discrete-choice demand model, which underlies much recent empirical work in IO. We add interactive fixed effects in the form of a factor structure on the unobserved product characteristics. The interactive fixed effects can be arbitrarily correlated with the observed product characteristics (including price), which accommodates endogeneity and, at the same time, captures strong persistence in market shares across products and markets. We propose a two-step least squares-minimum distance (LS-MD) procedure to calculate the estimator. Our estimator is easy to compute, and Monte Carlo simulations show that it performs well....

  11. Input-to-State Safety with Control Barrier Functions

    Kolathaya, Shishir; Ames, Aaron D.
    This letter presents a new notion of input-to-state safe control barrier functions (ISSf-CBFs), which ensure safety of nonlinear dynamical systems under input disturbances. Similar to how safety conditions are specified in terms of forward invariance of a set, input-to-state safety (ISSf) conditions are specified in terms of forward invariance of a slightly larger set. In this context, invariance of the larger set implies that the states stay either inside or very close to the smaller safe set; and this closeness is bounded by the magnitude of the disturbances. The main contribution of the letter is the methodology used for obtaining...

  12. Direct Collocation for Dynamic Behaviors with Nonprehensile Contacts: Application to Flipping Burgers

    Kolathaya, Shishir; Guffey, William; Sinnet, Ryan W.; Ames, Aaron D.
    To realize robotic systems in real-world settings, e.g., in restaurants, it will be necessary to achieve dynamic manipulation of non-trivial objects. In this context, this paper discusses methodologies used to realize trajectories in a robotic arm platform; specifically, applied to flipping burgers as an example of nonprehensile object manipulation. Flipping of burgers involves a series of tasks—going to the burger location, scooping, picking up and flipping. Since the goal is to obtain these trajectories in a reasonably fast manner, we employ direct collocation based multi-segmented trajectory optimization. We will first describe the problem setup, and then describe the constraints, decision...

  13. Assessing Wrist Movement with Robotic Devices

    Rose, Chad G.; Pezent, Evan; Kann, Claudia K.; Deshpande, Ashish D.; O'Malley, Marcia K.
    Robotic devices have been proposed to meet the rising need for high intensity, long duration, and goal-oriented therapy required to regain motor function after neurological injury. Complementing this application, exoskeletons can augment traditional clinical assessments through precise, repeatable measurements of joint angles and movement quality. These measures assume that exoskeletons are making accurate joint measurements with a negligible effect on movement. For the coupled and coordinated joints of the wrist and hand, the validity of these two assumptions cannot be established by characterizing the device in isolation. To examine these assumptions, we conducted three user-in-the-loop experiments with able-bodied participants. First,...

  14. The influence of CO_2 forcing on North American monsoon moisture surges

    Pascale, Salvatore; Kapnick, Sarah B.; Bordoni, Simona; Delworth, Thomas L.
    Widespread multiday convective bursts in the southwestern United States during the North American monsoon are often triggered by Gulf of California moisture surges (GoC surges). However, how GoC surges, and the amount and intensity of associated precipitation, will change in response to CO_2-induced warming remains little known, not least because the most widely available climate models do not currently resolve the relevant mesoscale dynamics due to their coarse resolution (100 km or more). In this study, a 50-km resolution global coupled model (FLOR) is used to address this question. It is found that the mean number of GoC surge events...

  15. Metathesis and Decomposition of Fischer Carbenes of Cyclometalated Z-Selective Ruthenium Metathesis Catalysts

    Ahmed, Tonia S.; Grandner, Jessica M.; Taylor, Buck L. H.; Herbert, Myles B.; Houk, K. N.; Grubbs, Robert H.
    The addition of vinyl ethers to Z-selective, cyclometalated ruthenium metathesis catalysts generates Fischer carbene complexes. Although Fischer carbenes are usually thought to be metathesis inactive, we show that Fischer carbenes are metathesis active under certain circumstances. These species were found to decompose facilely to Ru hydride complexes, as identified by both experiment and computation. Since vinyl ethers are often used to quench metathesis reactions implementing Ru-based metathesis catalysts, their decomposition to hydrides can have a deleterious effect on the desired stereochemistry of the olefin product.

  16. Characterization of hydrogen in basaltic materials with laser‐induced breakdown spectroscopy (LIBS) for application to MSL ChemCam data

    Thomas, N. H.; Ehlmann, B. L.; Anderson, D. E.; Clegg, S. M.; Forni, O.; Schröder, S.; Rapin, W.; Meslin, P.-Y.; Lasue, J.; Delapp, D. M.; Dyar, M. D.; Gasnault, O.; Wiens, R. C.; Maurice, S.
    The Mars Science Laboratory rover, Curiosity, is equipped with ChemCam, a Laser‐Induced Breakdown Spectroscopy (LIBS) instrument, to determine the elemental composition of nearby targets quickly and remotely. We use a laboratory sample set including prepared mixtures of basalt with systematic variation in hydrated mineral content and compositionally well‐characterized, altered basaltic volcanic rocks to measure hydrogen by characterizing the H‐alpha emission line in LIBS spectra under martian environmental conditions. The H contents of all samples were independently measured using thermogravimetric analysis. We found that H peak area increases with weight percent H for our laboratory mixtures with basaltic matrices. The increase...

  17. Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

    Moreno-Hernandez, Ivan A.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO_x) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO_x layer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm^(−2) of simulated sunlight. The SnO_x layer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0 m KOH(aq) or 1.0 m H_2SO_4(aq). Ideal regenerative solar‐to‐O_2(g) efficiencies of 4.1% and 3.7%, respectively, are obtained in...

  18. The planet formation imager

    Monnier, John D.; Vahala, Kerry
    The Planet Formation Imager (PFI, www.planetformationimager.org) is a next-generation infrared interferometer array with the primary goal of imaging the active phases of planet formation in nearby star forming regions. PFI will be sensitive to warm dust emission using mid-infrared capabilities made possible by precise fringe tracking in the near-infrared. An L/M band combiner will be especially sensitive to thermal emission from young exoplanets (and their disks) with a high spectral resolution mode to probe the kinematics of CO and H_2O gas. In this paper, we give an overview of the main science goals of PFI, define a baseline PFI architecture...

  19. An Allosteric Inhibitor of KRas Identified Using a Barcoded Rapid Assay Microchip Platform

    McCarthy, Amy Michelle; Kim, Jungwoo; Museth, A. Katrine; Henning, Ryan K.; Heath, John E.; Winson, Emma; Oh, Joseph J.; Liang, Jingxin; Hong, Sunga; Heath, James R.
    Protein catalyzed capture (PCC) agents are synthetic antibody surrogates that can target a wide variety of biologi-cally relevant proteins. As a step towards developing a high-throughput PCC pipeline we report on the preparation of a barcoded rapid assay platform, used here for the analysis of hits from an in situ click screen of a macrocycle peptide library against allosteric epitopes of the KRas protein. The platform utilizes patterned, micrometer scale barcodes composed of orthogonal ssDNA strands on a glass slide. The slide is partitioned into microwells, each of which contains multiple copies of the full barcode. Biotinylated candidate PCCs from...

  20. Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water

    Eugene, Alexis J.; Pillar, Elizabeth A.; Colussi, Agustín J.; Guzmán, Marcelo I.
    Understanding the acid-base behavior of carboxylic acids on aqueous interfaces is a fundamental issue in Nature. Surface processes involving carboxylic acids such as acetic and pyruvic acids play roles in 1) the transport of nutrients through cell membranes, 2) the cycling of metabolites relevant to the origin of life, and 3) the photooxidative processing of biogenic and anthropogenic emissions in aerosols and atmospheric waters. Here we report that 50% of gaseous acetic acid and pyruvic acid molecules transfer a proton to the surface of water at pH 2.8 and 1.8 units lower than their respective acidity constants pKa = 4.6...

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