Sediments diffusion

The contribution of atmospheric dust to surface dust depends on the dust falling to the earth. This occurs either as dry dust fall or wet washout with rain, snow or hail (1-6,8-10). Dry dust fall occurs by s imentation, impaction, interception or diffusion. Sedimentation, the fall under gravity, may be estimated using Stoke s law which relates the density and diameter of particles to their falling velocity. A particle of density 1.0 g cm"3 and diameter around 0.1 pm would fall with a velocity of around 9 x 10" cm s" ... 

The concentration of small ions in the atmosphere is determined by 1) the rate of ion-pair production by the cosmic rays and radioactive decay due to natural radioactive substances, 2) recombination with negative ions, 3) attachment to condensation nuclei, 4) precipitation scavenging, and 5) transport processes including convection, advection, eddy diffusion, sedimentation, and ion migration under the influence of electric fields. A detailed differential equation for the concentration of short-lived Rn-222 daughter ions including these terms as well as those pertaining to the rate of formation of the... 

Roering J.J., Kirchner J.W., and Dietrich W.E. (1999) Evidence for nonlinear, diffusive sediment transport on hillslopes and implications for landscape morphology. Water Resources Res. 35, 853-870. 

Explain the difference between diffusive sediment-water exchange and the sediment-water interaction caused by ripples on a sandy river bottom. 

Passive Sampling. These techniques provide for the accumulation of a contaminant on a substrate on the basis of the principles of diffusion, sedimentation, adsorption, or absorption. 

K 13 Kuhn, W., u. H. Kuhn Diffusion, Sedimentation und Viscositat beiLosungen verzweigter Fadenmolekule. Helv. Chim. Acta 30,1233 (1947). 

Method Viscosity Diffusion Sedimentation NMR DSC Dielectric Isopiestic Range... 

Tables 1—3 give the values of molecular parameters characterizing the equilibrium rigidity and hydrodynamic properties of molecules of some rigid-chain polymers obtained from viscometric and diffusion-sedimentation measurements of their dilute solutions.

Diffusion/sedimentation in the cell culture medium down to... 

Diffusion/ Sedimentation in the Cell Culture Medium and the Effective Dose... 

The application of CFD in the modeling of solid-liquid mixing is fairly recent. In 1994, Bakker et al. developed a two-dimensional computational approach to predict the particle concentration distribution in stirred vessels. In their model, the velocity field of the liquid phase is first simulated taking into account the flow turbulence. Then, using a finite volume approach, the diffusion-sedimentation equation along with the convective terms is solved, which includes Ds, a... 

In this chapter, we consider Brownian diffusion, sedimentation, migration in an electric Reid, and thermophoresis. The last term refers to particle movement produced by a temperature gradient in the gas. We consider also the London-van der Waals forces that are important when a particle approaches a surface. The analysis is limited to particle transport in stationary —that is. nonllowing— gases. I ransporl in flow systems is discussed in the chapters which follow. 

Condition (3.1.4) also corresponds to diffusion sedimentation of aerosol and colloid particles, and the entrapment phenomenon [139, 499] can be taken into account by assuming that the distance between the surface , = 0 and the sedimentation surface is equal to the mean radius of sediment particles. 

Let us consider diffusion to the surface of a circular cylinder of radius a in a flow with velocity Ui directed along the normal to the cylinder axis. This is a model problem used in chemical engineering for calculating mass transfer to prolate particles it is used even more widely in mechanics of aerosols for analyzing diffusion sedimentation of aerosols on fibrous filters [139,461]. 

The removal of aerosol particles under dry weather conditions is caused by turbulent diffusion and gravitational sedimentation, which transport particles to the Earth s surface, as well as by impaction on vegetation, buildings and other objects. Turbulent diffusion itself does not remove particles (Twomey, 1977). Soil and other surfaces are bordered by a thin laminar layer ( 1 mm thick) across which particles must be transported by other processes (e.g. phoretic forces, molecular diffusion, sedimentation). 

Density gradient ultra-centrifugation can be used to determine the buoyant density of DNA and quadruplexes. Sedimentation experiments can also be used to measure sedimentation coefficients and molecular weights. Bead modeling on atomic-level model structures can be used to calculate theoretical hydrodynamic properties of DNA the structures. The calculated hydrodynamic properties (translational diffusion, sedimentation coefficients (S) and correlation times) in conjunction with the program HYDROPRO has been used to help differentiate between known or inferred structural conformations measured by analytical ultra-centrifugation. 

Other methods of molecular-weight determination, such as diffusion, sedimentation, flow birefringence, and light scattering (all of which are considered later in this chapter) involve terms related to the size and shape of the molecules and give a... 

The thermodynamic interactions and the size of polymer coil enter dependencies that describe the transport behavior of polymer solutions, viz. viscosity, diffusion, sedimentation, etc. To complete this short summary, the intrinsic viscosity should be mentioned. 

Similar to the major redox-sensitive ions, discussed before, the quantity and distribution of AVS in sediments depend upon, for example OM input, sediment burial, sulphate diffusion, sediment mixing and sediment temperature (Morse et al., 1987 Matisoff and Holdren, 1995). Leonard et al. (1993) have shown that variations in AVS concentrations in lake sediments are directly correlated with temperature of the over-lying lake water. Fig. 5 clearly shows that a higher sulphate reduction rate is reflected in AVS present closer to the SWI. Large fluctuations in AVS have also been measured in sediments deposited in lakes with a seasonally anoxic water column (Howard and Evans, 1993). 

A simple one-dimensional diffusion sedimentation model works quite well for solids distribution. This model shows that large settling velocity solids at high concentration are much more difficult to homogenize than those with a low settling velocity at low concentration. Experimental data fit this model quite well and probably help explain the old rule of thumb, solids with a settling velocity less than 5 mm/s represent an easy suspension problem and if greater than 30 mm/s, a difficult one . ... 

The slower the migration of dissolved substances in it (diffusion, sedimentation, ion migration). 

Joshi et al. (30) proposed reactor models based on the shrinking core mechanism. Since the particles take part in the reaction their role was evaluated based on the residence time distribution. For extremely fine pyrite particles, (< 100 ym), it has been shown (31) that the RTD of the solid and liquid phases can be asstimed to be identical and the RTD of the solid phase is given by the diffusion-sedimentation model. Various rate controlling steps that were considered are (1) gas-liquid mass transfer (2) liquid-solid mass transfer (3) ash diffusion (4) chemical reaction and, (5) intraparticle diffusional resistance (for particles encased in the coal matrix). 

The thermodynamic interactions and the size of polymer coil also enter dependencies that describe the transport behavior of polymer solutions, viz., viscosity, diffusion, sedimentation, etc. To complete this short summary, the viscosity relations should be mentioned. Defining rj as the solution viscosity and as the solvent viscosity, the following, relative (rjf), specific (rj f), and intrinsic ([77]) viscosities, are typically expressed as... 

The experimental methods used for the study of thermodynamic parameters such as Xi and 0 of a polymer in dilute solutions are numerous. Among them are intrinsic viscosity, light scattering, diffusion, sedimentation, vapor pressure, and phase equilibrium. Here we discuss vapor pressure and phase equilibrium, leaving the other methods for later chapters. 

Like intrinsic viscosity and diffusion, sedimentation is a frictional property. It is the transport of mass from the surface toward the bottom. In ultracentrifuge sedimentation, which is our major subject in this chapter, macromolecules in solution are forced to sediment by a centrifugal force that is 100,000 times greater than gravitational force. Sedimentation can be described by Pick s two laws with some modification, since sedimentation is often accompanied by diffusion. The modified equation of Pick s first law is... 

The axial dispersion of solids can be measured using a steady state diffusion sedimentation technique. This method was applied e.g. by Kato et al. [27] and recently by Brian and Dyer [59]. At zero liquid rates, if the particles are only suspended by the gas flow, a solid concentration profile will be established ... 

One of Flory s closest personal friends, Leo Mandelkern (1922-2006), joined him at Cornell in 1949 as a Research Associate after completing his Ph.D. at Cornell with Franklin Long. Mandelkern benefitted from interactions with Debye, Kirkwood, Scheraga and Flory. He became one of polymer science s leading experts on crystalline polymers, an area he pioneered with Flory. He went on to spend a decade at the National Bureau of Standards from 1952-1962. One area of special interest at Cornell was the use of frictional methods like diffusion, sedimentation and intrinsic viscosity to characterize polymer chains in solution [32] (Fig. 2.7). 

Hydrodynamic properties. The basic hydrodynamic characteristics of a macromolecule are its translational and rotational friction, experimentally studied by methods of diffusion, sedimentation, and viscometry. The chain necklace model [11], which has been very satisfactorily confirmed by the experimental data, is widely used in the hydrodynamics of rigid-chain molecules. 

Processes such as diffusion, sedimentation and viscous flow involve the motion of individual molecules. When a polymer molecule moves through a dilute solution it undergoes frictional interactions with solvent molecules. The nature and effects of these frictional interactions depend upon the size and shape of the polymer molecule as modified by its thermodynamic interactions with solvent molecules. Thus the chain dimensions can be evaluated from measurements of the frictional properties of a polymer molecule. 

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