Electro-osmosis flow direction

In the Nemst-Planck equations used the activity coefficients were neglected a term accounting for the electro-osmosis, however, is present. Calculated and measured concentration profiles could be made to inter-correspond by adapting the term for water transport. The values indirectly determined by electro-osmotic flow were now found to agree with those measured directly. 

We can observe electro-osmosis directly with an optical microscope using liquids, which contain small, yet visible, particles as markers. Most measurements are made in capillaries. An electric field is tangentially applied and the quantity of liquid transported per unit time is measured (Fig. 5.13). Capillaries have typical diameters from 10 fim up to 1 mm. The diameter is thus much larger than the Debye length. Then the flow rate will change only close to a solid-liquid interface. Some Debye lengths away from the boundary, the flow rate is constant. Neglecting the thickness of the electric double layer, the liquid volume V transported per time is... 

Figure 4.6. Electro-osmosis In a plug, contained In a capillary between two walls Wj and Wj. The applied electric field exerts a force to the left on the mobile positive charges In the liquid, leading to a liquid flow In that direction. For simplicity the countercheu ge Is only represented by plus signs.

Electro-osmosis in a closed cell leads to a hydrodynamic pressure which, in turn, causes a Poiseuille-type back flow (sec. 1.6.4d and fig. 1.6.10), leading to a velocity profile as in fig. 4.8. For the, most common, cylindrical cell, the resulting velocity profile is as in fig. 4.15. The mathematical elaboration is as follows. Let 2 be the axial direction in the cylinder and r the radial one, then the fluid velocity in the z-direction at a distance r from the axis can be written as... 

Safe and effective delivery of peptides has also been successfully demonstrated in human studies using iontophoresis, a technique that uses mild electric current to facilitate transport of molecules across the skin. ° Iontophoresis works primarily by a combination of two forces, electro-repulsion of charged drug molecule away from the electrode and into the skin, and electroosmosis, a convective solvent flow in the direction of the counter-ion transport. In general, cationic proteins and peptides are delivered more efficiently than anionic molecules because electro-osmosis works in the same direction as electro-migration for cationic species. 

It is best to match the mobility of the reagent to the average mobilities of the solutes to minimize electrodispersion, which causes band broadening [7]. When anions are determined, a cationic surfactant is added to the BGE to slow or even reverse the electro-osmotic flow (EOF). When the EOF is reversed, both electrophoresis and electro-osmosis move in the same direction. Anion separations are performed using reversed polarity. 

The fused-silica surface also provides another mechanism, electro-osmosis, which drives solutes through the tube under the influence of an electric field. The principle of electro-osmotic flow (EOF) is illustrated in Fig. 1. The inner wall of the capillary contains silanol groups on the surface that become ionized as the pH is raised above about 3.0. This creates an electrical double layer in the presence of an applied electric field so that the positively charged species of the buffer which are surrounded by a hydrated layer carry solvent toward the cathode (negatively charged electrode). This results in a net movement of solvent toward the cathode that will carry solutes in the same direction as if the solvent were pumped through the capillary. This electrically driven solvent pumping mechanism results in a flat flow profile in contrast to... 

When the solid phase is fixed (e.g., as a capillary, membrane, or porous plug), an electric field induces a flow of liquid termed electro-osmosis. The character of the flow depends on the construction of the apparatus. For example, in an electrophoretic cell, the liquid flows in one direction near the walls and in the opposite direction in the center of the cell, and the net flow across the cell cross-section is zero (Figure 2.2). Electro-osmosis can also be demonstrated as a difference in pressure (height of a water column) generated as a result of an electric field applied to a capillary, membrane, or porous plug. 

Electro-osmosis, which is a troublesome effect in this system, can be counteracted by carefully adjusting buffer flow in the direction of electrophoretic migration. The separation is usually started in such a way that the sample is placed in the column (together with some dye to visualize the movement) by making use of the reservoir buffer. Only then is the electricity switched on. 

Figure 16. Experimental system for measuring cadmium or zinc movement throngh the barrier, conducted in parallel, with and without electric field (Ruggeri, 2005). Arrows show the direction of the hydraulic flux, and electroosmotic and ion migratory flow. The dark area is the cadmium-contaminated soil. EO, electro-osmosis EM, electro-migration (ion migration).

Can the phenomenon of H" "-flux mediated electro-osmosis provide the biodynamic principle which we have been looking for It is clear that cellular transport processes such as axonal transport and cytoplasmic streaming could directly result from proto-osmosis if the ATPases inject protons into the filamentous systems. Ciliary movements could possibly result from a periodic proto-osmotic insurge of fluid in the cilia. A relative (contractile) movement of two sets of filaments could be caused by a proto-osmotic loop flow between the pairs of the filaments by means of the associated visous drag couple. But the question of the relevance of the effect can be solved only if it can be shown that the magnitude of the effect is sufficient, quantitatively, to explain at least one biodynamic phenomenon. The best-studied system in this context, from structural and biochemical aspects, is that of muscles. The necessary quantitative data for the comparision of theoretical result with experiments is available, but it has to be first confirmed that the conditions for the proposed electro-osmotic flow obtain in muscles. 

Steady states, as we have seen in Part One, are obtained when fluxes in opposite directions are involved. In electro-osmosis, hydrodynamic flow is opposed by electro-osmotic flux. In thermo-osmosis, hydrodynamic flow is opposed by thermo-osmotic flux. In case of chemical reactions, such situations can arise when positive feedback is opposed by negative feedback. For example, when autocatalysis is opposed by inhibitory reaction, steady state can be attained. However, the reaction rates are non-linear and have only non-linear steady states in practice. We illustrate this point by the following example. 

Direction of groundwater flow - Flow due to electro-osmosis Anode series O Cathode series... 

Conclusion. We may expect transport phenomena by means of electric fields to spawn a plethora of new devices. Electro-osmosis enables us to select specific flow directions, paving the way toward intriguing experiments. However, as far as practical technological applications are concerned, capacitive effects hold great promise because of their superior robustness. 

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