Biomedical subjects

Moshe Sheintuch

Publications and source records attributed to Moshe Sheintuch.

13 recordsLinked to original sources

PubMed2006

Boundary-induced spatiotemporal complex patterns in excitable systems.

We show that inhomogeneous boundary conditions (BCs) in a distributed reaction-diffusion excitable system are a natural source of permanent perturbations that can induce wave trains, which can be characterized as mixed-mode temporal oscillations and, when a parameter is varied, admit a period-adding bifurcation. To that end we analyze: a pair of coupled excitable and oscillatory cells, a distributed FitzHugh-Nagumo model, and a distributed five-variable model that describes catalytic oxidation. The obtained results account for the recently reported experimental observations of mixed-mode oscillations showing a period-adding bifurcation during oxidation on a disk-shaped catalytic cloth with imposed cold temperature BC.

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PubMed2006

Predicting solute adsorption on activated carbon: phenol.

Activated carbon (AC), the most widely used adsorbent in water and in wastewater treatment, comprises a high surface area of very small, convoluted and interconnected pores. Despite the wide use of AC, there is little fundamental atomic-level understanding of its adsorption capacity and selectivity as well as its pore structure. The purpose of this work is to suggest the methodology for calculation of equilibrium adsorption capacity of common water organic pollutants and use it for phenol as a model. The effects of various functional groups, pore size, and coverage on thermodynamics of phenol adsorption from the gas phase and from water media are calculated using molecular mechanics (MM) and density functional theory (DFT) approaches.

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PubMed2006

Catalytic spatiotemporal thermal patterns during CO oxidation on cylindrical surfaces: experiments and simulations.

Dynamics of spatiotemporal thermal patterns during the catalytic CO oxidation over Pd supported on a glass-fiber catalytic cloth rolled into a tube of 20 mm diameter and 80 mm length has been studied in a continuous flow reactor by IR thermography. A specially designed aluminum mirror built in the reactor provided image of the entire surface of the horizontally held catalytic tube. With flow in the main axial direction and through the tube surface, we observed periodic motions of a pulse, which was born downstream and propagated upstream. The temperature pulse motion was accompanied by conversion oscillations of CO2. With flow in the main axial direction, parallel to the surface, we observed a stationary hot zone after an oscillatory transient. These patterns can be simulated with a plug-flow-reactor-like heterogeneous reactor model that incorporates previously determined kinetic and transport parameters.

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PubMed2005

Relationship between kinetic and thermodynamic characteristics of oxygen dissociative adsorption on close-packed metal surfaces.

The activation energies and rate constants characterizing dissociative oxygen adsorption on transition and noble metal surfaces are calculated using a previously developed analytical formalism. Three approaches are considered: classical and quantum nonadiabatic approaches as well as a classical adiabatic approach. The results show a nonlinear relationship between the activation energies and the corresponding reaction energies. The effects due to the quantum description of the O-O bond and due to the Condon approximation are discussed. Calculated activation energies are compared with published experimental data.

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PubMed2005

Stationary fronts due to weak thermal effects in models of catalytic oxidation.

We analyze the possible existence of an infinite number of stationary front solutions in a microkinetic model of a catalytic reaction coupled with weak enthalpy effects in the domain of kinetics bistability. The kinetic model incorporates three steps: dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. The infinitude of stationary front solutions emerges due to the lack of intercrystallites communication of surface species in supported catalysts; thermal conductions and gas-phase diffusion are the only means of interaction. Incorporation of surface species diffusion leads to a very slow front motion. We complement this analysis with simulations of stationary states on one- (wire and ring) and two-dimensional (disk) systems which may be subject to control or to fluid flow. These results account for certain experimental results and may have implications for various technological problems.

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PubMed2005

Enthalpy and entropy effects in hydrogen adsorption on carbon nanotubes.

Interaction energies and entropies associated with hydrogen adsorption on the inner and outer surfaces of zigzag single-wall carbon nanotubes (SWCNT) of various diameters are analyzed by means of molecular mechanics, density functional theory, and ab initio calculations. For a single molecule the strongest interaction, which is 3.5 greater than that with the planar graphite sheet, is found inside a (8,0) nanotube. Adsorption on the outer surfaces is weaker than that on graphite. Due to the steric considerations, both processes are accompanied by an extremely strong decline in entropy. Absence of specific adsorption sites and weak attractive interaction between hydrogen molecules within carbon nanotubes results in their close packing at low temperatures. Using the calculated geometric and thermodynamic parameters in Langmuir isotherms we predict the adsorption capacity of SWCNTs at room temperature to be smaller than 1 wt % even at 100 bar.

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PubMed2005

Moving waves and spatiotemporal patterns due to weak thermal effects in models of catalytic oxidation.

We analyze the behavior of a microkinetic model of a catalytic reaction coupled with weak enthalpy effects to show that under fixed gas-phase concentrations it can produce moving waves with an intrinsic length scale, when the underlying kinetics is oscillatory. The kinetic model incorporates dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. Three typical patterns may emerge in a one-dimensional system (a long wire or a ring): homogeneous oscillations, a family of moving waves propagating with constant velocities, and patterns with multiple source/sink points. Pattern selection depends on the ratio of the system length to the intrinsic wave length and the governing parameters. We complement these analysis with simulations that revealed a plethora of patterned states on one- and two-dimensional systems (a disk or a cylinder). This work shows that weak long-range coupling due to high feed rates maintains such patterns, while low feed rates or strong long-range interaction can gradually suppress the emerging patterns.

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PubMed2005

Quantum mechanical model for the dissociative adsorption of diatomic molecules on metal surfaces.

A quantum mechanical nonadiabatic theory of dissociative adsorption of diatomic molecules X2 on metal surface is presented. The following reaction coordinates are used to construct crossing diabatic potential energy surfaces (PES): the distance y between the atoms of the X2 molecule, the distance x of the X2 molecular axis from the surface, the set of coordinates describing possible displacements of metal atoms under adsorption. Expression for the rate constant is derived using the model potentials describing vibrations along these coordinates. The calculated dependency of the rate constant W on the reaction heat DeltaE is compared with that in classical approximation. It is shown that quantum effects lead to a weaker dependence of W on DeltaE as compared to that for classical one.

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PubMed2004

Stabilizing the absolutely or convectively unstable homogeneous solutions of reaction-convection-diffusion systems.

We study the problem of stabilization of a homogeneous solution in a two-variable reaction-convection-diffusion one-dimensional system with oscillatory kinetics, in which moving or stationary patterns emerge in the absence of control. We propose to use a formal spatially weighted feedback control to suppress patterns in an absolutely or convectively unstable system and pinning control for a convectively unstable system. The latter approach is very effective and may require only one actuator to adjust feed conditions. In the former approach, the positive diagonal elements of the appropriate dynamics matrix are shifted to the left-hand part of the complex plane to ensure linear (asymptotic) stability of the system according to Gershgorin criterion. Moreover, we construct a controller that (with many actuators) will approach the global stability of the solution, according to Liapunov's direct method. We apply two alternative approaches to reveal the unstable modes: an approximate one that is based on linear stability analysis of an unbounded system, and an exact one that uses a traditional eigenstructure analysis of bounded systems. The number of required actuators increases dramatically with system size and with the distance from the bifurcation point. The methodology is developed for a system with learning cubic kinetics and is tested on a more realistic cross-flow reactor model.

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PubMed2003

Asymptotic solutions of stationary patterns in convection-reaction-diffusion systems.

We study and map the possible stationary patterns that emerge in a convection-reaction-diffusion (CRD) system using a learning polynomial kinetics. We classify the patterns according to the kinetic model (oscillatory, bistable, or intermediate), the instability nature of the bounded system (convective or absolute), the applied boundary conditions and the system length. This analysis presents a unifying approach to various pattern-inducing mechanisms such as DIFICI (differential flow induced chemical instability), which predicts moving patterns in systems with wide difference of convective rates, and differential capacity patterns, which predicts stationary patterns in cross-flow reactors with a large heat capacity. Previous studies of CRD systems have considered only oscillatory kinetics. Nonlinear analysis, which follows the front motion by approximating its velocity, accounts for the stability of the stationary, whether spatially periodic or other, patterns. The most dominant state is the large-amplitude stationary spatially periodic pattern. With oscillatory kinetics these emerge in the convectively unstable domain above the amplification threshold. The domain of absolute instability, which is determined analytically for unbounded systems, is divided in the bounded system into two subdomains with moving DIFICI waves or stationary patterns. With bistable kinetics the large-amplitude stationary patterns can be sustained only within a narrow subdomain but other stationary patterns, that incorporate several fronts upstream and an "almost homogeneous" tail downstream, can be sustained as well. With intermediate kinetics the large-amplitude axisymmetric stationary patterns may coexist with small-amplitude stationary nonaxisymmetric patterns.

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PubMed2002

Pinning stationary planar fronts in diffusion-convection-reaction systems.

This paper considers various strategies for controlling a stationary planar front solution, in a rectangular domain with a diffusion-reaction distributed system, by pinning the solution to one or few points and using actuators with the simplest possible spatial dependence. We review previous results obtained for one-dimensional diffusion-reaction (with or without convection) systems, for which we applied two approaches: an approximate model reduction to a form that follows the front position while approximating the front velocity, and linear stability analysis. We apply the same two approaches for the planar fronts. The approximate model reduction allows us to analyze qualitatively various control strategies and to predict the critical width below which the control mode of the one-dimensional system is sufficient. These results are corroborated by linear analysis of a truncated model with the spectral methods representation, using concepts of finite and infinite zeros of linear multidimensional systems.

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PubMed2002

Spatially "chaotic" solutions in reaction-convection models and their bifurcations to moving waves.

The emergence of stationary spatially multiperiodic or even spatially chaotic patterns is analyzed for a simple model of convection, reaction, and conduction in a cross-flow reactor. Spatial patterns emerge much like dynamic temporal patterns in a mixed system of the same kinetics. Moving waves are formed in an unbounded system but they are transformed into stationary spatially inhomogeneous patterns in a bounded system. The sequence of period doubling bifurcations is determined numerically. The incorporation of a slow nondiffusing inhibitor leads to chaotic spatiotemporal patterns.

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PubMed1999

Using weighted global control for stabilizing patterned states.

A general method to control a desired patterned state in reaction-diffusion processes is presented. Weighted global control is aimed to keep weighted spatially averaged properties of state variable at preset values. It is shown that weighted global control creates a stable direction in the global space of system states and affects system dynamics globally. We apply it for a specific two-component reaction-diffusion system and show that the desired pattern is attainable for a wide range of the control parameters. (c) 1999 American Institute of Physics.

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