Browsing by Subject "diffusion"

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A Diffusion Theory for Stochastic Storage Analysis
(University of Texas at Austin, 1988-03) Buchberger, S.G.; Maidment, D.R.
Anomalous structure and dynamics of the Gaussian-core fluid
(2009-04) Krekelberg, William P.; Kumar, Tanuj; Mittal, Jeetain; Errington, Jeffrey R.; Truskett, Thomas M.; Krekelberg, William P.; Kumar, Tanuj; Truskett, Thomas M.
It is known that there are thermodynamic states for which the Gaussian-core fluid displays anomalous properties such as expansion upon isobaric cooling (density anomaly) and increased single-particle mobility upon isothermal compression (self-diffusivity anomaly). Here, we investigate how temperature and density affect its short-range translational structural order, as characterized by the two-body excess entropy. We find that there is a wide range of conditions for which the short-range translational order of the Gaussian-core fluid decreases upon isothermal compression (structural order anomaly). As we show, the origin of the structural anomaly is qualitatively similar to that of other anomalous fluids (e.g., water or colloids with short-range attractions) and is connected to how compression affects static correlations at different length scales. Interestingly, we find that the self-diffusivity of the Gaussian-core fluid obeys a scaling relationship with the two-body excess entropy that is very similar to the one observed for a variety of simple liquids. One consequence of this relationship is that the state points for which structural, self-diffusivity, and density anomalies of the Gaussian-core fluid occur appear as cascading regions on the temperature-density plane; a phenomenon observed earlier for models of waterlike fluids. There are, however, key differences between the anomalies of Gaussian-core and waterlike fluids, and we discuss how those can be qualitatively understood by considering the respective interparticle potentials of these models. Finally, we note that the self-diffusivity of the Gaussian-core fluid obeys different scaling laws depending on whether the two-body or total excess entropy is considered. This finding, which deserves more comprehensive future study, appears to underscore the significance of higher-body correlations for the behavior of fluids with bounded interactions.
Antibody-Independent Isolation of Circulating Tumor Cells by Continuous-Flow Dielectrophoresis
(2013-01) Shim, Sangjo; Stemke-Hale, Katherine; Tsimberidou, Apostolia M.; Noshari, Jamileh; Anderson, Thomas E.; Gascoyne, Peter R. C.; Shim, Sangjo; Noshari, Jamileh; Anderson, Thomas E.; Gascoyne, Peter R. C.
Circulating tumor cells (CTCs) are prognostic markers for the recurrence of cancer and may carry molecular information relevant to cancer diagnosis. Dielectrophoresis (DEP) has been proposed as a molecular marker-independent approach for isolating CTCs from blood and has been shown to be broadly applicable to different types of cancers. However, existing batch-mode microfluidic DEP methods have been unable to process 10 ml clinical blood specimens rapidly enough. To achieve the required processing rates of 106 nucleated cells/min, we describe a continuous flow microfluidic processing chamber into which the peripheral blood mononuclear cell fraction of a clinical specimen is slowly injected, deionized by diffusion, and then subjected to a balance of DEP, sedimentation and hydrodynamic lift forces. These forces cause tumor cells to be transported close to the floor of the chamber, while blood cells are carried about three cell diameters above them. The tumor cells are isolated by skimming them from the bottom of the chamber while the blood cells flow to waste. The principles, design, and modeling of the continuous-flow system are presented. To illustrate operation of the technology, we demonstrate the isolation of circulating colon tumor cells from clinical specimens and verify the tumor origin of these cells by molecular analysis. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774304]
Broadband boundary effects on Brownian motion
(2015-12) Mo, Jianyong; Simha, Akarsh; Raizen, Mark G.; Mo, Jianyong; Simha, Akarsh; Raizen, Mark G.
Brownian motion of particles in confined fluids is important for many applications, yet the effects of the boundary over a wide range of time scales are still not well understood. We report high-bandwidth, comprehensive measurements of Brownian motion of an optically trapped micrometer-sized silica sphere in water near an approximately flat wall. At short distances we observe anisotropic Brownian motion with respect to the wall. We find that surface confinement not only occurs in the long time scale diffusive regime but also in the short time scale ballistic regime, and the velocity autocorrelation function of the Brownian particle decays faster than that of a particle in bulk fluid. Furthermore, at low frequencies the thermal force loses its color due to the reflected flow from the no-slip boundary. The power spectrum of the thermal force on the particle near a no-slip boundary becomes flat at low frequencies. This detailed understanding of boundary effects on Brownian motion opens a door to developing a 3D microscope using particles as remote sensors.
Chandra Grating Spectroscopy Of Three Hot White Dwarfs
(2012-10) Adamczak, J.; Werner, K.; Rauch, T.; Schuh, S.; Drake, J. J.; Kruk, J. W.; Adamczak, J.
Context. High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T-eff = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG 1159-035) defines the location of the blue edge of the GWVir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models. Such models are therefore reliable tools to investigate the interior structure of GW Vir variables. Conclusions. Our soft X-ray study reveals that the understanding of metal abundances in hot DA white dwarf atmospheres is still incomplete. On the other hand, model atmospheres of hydrogen-deficient PG 1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.
Chaos in the honeycomb optical-lattice unit cell
(2016-01) Porter, Max D.; Reichl, L. E.; Porter, Max D.; Reichl, L. E.
Natural and artificial honeycomb lattices are of great interest because the band structure of these lattices, if properly constructed, contains a Dirac point. Such lattices occur naturally in the form of graphene and carbon nanotubes. They have been created in the laboratory in the form of semiconductor 2DEGs, optical lattices, and photonic crystals. We show that, over a wide energy range, gases (of electrons, atoms, or photons) that propagate through these lattices are Lorentz gases and the corresponding classical dynamics is chaotic. Thus honeycomb lattices are also of interest for understanding eigenstate thermalization and the conductor-insulator transition due to dynamic Anderson localization.
Chaotic dynamics in a two-dimensional optical lattice
(2014-01) Horsley, Eric; Koppell, Stewart; Reichl, L. E.; Horsley, Eric; Koppell, Stewart; Reichl, L. E.
The classical nonlinear dynamics of a dilute gas of rubidium atoms in an optical lattice is studied for a range of polarizations of the laser beams forming the lattice. The dynamics ranges from integrable to chaotic, and mechanisms leading to the onset of chaos in the lattice are described.
Chemical Vapour Deposition of Amorphous Ru(P) Thin Films from Ru Trialkylphosphite Hydride Complexes
(2012-09) McCarty, W. Jeffrey; Yang, Xiaoping; Anderson, Lauren J. D.; Jones, Richard A.; McCarty, W. Jeffrey; Yang, Xiaoping; Anderson, Lauren J. D.; Jones, Richard A.
The ruthenium phosphite hydride complexes H2Ru(P(OR)(3))(4) (R = Me (1), Et (2), Pr-i (3)) were used as CVD precursors for the deposition of films of amorphous ruthenium-phosphorus alloys. The as-deposited films were X-ray amorphous and XPS analysis revealed that they were predominantly comprised of Ru and P in zero oxidation states. XPS analysis also showed the presence of small amounts of oxidized ruthenium and phosphorus. The composition of the films was found to depend on ligand chemistry as well as the deposition conditions. The use of H-2 as the carrier gas had the effect of increasing the relative concentrations of P and O for all films. Annealing films to 700 degrees C under vacuum produced films of polycrystalline hcp Ru while a flowing stream of H-2 resulted in polycrystalline hcp RuP.
Commitment and Diffusion: How and Why National Constitutions Incorporate International Law
(2008) Elkins, Zachary; Ginsburg, Tom; Chernykh, Svitlana
Drafters of new constitutions face a bewildering array of choices as they seek to design stable and workable political institutions for their societies. One such set of choices concerns the status of interna- tional law in the domestic legal order. In a global era, with an ex- panding array of customary and treaty norms purporting to regulate formerly domestic behavior, this question takes on political salience. This paper seeks to describe the phenomenon of constitutional incor- poration of international law in greater detail and provide a prelimi- nary empirical test of the competing explanations for it. First, the dis- cussion focuses on the concepts of monism and dualism, which have become conventional terms used by lawyers to describe the interac- tion of domestic and international legal systems. Second, a theory of commitments as well as the advantages and disadvantages of interna- tional law are set forth. Third, empirical implications are developed for the precommitment and diffusion theories, which are then tested. Findings show that adopting international law is a useful strategy for democracies to lock in particular policies, encourage trust in govern- ments and state regimes, and bolster global reputations.
Constraining the Surface Inhomogeneity and Settling Times of Metals on Accreting White Dwarfs
(2008-10) Montgomery, M. H.; Thompson, S. E.; von Hippel, Ted; Montgomery, M. H.; von Hippel, T.
Due to the short settling times of metals in DA white dwarf atmospheres, any white dwarfs with photospheric metals must be actively accreting. It is therefore natural to expect that the metals may not be deposited uniformly on the surface of the star. We present calculations showing how the temperature variations associated with white dwarf pulsations lead to an observable diagnostic of the surface metal distribution, and we show what constraints current data sets are able to provide. We also investigate the effect that time-variable accretion has on the metal abundances of different species, and we show how this can lead to constraints on the gravitational settling times.
CS 22964-161: A Double-Lined Carbon- And S-Process-Enhanced Metal-Poor Binary Star
(2008-04) Thompson, Ian B.; Ivans, , Inese I.; Bisterzo, Sara; Sneden, Christopher; Gallino, Roberto; Vauclair, Sylvie; Burley, Gregory S.; Shectman, Stephen A.; Preston, George W.; Sneden, Christopher
A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low-metallicity Galactic halo object CS 22964-161. We have discovered that CS 22964-161 is a double-lined spectroscopic binary and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the "Spite plateau.'' The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964-161 abundance set with nucleosynthesis predictions of AGB stars, discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system.
Customization Or Conformity? An Institutional And Network Perspective On The Content And Consequences Of TQM Adoption
(1997-06) Westphal, J. D.; Gulati, R.; Shortell, S. M.; Westphal, James D.
This study develops a theoretical framework that integrates institutional and network perspectives on the form and consequences of administrative innovations. Hypotheses are tested with survey and archival data on the implementation of total quality management (TQM) programs and the consequences for organizational efficiency and legitimacy in a sample of over 2,700 U.S. hospitals. The results show that early adopters customize TOM practices for efficiency gains, while later adopters gain legitimacy from adopting the normative form of TQM programs. The findings suggest that institutional factors moderate the role of network membership in affecting the form of administrative innovations adopted and provide strong evidence for the importance of institutional factors in determining how innovations are defined and implemented. We discuss implications for theory and research on institutional processes and network effects and for the literatures on innovation adoption and total quality management.(.)
Diffusion and the Constitutionalization of Europe
(Comparative Political Studies, 2010) Elkins, Zachary
This article begins with a rather forceful defense of the explanatory role of formal institutions—and, in particular, constitutions—in the study of democratization. Important aspects of constitutions play a significant part in shaping the quality, type, and survival of institutional arrangements in new democracies. With this assumption, the article turns seriously to theories of constitutional design, any of which must grapple with the overwhelming prima facie evidence of constitutional diffusion. It is well known that constitutional ideas travel easily across contexts. However, scholars until now have lacked even basic empirical evidence regarding the patterns of constitutional similarity across time and space. This article introduces exactly this sort of evidence in the context of 19th-century Europe, employing a new data set expressly designed for such a purpose. The analysis uncovers a number of new insights regarding the spread of constitutional ideas in Europe, insights that disturb some of the classic narratives of democratization in these cases.
Diffusion is no Illusion: Neighbor Emulation in the Third Wave of Democracy
(Comparative Political Studies, 2006) Brinks, Daniel M; Coppedge, Michael
This article develops and tests a specific model of the role of diffusion as a determinant of the magnitude and direction of regime change, using a database covering the world from 1972 to 1996. The authors find that countries tend to change their regimes to match the average degree of democracy or nondemocracy found among their contiguous neighbors and that countries in the U.S. sphere of influence tended to become more democratic in the period examined. They also confirm that countries tend to follow the direction in which the majority of other countries in the world are moving. Their model builds on several findings in the diffusion literature but adds methodological improvements and includes more extensive controls for other variables that have been found to affect regime change—including levels of development, presidentialism, and regional differences—offering further support for some and challenging other findings of the regime change literature.
Equilibrium and non-equilibrium molecular absorption: A study of the Ising Model and the infinite parking limit problem
(2012-12-07) Beckman, Erin; John Stanton; Lorenzo Sadun
Randomness is present in so many everyday systems that we often forget its importance in both mathematical and chemical situations. Chemical reactions depend on random interactions and collisions, the movement of particles is often randomly determined, and randomness plays a role in the way in which diffusing particles interact with a solid surface. Looking specifically into this last situation, we know that there are many ways in which diffusing particles can interact with solids. Particles can diff use through either water or air, and at low concentrations, this process is well-modeled by random processes. When looking at this situation, there are two distinct types of molecular absorption to consider: equilibrium or nonequilibrium absorption. That is, when the diffusing particle comes into contact with the surface, it can either stick exactly where it lands or move around a little to come to a more stable equilibrium arrangement. These processes might look very similar on the macroscopic level, but on the microscopic level, they are studied using very different mathematical techniques. Sometimes the process is modeled by a simpler problem in order to use more rigorous mathematics to address things which are very complicated. Other times, it is more beneficial to solve a problem computationally with programming. These different approaches each have benefits and are both used in comprehensive studies of natural processes.To get a feel for these two different ways of studying this problem, we will examine two different models: an equilibrium model and a non-equilibrium model. One of the most common models used to study equilibrium systems is the Ising model. We start by thinking of the Ising model as a infinite system with boundary conditions, and then we consider the limit as the size of the grid goes to infinity. The value of the spin at each lattice site depends on two different things-the values of the neighboring lattice sites and any applied external magnetic fields. The contributions of each of these pieces depends on the system and is controlled by constants. It was originally proposed by Ising to model the spontaneous magnetization in ferromagnetic substances. The restriction on the spin states and the ordered pattern of the grid points allow this model to be analyzed mathematically, while still giving interesting information about the behavior of many different chemical systems. As the number of points in the grid goes to infinity, we use this model to look at phase transitions, switches between one defined state and another.
Fine Grid Asteroseismology Of G117-B15A And R548
(2008-03) Bischoff-Kim, Agnes; Montgomery, Michael H.; Winget, D. E.; Bischoff-Kim, Agnes; Montgomery, Michael H.; Winget, D. E.
We now have a good measurement of the cooling rate of G117-B15A. In the near future, we will have equally well determined cooling rates for other pulsating white dwarfs, including R548. The ability to measure their cooling rates offers us a unique way to study weakly interacting particles that would contribute to their cooling. Working toward that goal, we perform a careful asteroseismological analysis of G117-B15A and R548. We study them side by side because they have similar observed properties. We carry out a systematic, fine grid search for best-fit models to the observed period spectra of those stars. We freely vary four parameters: the effective temperature, the stellar mass, the helium layer mass, and the hydrogen layer mass. We identify and quantify a number of uncertainties associated with our models. Based on the results of that analysis and fits to the periods observed in R548 and G117-B15A, we clearly define the regions of the four-dimensional parameter space occupied by the best-fit models.
Generalized Rosenfeld scalings for tracer diffusivities in not-so-simple fluids: Mixtures and soft particles
(2009-12) Krekelberg, William P.; Pond, Mark J.; Goel, Gaurav; Shen, Vincent K.; Errington, Jeffrey R.; Truskett, Thomas M.; Krekelberg, William P.; Pond, Mark J.; Goel, Gaurav; Truskett, Thomas M.
Rosenfeld [Phys. Rev. A 15, 2545 (1977)] originally noticed that casting the transport coefficients of simple monatomic equilibrium fluids in a specific dimensionless form makes them approximately single-valued functions of excess entropy. This observation has predictive value because, while the transport coefficients of dense fluids can be difficult to estimate from first principles, the excess entropy can often be accurately predicted from liquid-state theory. In this work, we use molecular simulations to investigate whether Rosenfeld's observation is a special case of a more general scaling law relating the tracer diffusivities of particles in mixtures to the excess entropy. Specifically, we study the tracer diffusivities, static structure, and thermodynamic properties of a variety of one- and two-component model fluid systems with either additive or nonadditive interactions of the hard-sphere or Gaussian-core form. The results of the simulations demonstrate that the effects of mixture concentration and composition, particle-size asymmetry and additivity, and strength of the interparticle interactions in these fluids are consistent with an empirical scaling law relating the excess entropy to a dimensionless (generalized Rosenfeld) form of tracer diffusivity, which we introduce here. The dimensionless form of the tracer diffusivity follows from knowledge of the intermolecular potential and the transport/thermodynamic behavior of fluids in the dilute limit. The generalized Rosenfeld scaling requires less information and provides more accurate predictions than either Enskog theory or scalings based on the pair-correlation contribution to the excess entropy. As we show, however, it also suffers from some limitations especially for systems that exhibit significant decoupling of individual component tracer diffusivities.
Global Competition, Institutions, And The Diffusion Of Organizational Practices: The International Spread Of Iso 9000 Quality Certificates
(2002-06) Guler, I.; Guillen, M. F.; MacPherson, J. M.; Macpherson, John Muir
We use panel data on ISO 9000 quality certification in 85 countries between 1993 and 1998 to better understand, the cross-national diffusion of an organizational practice. Following neoinstitutional theory, we focus on the coercive, normative, and mimetic effects that result from the exposure of firms in a given country to a powerful source of critical resources, a common pool of relevant technical knowledge, and the experiences of firms located in other countries. We use social network theory to develop a systematic conceptual understanding of how firms located in different countries influence each other's rates of adoption as a result of cohesive and equivalent network relationships. Regression results provide support for our predictions that states and foreign multinationals are the key actors responsible for coercive isomorphism, cohesive trade relationships between countries generate coercive and normative effects, and role-equivalent trade relationships result in learning-based and competitive imitation.
HE 1327-2326, An Unevolved Star With Fe/H <-5.0. III. Does Its Atmosphere Reflect Its Natal Composition?
(2009-06) Korn, A. J.; Richard, O.; Mashonkina, L.; Bessell, Michael S.; Frebel, Anna; Aoki, Wako; Frebel, Anna
Based on spectroscopic constraints derived from nonlocal thermodynamic equilibrium line formation, we explore the likely range of stellar parameters (T-eff and log g) for the hyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmer line profiles and the Ca I/II ionization equilibrium, a subgiant stage of evolution is indicated. This result is further supported by spectrophotometric observations of the Balmer jump. If a higher T-eff value was used (as favored by some photometric calibrations), the spectroscopic analysis would indicate a turnoff-point stage of evolution. Using a stellar-structure code that treats the effects of atomic diffusion throughout the star in detail, we evolve a low-mass model star to reach the Hertzsprung-Russell-diagram position of HE 1327-2326 after roughly 13 Gyr. While the surface abundances are modified significantly (by more than 1 dex for the case of uninhibited diffusion), such corrections cannot resolve the discrepancy between the abundance inferred from the nondetection of the Li I resonance line at 6707 angstrom and the Wilkinson Microwave Anisotropy Probe based primordial lithium abundance. As there are numerous processes that can destroy lithium, any cosmological interpretation of a lower-than-expected lithium abundance at the lowest metallicities will have to await sample sizes of unevolved HMP stars that are 1 order of magnitude larger. The situation remains equally inconclusive concerning atomic-diffusion corrections. Here, attempts have to be made to better constrain internal mixing processes, both observationally and by means of sophisticated modeling. With constraints on additional mixing processes taken from a recent globular-cluster study, the likeliest scenario is that HE 1327-2326's surface abundances have undergone mild depletion (of order 0.2 dex).
On Waves, Clusters, and Diffusion: A Conceptual Framework
(2005) Elkins, Zachary; Simmons, Beth
This article makes a conceptual and theoretical contri- bution to the study of diffusion. The authors suggest that the concept of diffusion be reserved for processes (not outcomes) characterized by a certain uncoordinated interdependence. Theoretically, the authors identify the principal sources of clustered policy reforms. They then clarify the characteristics specific to diffusion mecha- nisms and introduce a categorization of such processes. In particular, they make a distinction between two types of diffusion: adaptation and learning. They argue that this categorization adds conceptual clarity and distinguishes mechanisms with distinct substantive consequences.