Surface structure transport

We now describe a relatively simple MD model of a low-index crystal surface, which was conceived for the purpose of studying the rate of mass transport (8). The effect of temperature on surface transport involves several competing processes. A rough surface structure complicates the trajectories somewhat, and the diffusion of clusters of atoms must be considered. In order to simplify the model as much as possible, but retain the essential dynamics of the mobile atoms, we will consider a model in which the atoms move on a "substrate" represented by an analytic potential energy function that is adjusted to match that of a surface of a (100) face-centered cubic crystal composed of atoms interacting with a Lennard-Jones... 

In analyzing the kinetics of surface reactions, it will be illustrated that many of these processes are rate-controlled at the surface (and not by transport). Thus, the surface structure (the surface speciation and its microtopography) determine the kinetics. Heterogeneous kinetics is often not more difficult than the kinetics in homogeneous systems as will be shown, rate laws should be written in terms of concentrations of surface species. 

There can be, however, no doubt that in catalytic processes, purely physical factors play an important role, in addition to the chemical valence forces. This is particularly true for the solid catalysts of heterogeneous reactions for which the properties of surfaces, as the seats of catalytic action are of prime importance. The total surface areas, the fine structure of the surfaces, the transport of reactants to and from surfaces, and the adsorption of the reactants on the surfaces, can all be considered as processes of a predominantly physical nature which contribute to the catalytic overall effect. Any attempt, however, to draw too sharp a line between chemical and physical processes would be futile. This is illustrated clearly by the fact that the adsorption of gases on surfaces can be described either as a mere physical condensation of the gas molecules on top of the solid surface, as well as the result of chemical affinities between adsorbate and adsorbent. Every single case of adsorption may lie closer to either one of the hypothetical extremes of a purely physcial or of a purely chemical adsorption, and it would be misleading to maintain an artificial differentiation between physical and chemical factors. 

The best characterized multidrug transporter is P-gp which is a membrane protein encoded by the mdrl gene. The most intriguing feature of P-gp is its ability to interact with a large number of structurally and functionally different amphiphilic compounds. P-gp expression can be acquired during the course of treatment (e.g., in leukemias, lymphomas, ovarian carcinomas) or it is constitutive (e.g., in colorectal and renal cancers). In normal tissues of mammals P-gp is localized on the luminal surface of transporting epithelia in liver, kidney, small intestine, testes, and blood-brain barrier. 

Whether particles sinter by migration or by an atomic interparticle transport process is less important in a catalytic sense than is the rate at which particles reconstruct. If reconstruction is slow, as it might be with large particles, then unusual surface structures will be present and these may affect catalytic properties. Wynblatt and Gjostein23 have estimated the relative... 

In spite of the advances made by these researchers, it remains unclear how membrane surfaces undergo restructuring and how these changes influence the catalytic and transport properties of the material. Furthermore, there is a need to link surface structure and composition with long-term performance of palladium membranes under continuous reaction conditions. One... 

The choice of dimension D, depends on the value of relation dhldm [9], At dm<0,6dh interaction of diffusant molecules with walls of free volume microvoid is small and transport process is controlled by fractal dimension of structure (structural transport). At dm<0,6dh on transport processes has strong influence interaction of diffusant molecules with walls of free volume microvoid, which are polymeric macromolecules surface with dimension >/(/)/ is the dimension of excess energy localization regions) [10], In this case Dt=Df (molecular transport) [9] is adopted. 

The motion of atoms both in and on the surface can also occur through surface diffusion. The concept of atom transport along the surface plane is important in equihbrium surface structure, nucleation and growth of thin films, and surface reactivity. 

The very shallow ( 1 txm) layer of air in contact with the surfaces, where stream lines for air-flow are approximately parallel to the surface and turbulence is suppressed by the viscosity of the air, is known as the laminar boundary layer. The thickness of this layer is determined by physical characteristics of the object (size, shape and surface structure) and by the velocity of air outside the laminar boundary layer. Although gases and small particles are transported by the same mechanism and at about the same rates in the free atmosphere, their transport through the laminar boundary layer differs considerably. Gases must diffuse through this layer by molecular diffusion, the concentration gradient and diffusion coefficient for the gas in question controlling the rate of transfer. 

Readsorption enhancements caused by slow removal of a-olefins from catalyst pellets and interpellet voids are described by Eqs. (8)-(14). In these equations, transport rates are described in terms of the physical structure of the support and of the reactive and diffusive properties of reactant and products in molten hydrocarbons. Chain growth and termination rate constants are assumed to be independent of chain size and of surface structure and chemical properties. The readsorption probability (jSr) is also assumed... 

The application of STM to the studies of adsorption and surface reactions in the last two decades has revealed important aspects of the dynamic nature of catalytic metal surfaces. In addition to a local restructuring of metal surfaces with respect to their bulk atomic structure upon adsorption (e.g. the flexible surface), mass transport of... 

Use Study of atomic-scale structure of surfaces, drawing molecular-sized markings on surfaces and transport of atoms and molecules. 

The distributed array of drag elements in vegetation canopies creates a mean wind profile that contains an elevated shear layer centred near the canopy top that more closely approximates a plane mixing layer than a wall layer. This velocity stmcture is responsible for turbulence characteristics that differ substantially from those over a smooth surface. Velocity spectra are sharply peaked, streamwise and vertical velocities have probability densities that are strongly skewed, streamwise and vertical velocities are correlated more strongly that would be expected over a smoother surface, and transport is dominated by coherent flow structures with sweeps more important than ejections. 

Species emitted to snowpack interstitial air must be transported out of the snow into the atmosphere to have an atmospheric impact. The most efficient mechanism is ventilation by wind, which depends on wind speed, snow surface structure and permeability. Hansen et al." predict little surface wind change in polar regions. Snow surface structure depends mostly on wind, but speculating on its evolution is beyond our scope. We discussed that snow permeability will in general decrease with warming. The residence time of species produced in interstitial air will therefore increase under a warmer climate, reducing the chances of reactive species to escape before they react in the snowpack. 

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