Gradient-driven processes

In the drift tube, ions also disperse in the tube through diffusion. Diffusion is a concentration gradient-driven process. This diffusion process depends on the nature of the ions and the drift tube temperature that produce the concentration gradient of the ions as they migrate within the tube. The concentration gradient tends to broaden the detection peaks for respective ions in an ion mobility spectrum. This diffusion process is one of the reasons why the longer drift-time ion peaks are broader. 

In a pH gradient-driven processes, it is necessary to have in the more alkaline solution. If there were no in the receiving solution, we would have only... 

Let us examine how these concepts can be applied in practice by taking up a familiar example of a gradient-driven process, that of the conducHon of heat. 

Two common types of one-dimensional flow regimes examined in interfacial studies Poiseuille and Couette flow [37]. Poiseuille flow is a pressure-driven process commonly used to model flow through pipes. It involves the flow of an incompressible fluid between two infinite stationary plates, where the pressure gradient, Sp/Sx, is constant. At steady state, ignoring gravitational effects, we have... 

Diffusion That form of mass transport in which motion occurs in response to a gradient in concentration or composition, itself caused by a gradient of the chemical potential fi. Diffusion is ultimately an entropy-driven process. 

Under osmotic pressure gradients between the two aqueous phases of W/OAV emulsions, water may migrate either from the internal to the external phase or vice versa, depending on the direction of the osmotic pressure gradient. This process is entropically driven and is another manifestation of compositional ripening. Such... 

Diffusion in general, not only in the case of thin films, is a thermodynamically irreversible self-driven process. It is best defined in simple terms, such as the tendency of two gases to mix when separated by a porous partition. It drives toward an equilibrium maximum-entropy state of a system. It does so by eliminating concentration gradients of, for example, impurity atoms or vacancies in a solid or between physically connected thin films. In the case of two gases separated by a porous partition, it leads eventually to perfect mixing of the two. 

The general approach of graded radiation exposure can also be used to examine light driven processes such as photopolymerization [19]. For example, Lin-Gibson and coworkers used this library technique to examine structure-property relationships in photopolymerized dimethacrylate networks [38] and to screen the mechanical and biocompatibility performance of photopolymerized dental resins [39]. In another set of recent studies, Johnson and coworkers combined graded light exposure with temperature and composition gradients to map and model the photopolymerization kinetics of acrylates, thiolenes and a series of co-monomer systems [40 2]. 

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

Dialysis continues to meet certain specialized applications, particularly those in biotechnology and the life sciences. Delicate substances can be separated without damage because dialysis is typically performed under mild conditions ambient temperature, no appreciable transmembrane pressure drop, and low-shear flow. While slow compared with pressure-driven processes, dialysis discriminates small molecules from large ones reliably because the absence of a pressure gradient across the membrane prevents convective flow through defects in the membrane. This advantage is significant for two... 

The first method employs the ballistic gun (2,3), where cells are exposed to ballistic bombardment by microparticles coated with the molecules of choice (e g., DNA). The second method is based on exposing the cells to ultrasound leading to an increased transmembrane transport (4). The third approach is based on an electrically driven process (electroporation), where cells are exposed to high-electric fields for short durations of micro- to milliseconds (5). This exposure leads to induction of short-lived permeability changes in the membrane ( pores ) enabling the diffusion of molecules across the membrane along their electrochemical gradients. 

A diffusion flow against its conjugate gradient driven by the dissipation of another diffusional process would be called incongruenf diffusion. For example, the flow of the /th component across a membrane may be expressed by... 

During the last thirty years, intensive investigations by numerous laboratories converted photophosphorylation from a highly debatable and marginally detectable process to a well-established and well-dissected reaction. We have today a wealth of information about the overall photochemical steps, the electron transport reactions driven by it, the electrochemical gradient driven by the electron transport, and the overall reaction responsible for ATP synthesis by the enzyme-bound ATP... 

The processes that use energy from changes in potential or from energetically favorable chemical processes are called active transport processes. They can move substances against a concentration gradient using the additional contribution from AG which is not available in diffusion-driven processes. Students should be aware that both of these mechanisms can also be used to oppose transfer as well. Hence, electrical differences across a membrane may oppose a transfer instead of favor it and formation of ATP from ADP + Pi may be too energetically unfavorable of a barrier to allow a transfer to occur. 

Dialysis, a concentration gradient-driven separation process on the basis of molecular diffusion through a membrane. Dialysis is a suitable technique to exchange the buffer in which a protein of interest is dissolved or to decrease the salt concentration of a protein solution. 

The other method of diffusion of a chemical through a liquid phase, molecular diffusion, is driven by concentration gradients. It is normally orders of magnitude slower in natural waters than eddy-driven processes, unless the water body is abnormally still and uniform in temperature (Lerman, 1971). Such situations are found only in isolated settings such as groundwaters and sediment interstitial waters. Even here, however, empirical measurements often indicate that actual dispersion exceeds that calculated from molecular diffusion alone. 

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