Electro-osmosis pressure

In PEMFC systems, water is transported in both transversal and lateral direction in the cells. A polymer electrolyte membrane (PEM) separates the anode and the cathode compartments, however water is inherently transported between these two electrodes by absorption, desorption and diffusion of water in the membrane.5,6 In operational fuel cells, water is also transported by an electro-osmotic effect and thus transversal water content distribution in the membrane is determined as a result of coupled water transport processes including diffusion, electro-osmosis, pressure-driven convection and interfacial mass transfer. To establish water management method in PEMFCs, it is strongly needed to obtain fundamental understandings on water transport in the cells. 

Electro osmosis often accompanies electrophoresis. It is the transport of Hquid past a surface or through a porous soHd, which is electricaHy charged but immovable, toward the electrode with the same charge as that of the surface. Electrophoresis reverts to electroosmotic flow when the charged particles are made immovable if the electroosmotic flow is forcibly prevented, pressure builds up and is caHed electroosmotic pressure. 

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Once a matrix of particles is formed, whether filter cake, thickened underflow, or soil, applying a current to the fluid causes a movement of ions in the water and, with the ions, water of hydration. The phenomenon is called electro osmosis. The pressure generated on the fluid is given by (127) ... 

Consider that a potential difference is applied across a glass capillary tube filled with an electrolytic solution (Fig. 6.134). What would one expect Of course, one would expect a current to flow through the capillary according to Ohm s law. In practice, however, a remarkable and unexpected phenomenon is observed. In addition to the current, the solution itself begins to flow—the phenomenon of electro-osmosis. Liquid flow is generally associated with the application of a pressure gradient, but in this case it appears that a potential difference is doing the job normally achieved by a pressure difference. 

One can transcribe the phenomenon in the form of an equation following the same thinking as for electro-osmosis. One says A current density j results not only from an electric field but also from a pressure difference AP, and, for small X and AP,... 

CEC has recently become an alternative to HPLC. A capillary is filled or its internal wall covered with a porous sorbent. The free volume remaining in the capillary is filled with an electrolyte. High voltage (on the order of ten kV) is applied across the length of the capillary. Sample plugs are introduced at one end. Sample components are carried to the other end due to electro-osmosis and - in the case of ions - also electrophoresis. In CEC the more important effect is electro-osmosis, which is essentially a flow mechanism of the electrolyte solution without the need for applied pressure. The separation of the sample components occurs mainly due to phase distribution between the stationary phase and the flowing electrolyte. Thus CEC is very similar to HPLC in a packed capillary except that the flow is not pressure driven and that ionic analytes undergo electrophoresis additionally to phase separation. 

Electro-osmosis - the movement of liquid relative to a stationary charged surface (e.g. a capillary or porous plug) by an applied electric field (i.e. the complement of electrophoresis). The pressure necessary to counterbalance electro-osmotic flow is termed the electro-osmotic pressure. 

It can be seen when comparing (10) to (4) that the equations are of the same form with identical transfer functions multiplied by different constants. However, we want to compare the FDSP case to the electro-osmosis closed capillary case where the same ratio of AV/ AP can be compared. To get this ratio for the closed capillary the electro-osmosis volume flow must be balance by counter flow which is caused by a pressure build-up at the ends of the capillary. The counter flow is Poiseuille flow in a capillary and whose solution is... 

Water transport in electrodialysis from the diluate to the concentrate process stream can affect the process efficiency significantly. If a convective flux as a result of pressure differences between flow streams can be excluded there are still two sources for the transport of water from the diluate to the concentrate solution. The first one is the result of osmotic-pressure differences between the two solutions, and the second is due to electro-osmosis that results from the coupling of water to the ions being transported through the membrane due to the driving force of an electrical potential. 

Current utilization In practical application electrodialysis is affected by incomplete current utilization. The reasons for the incomplete current utilization are poor membrane permselectivity, parallel current through the stack manifold, and water transport by convection and due to osmosis and electro-osmosis. In a well-designed stack with no pressure difference between diluate and the concentrate convective water transport is negligibly low and also the current through the manifold can be neglected. Under these conditions the overall current utilization is given by ... 

For a given tube or diaphragm, q is constant and so the difference of pressure P maintained as a result of electro-osmosis should be proportional to the applied voltage E and independent of the dimensions of the diaphragm, as has been found by experiment. ... 

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... 

Electro-osmosis is the counterpart of electrophoresis in that now the materials to be studied are stationary, whereas the liquid moves at a given velocity, driven by an applied field. In streaming potential measurements an applied pressure difference is the driving force in that case a potential difference is measured. In practice, two ways are open working with capillaries or with plugs. 

The flow of liquid caused by electro-osmosis displays a pluglike profile because the driving force is uniformly distributed along the capillary tube. Consequently, a uniform flow velocity vector occurs across the capillary. The flow velocity approaches zero only in the region of the double layer very close to the capillary surface. Therefore, no peak broadening is caused by sample transport carried out by the electro-osmotic flow. This is in contrast to the laminar or parabolic flow profile generated in a pressure-driven system, where there is a strong pressure drop across the capillary caused by frictional forces at the liquid-solid boundary. A schematic representation of the flow profile due... 

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. 

Figure 1. A schematic diagram of dewatering of clay by the combined processes of electro-osmosis and pressure. ...

From Eq. (8), it is easy to see the explanation of a persistent puzzle in the EOD literature that experimental efficiencies translate to high and obviously untenable amounts of water per ion the measured current accounts only for Ln, whereas most of the water is removed by L21 (electro-osmosis) and, in the presence of applied hydrostatic pressure, by J-ai-... 

Ericson, C., Holm, J., Ericson, T. and Hjerten, S. (2000) Electro-osmosis and pressure-driven chromatography in chips using continuous beds. Anal Chem, 72 (1), 81-87. 

The streaming potential is an electric potential difference induced by the response of an electrolyte solution near an electrified interface to an applied uniformpressure gradient. As m electro-osmosis, only the liquid phase moves in response to the pressure gradient. The convection current, I, established through this liquid motion can be deduced from Eq. 3.32 in the special case where f x) = 0, dPIdz = constant ... 

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