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  • 1
    Electronic Resource
    Electronic Resource
    A critical comparison of equilibrium, non-equilibrium and boundary-driven molecular dynamics techniques for studying transport in microporous materials (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 8112-8124 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transport in an idealized model with variable pore diameter as well as an AlPO4-5 zeolite is examined using three different molecular dynamics techniques: (1) equilibrium molecular dynamics (EMD); (2) external field nonequilibrium molecular dynamics (EF–NEMD); and (3) dual control volume grand canonical molecular dynamics (DCV–GCMD). The EMD and EF–NEMD methods yield identical transport coefficients for all the systems studied. The transport coefficients calculated using the DCV–GCMD method, however, tend to be lower than those obtained from the EMD and EF–NEMD methods unless a large ratio of stochastic to dynamic moves is used for each control volume, and a streaming velocity is added to all inserted molecules. Through development and application of a combined reaction–diffusion–convection model, this discrepancy is shown to be due to spurious mass and momentum transfers caused by the control volume equilibration procedure. This shortcoming can be remedied with a proper choice of streaming velocity in conjunction with a well-maintained external field, but the associated overhead makes it much less efficient than either the EMD or EF–NEMD techniques. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1407002
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  • 2
    Electronic Resource
    Electronic Resource
    Diffusion of inert gases in silica sodalite: Importance of lattice flexibility (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9519-9527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is known that some zeolites are able to adsorb sorbates with diameters larger than their pore size. This ability is usually explained by the flexibility of zeolite lattices. In this paper we quantify this explanation by incorporating lattice flexibility into transition state theory to compute transport rates of inert gas molecules (Ne, Ar and Kr) through narrow windows of silica sodalite cages. We find that the diffusion rates obtained with the flexible zeolite lattice are in much better agreement with experiment than those with a rigid lattice, as stretching of the zeolite window considerably reduces the activation barrier. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1414373
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  • 3
    Electronic Resource
    Electronic Resource
    Does lattice vibration drive diffusion in zeolites? (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 3776-3789 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method of estimation of the effect of lattice vibration as a driving force for sorbate diffusion in zeolites is proposed. A realistic lattice model is employed to cut off unrealistic long vibrational modes and eliminate feedback due to lattice periodicity. A generalized Langevin equation for sorbate motion is then derived with the magnitude of the lattice vibration captured by two parameters, μ and ν, which can be readily computed for any system. The effect of lattice vibration is then estimated for a variety of sorbate–zeolite pairs. Lattice vibration is found to be a negligible driving force for some systems (e.g., methane and xenon in silicalite) and an important driving force for other systems. In the latter case, the lattice vibration can provide either linear stochastic Langevin-type force (e.g., for benzene in silicalite) or nonlinear deterministic force (e.g., for argon in sodalite). © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1343072
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  • 4
    Electronic Resource
    Electronic Resource
    Efficient viscosity estimation from molecular dynamics simulation via momentum impulse relaxation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 2079-2087 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new momentum impulse relaxation method for obtaining the shear viscosity of Newtonian fluids using molecular dynamics simulations is introduced. The method involves the resolution of a decaying coarse-grain Gaussian velocity profile in a properly thermostated simulation box. This localized velocity profile, along with a modification of the periodic boundary conditions, allows computations in a periodic box with minimal phonon feedback due to periodicity. The short-time decay of the small-amplitude velocity profile yields shear viscosities for atomic and molecular species that are in quantitative agreement with those obtained using conventional techniques, but with more than an order of magnitude reduction in computational effort. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.482019
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  • 5
    Electronic Resource
    Electronic Resource
    Low-dimensional spatio-temporal thermal dynamics on nonuniform catalytic surfaces (1993)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 1055-1064 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100107a013
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  • 6
    Electronic Resource
    Electronic Resource
    Nanoscale pore formation dynamics during aluminum anodization (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Chaos 12 (2002), S. 240-251 
    Details
    ISSN: 1089-7682
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical analysis of nanoscale pore formation during anodization reveals its fundamental instability mechanism to be a field focusing phenomenon when perturbations on the minima of the two oxide interfaces are in phase. Lateral leakage of the layer potential at high wave number introduces a layer tension effect that balances the previous destabilizing effect to produce a long-wave instability and a selected pore separation that scales linearly with respect to voltage. At pH higher than 1.77, pores do not form due to a very thick barrier layer. A weakly nonlinear theory based on long-wave expansion of double free surface problem yields two coupled interface evolution equations that can be reduced to one without altering the dispersion relationship by assuming an equal and in-phase amplitude for the two interfaces. This interface evolution equation faithfully reproduces the initial pore ordering and their dynamics. A hodograph transformation technique is then used to determine the interior dimension of the well-developed pores in two dimensions. The ratio of pore diameter to pore separation is found to be a factor independent of voltage but varies with the pH of the electrolyte. Both the predicted pH range where pores are formed and the predicted pore dimensions are favorably compared to experimental data. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1436499
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  • 7
    Electronic Resource
    Electronic Resource
    Pattern selection during electropolishing due to double-layer effects (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Chaos 9 (1999), S. 62-77 
    Details
    ISSN: 1089-7682
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We extend our earlier study of nanoscale pattern formation during electropolishing [Nanotechnology 7, 360 (1996); Phys. Rev. B 56, 12 608 (1997)]. The patterns are attributed to preferential adsorption of organic molecules on the convex portion of the electrode due to its enhanced electric field. This local enhancement occurs because of the effect of surface curvature on the double-layer potential drop. By allowing for transport correction to the double-layer potential drop at thermodynamic equilibrium, we estimate this anodic overpotential to be in the realistic mV range and hence verify the Debye–Hückel approximation used in our model. This small anodic overpotential suggests that pattern formation is a generic electropolishing phenomenon whose only requirement is that the polarizability of the organic additive relative to water must lie within a range specified by our theory. We verify this prediction experimentally with a variety of electrolyte solutions. The voltage ranges for specific hexagonal and ridge patterns are well correlated by our model with only a single parameter. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.166380
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  • 8
    Electronic Resource
    Electronic Resource
    Subharmonic instabilities of finite-amplitude monochromatic waves (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 505-523 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Finite-amplitude monochromatic waves in free-shear layers, planar jets, gravity–capillary waves, free-convection boundary layers, and falling films are often first destabilized by disturbances twice or two-thirds their wavelength or period. The transition to the 1/2 mode allows energy transfer to modes with lower wave numbers or lower frequencies and it precedes the onset of wide-spectrum turbulence in the above systems. A center-unstable manifold theory is used to derive the onset criteria for these subharmonic instabilities in terms of a critical amplitude of the fundamental or a critical value of a control parameter. It is shown that, in contrast to classical results, a finite-amplitude wave is always unstable to disturbances with 1/2 its wave number (frequency) if the subharmonic is linearly unstable. This instability is either oscillatory or static depending on the phase difference (frequency mismatch) between the fundamental and the subharmonic and on the amplitude of the fundamental. For a given phase difference, there exists a critical amplitude beyond which the static instability triggers a most efficient energy transfer to the 1/2 mode and causes it to grow monotonically. Inefficient oscillatory interaction occurs below this critical amplitude of the fundamental. In contrast, the fundamental −3/2 interaction is always oscillatory except when there is perfect resonance (no frequency mismatch). The theory presented here contains a rigorous and explicit derivation of amplitude equations for spatially evolving open-flow systems without recourse to Gaster transformation or multiscale expansions. This new formulation is partially demonstrated on the inviscid shear layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858324
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  • 9
    Electronic Resource
    Electronic Resource
    A generalized sideband stability theory via center manifold projection (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1364-1379 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A center manifold theory is used to determine all modes that contribute significantly to the leading-order sideband stability of finite-amplitude monochromatic waves. The classical multiscale theories based on the Ginzburg–Landau equation are extended away from near-critical conditions and are shown to have omitted an important contribution from nonlinear interactions with low wave-number modes. Stability bounds on stable monochromatic waves are reported for dispersive systems that extend the classical Eckhaus bound for nondispersive systems and the Lange–Newell and Benjamin–Feir stability conditions for monochromatic waves with critical wave numbers. These new stability bounds are verified numerically by computing the evolving spectrum of a model equation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857586
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  • 10
    Electronic Resource
    Electronic Resource
    Transport of gas bubbles in capillaries (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1642-1655 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The pressure drop and wetting film thickness for isolated bubbles and bubble trains moving in circular and square capillaries are computed. An arclength-angle formulation of a composite lubrication equation allows for the numerical matching of the lubrication solution of the transition region to the static profiles away from the channel wall. This technique is shown to extend the classical matched asymptotic analysis of Bretherton for circular capillaries to higher capillary numbers Ca. More importantly, it allows the study of finite bubbles, which are shown to resemble infinitely long bubbles in film thickness and pressure drop if their lengths exceed the channel width. The numerical study of bubble trains, verified by a matched asymptotic analysis, shows a surprising result that the pressure drop across one member bubble is identical to that of an isolated bubble at low capillary numbers. This analysis of square capillaries neglects azimuthal flow and is only valid for Ca〉3.0×10−3. Nevertheless the film radius and pressure drop of a bubble traveling in a square capillary above this capillary number are computed. These results are conveniently summarized in a correlation for the apparent viscosity of bubbles as a function of foam texture and capillary geometry and dimension.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857530
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