Transport and External Field Effects

Convection is a much more efficient process than diffusion for transporting heat and matter. To appreciate that convection is a more effective mode of material transport, one need only consider what would happen if smoke in a fireplace were removed solely by diffusion. In a short time, the room would fill as the smoke particles dispersed randomly. Instead, if things work properly, the smoke goes up the chimney as the exothermic combustion reactions in the fire produce heat, which decreases the density of the gases and allows them to float up the flue because of buoyancy. We understand this idea of buoyancy intuitively when we say that heat rises.  

Bazsa and Epstein (1985) found that fronts in the nitric acid oxidation of iron(II) propagated as much as six times faster moving down a vertical tube than traveling up the same tube. This front velocity anisotropy, which we shall discuss in more detail later in this chapter, was a function of the tube diameter and could be eliminated by adding silica gel to the solution. 

The density of a solution is a function of its temperature and composition. We can define coefficients that relate the density p to changes in temperature T and 

If Fq is the initial volume of the reaction mixture, the volume of reaction AF can be related to the change in density Ap. caused by the change in chemical composition by 

From a and the heat capacity Cp of the reaction solution, we can calculate the density change Apj caused by the enthalpy change of an adiabatic reaction  

---

The motion of particles in a fluid is best approached tlirough tire Boltzmaim transport equation, provided that the combination of internal and external perturbations does not substantially disturb the equilibrium. In otlier words, our starting point will be the statistical themiodynamic treatment above, and we will consider the effect of botli the internal and external fields. Let the chemical species in our fluid be distinguished by the Greek subscripts a,(3,.. . and let f (r, c,f)AV A be the number of molecules of type a located m... 

The energy dissipation of a system containing free charges subjected to electric fields Is well known but this Indicates a non-equilibrium situation and as a result a thermodyanmlc description of the FDE Is Impossible. Within the framework of interionic attraction theory Onsager was able to derive the effect of an electric field on the Ionic dissociation from the transport properties of the Ions In the combined coulomb and external fields (2). It is not improper to mention here the notorious mathematical difficulty of Onsager s paper on the second Wien effect. 

Other energy terms encountered with particular flow conditions are work of expansion or viscous dissipation terms, primarily important in high-speed flow external field effects, mechanical or electrical, can also occur. Since they usually are of much less importance, they will not be considered here. Heat radiation in the reactor is often neglected, except in the case of fixed bed catalytic reactors operating at high temperatures, but then it is generally lumped with the heat conduction and a few more heat transport mechanisms into an "effective"... 

Spin-orbit(SO) coupling is an important mechanism that influences the electron spin state [1], In low-dimensional structures Rashba SO interaction comes into play by introducing a potential to destroy the symmetry of space inversion in an arbitrary spatial direction [2-6], Then, based on the properties of Rashba effect, one can realize the controlling and manipulation of the spin in mesoscopic systems by external fields. Recently, Rashba interaction has been applied to some QD systems [6-8]. With the application of Rashba SO coupling to multi-QD structures, some interesting spin-dependent electron transport phenomena arise [7]. In this work, we study the electron transport properties in a three-terminal Aharonov-Bohm (AB) interferometer where the Rashba interaction is taken into account locally to a QD. It is found that Rashba interaction changes the quantum interference in a substantial way. 

Field-flow fractionation is, in principle, based on the coupled action of a nonuniform flow velocity profile of a carrier liquid with a nonuniform transverse concentration profile of the analyte caused by an external field applied perpendicularly to the direction of the flow. Based on the magnitude of the acting field, on the properties of the analyte, and, in some cases, on the flow rate of the carrier liquid, different elution modes are observed. They basically differ in the type of the concentration profiles of the analyte. Three types of the concentration profile can be derived by the same procedure from the general transport equation. The differences among them arise from the course and magnitude of the resulting force acting on the analyte (in comparison to the effect of diffusion of the analyte). Based on these concentration profiles, three elution modes are described. 

Electrophoretic elution and "switch" monoclonal antibodies are combined in a new rapid recycle method an affinity-mediated membrane transport process reported by Dall-Bauman and Ivory (8). In this modeling paper, a "switch" monoclonal antibody incorporated into a supported liquid membrane is used to facilitate the transport of human growth hormone from a high-pH to a low-pH environment. Electrochemical effects, including Donnan equilibria between the membrane and external environments, and imposition of external electrical fields, significantly affected the flux of protein across the membrane. Experimental confirmation of the simulation results could introduce affinity-mediated transport as a powerful new biospecific separation method. 

Sancho-Garcia, J.C., Horowitz, G., Bredas, J.L., and ComU, J., Effect of an external electric field on the charge transport parameters in organic molecular semiconductors,... 

---