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Full-Text Articles in Physics

Generic Two-Phase Coexistence, Relaxation Kinetics, And Interface Propagation In The Quadratic Contact Process: Simulation Studies, Xiaofang Guo, Da-Jiang Liu, James W. Evans Jun 2007

Generic Two-Phase Coexistence, Relaxation Kinetics, And Interface Propagation In The Quadratic Contact Process: Simulation Studies, Xiaofang Guo, Da-Jiang Liu, James W. Evans

Physics and Astronomy Publications

The quadratic contact process is formulated as an adsorption-desorption model on a two-dimensional square lattice. It involves random adsorption at empty sites and correlated desorption requiring diagonally adjacent pairs of empty neighbors. We assess the model behavior utilizing kinetic Monte Carlo simulations. One finds generic two-phase coexistence between a low-coverage active steady state and a completely covered or "poisoned" absorbing steady state; i.e., both states are stable over a finite range of adsorption rates or "pressures." This behavior is in marked contrast to that for equilibrium phase separation. For spatially homogeneous systems, we provide a comprehensive characterization of the ...


Quadratic Contact Process: Phase Separation With Interface-Orientation-Dependent Equistability, Da-Jiang Liu, Xiaofang Guo, James W. Evans Feb 2007

Quadratic Contact Process: Phase Separation With Interface-Orientation-Dependent Equistability, Da-Jiang Liu, Xiaofang Guo, James W. Evans

Physics and Astronomy Publications

The quadratic contact process is implemented on a square lattice as a model with random adsorption and correlated desorption requiring empty pairs of diagonal neighbors. The model exhibits a discontinuous phase transition between an active state and an absorbing state, but equistability between these states depends on the orientation of the separating interface. Correspondingly, for a generalized class of models, we find phase coexistence over a finite region of their two-dimensional parameter space. This is in stark contrast to behavior in equilibrium systems.


Numerical Simulations Of Gas-Liquid Flow Dynamics In Bubble Columns, Deify Law, Francine Battaglia, Theodore J. Heindel Nov 2006

Numerical Simulations Of Gas-Liquid Flow Dynamics In Bubble Columns, Deify Law, Francine Battaglia, Theodore J. Heindel

Mechanical Engineering Conference Presentations, Papers, and Proceedings

There is great potential for using computational fluid dynamics (CFD) as a tool in scale-up and design of bubble columns. Full-scale experimentation in bubble columns is expensive and CFD is an alternative approach to study bubble column hydrodynamics. However, CFD can be computationally intensive as a predictive tool for a full three-dimensional geometry. In this paper, a 0.2 m diameter semi-batch bubble column is numerically simulated and the results are compared to experimental measurements performed by Rampure et al. [1]. The objectives are to examine and determine an appropriate set of numerical parameters and to determine if two-dimensional simulations ...


Chemical Diffusion Of Co In Mixed Co+O Adlayers And Reaction-Front Propagation In Co Oxidation On Pd(100), Da-Jiang Liu, James W. Evans Aug 2006

Chemical Diffusion Of Co In Mixed Co+O Adlayers And Reaction-Front Propagation In Co Oxidation On Pd(100), Da-Jiang Liu, James W. Evans

Physics and Astronomy Publications

Within the framework of a realistic atomistic lattice-gas model, we present the theoretical formulation and simulation procedures for precise analysis of the chemical diffusion flux of highly mobile CO within a nonuniform interacting mixed CO+O adlayer on a Pd(100) surface. The approach applies in both regimes of relatively immobile unequilibrated and fairly mobile near-equilibrated O adlayer distributions. Spatiotemporal behavior in surface reactions is controlled by chemical diffusion in mixed adlayers. Thus, we naturally integrate the above analysis with a previously developed multiscale modeling strategy to describe mesoscale reaction front propagation in CO oxidation on Pd(100). This treatment ...


Microelectromagnetic Ferrofluid-Based Actuator, Y. Melikhov, S. J. Lee, David C. Jiles, D. H. Schmidt, Marc D. Porter, Ruth Shinar May 2003

Microelectromagnetic Ferrofluid-Based Actuator, Y. Melikhov, S. J. Lee, David C. Jiles, D. H. Schmidt, Marc D. Porter, Ruth Shinar

Ames Laboratory Publications

Computer simulations were used to investigate the performance of a microscale ferrofluid-based magnetic actuator developed for liquid dispensing in microfluidic channels. The actuation was based on the movement of a ferrofluid plug in a magnetic field gradient generated by on-chip effectively infinite parallel conductors. The movement, positioning, and retaining of ferrofluid plugs with different lengths at various locations along a microfluidic channel were investigated for two cases. In case (a), the magnetic field gradient was generated by a single conductor; when the ferrofluid reached its equilibrium position, the current was switched off and the nearest neighbor conductor was energized. A ...


Fluctuations And Bistability In A "Hybrid" Atomistic Model For Co Oxidation On Nanofacets: An Effective Potential Analysis, Da-Jiang Liu, James W. Evans Oct 2002

Fluctuations And Bistability In A "Hybrid" Atomistic Model For Co Oxidation On Nanofacets: An Effective Potential Analysis, Da-Jiang Liu, James W. Evans

Mathematics Publications

An analysis on the existence of bistability in the hybrid reaction model for CO oxidation in finite systems is demonstrated. As such, numerical evidence to indicate the existence of an effective potential characterizing this behavior is provided. This potential is determined by exact kinetic Monte Carlo (KMC) simulation analysis, but reasonable estimate available from approximate master equations, and from Fokker-Planck equations which are obtained from further approximation of the master equations.


Chemical Diffusivity And Wave Propagation In Surface Reactions: Lattice-Gas Model Mimicking Co-Oxidation With High Co-Mobility, Michael Joseph Tammaro, James W. Evans Jan 1998

Chemical Diffusivity And Wave Propagation In Surface Reactions: Lattice-Gas Model Mimicking Co-Oxidation With High Co-Mobility, Michael Joseph Tammaro, James W. Evans

Mathematics Publications

We analyze the spatiotemporal behavior in a lattice-gas model for the monomer-dimer reaction on surfaces. This model, which mimics catalytic CO-oxidation, includes a mobilemonomer adspecies (representing CO), an immobile dissociatively adsorbed dimer species (representing O), and afinitereaction rate (for CO2 production). We characterize in detail the propagation of the chemical wave or reaction front produced when the stable reactive steady-state of the model displaces the metastable CO-poisoned state. In the regime of high CO-mobility, such propagation can be described directly within a “hydrodynamic” reaction-diffusion equation formalism. However, we show that the chemical diffusivity of CO is dependent ...


Nonequilibrium Percolative C(2×2) Ordering: Oxygen On Pd(100), James W. Evans Jan 1987

Nonequilibrium Percolative C(2×2) Ordering: Oxygen On Pd(100), James W. Evans

Physics and Astronomy Publications

Dissociative adsorption of oxygen on certain (100) metal surfaces has been modeled as random dimer adsorption onto diagonally adjacent empty sites of a square lattice subject to the additional constraint that all six neighboring sites must be empty (the 8‐site model). Here we adapt this model to analyze the nonequilibrium c(2×2) ordering recently observed for oxygen on Pd(100) at coverages up to saturation (>1/4 monolayer), under conditions of low temperature and high pressure where effects of diffusive mobility can be ignored. We do, however, propose that adsorption ...