Electroosmotic rise

Electroosmosis causes a change in the level of the liquid in communicating vessels, i.e. in the anodic and cathodic parts of a U-shaped tube. This effect, referred to as the electroosmotic rise, turns out to be very strong for example an applied voltage of 100 V may result in a change in liquid levels of up to 20 cm. Electroosmosis and the electroosmotic rise are thus related to the motion of the liquid with respect to the immobilized disperse phase (porous diaphragm). In the case of electroosmotic rise, at equilibrium the electroosmotic transfer of the liquid is compensated by its back flow due to the change in hydrostatic pressures in different arms of the U-shaped tube. 

Fig. V-13. The fluid velocity distribution profile in a capillary a -during filtration, b -during electroosmotic transfer, c - during electroosmotic rise...

The flow of medium leads to the appearance of difference in fluid levels in vessels attached to the capillary. The resulting pressure drop, Ap=pg A//, causes the counter-flow of dispersion medium, and the flow profile in the capillary is such as that shown in Fig. (V-13, c), i.e., near the walls and in the center of a capillary the medium moves in opposite directions. Under the steady-state conditions, when the net flux of medium is zero (QE + Qp=0), the height of electroosmotic rise, HE, is given by... 

H height of the disperse system volume (in sedimentation analysis) He height of electroosmotic rise... 

The movement (migration) of a charged species under the influence of an applied field is characterized by its electrophoretic mobility, fie, which has units of cm2 sec 1 V 1. Mobility is dependent not only on the charge density of the solute (the overall valence and size of the solute molecule), but also on the dielectric constant and viscosity of the electrolyte. Mobility is also strongly dependent on temperature, increasing by approximately 2% for each Kelvin rise in temperature.2 In the presence of electroosmotic flow (Section 4.3.3), the apparent mobility is the sum of the electrophoretic mobility of the analyte, /ze, and the mobility of the electroosmotic flow, /XGO. 

As expected, the confinement of phosphoric acid in the PBI matrix does not give rise to any relevant electroosmotic drag. Of course, the main reason is the fact that proton conductivity is dominated by structure diffusion, that is the transport of protonic charge carriers and phosphoric acid are effectively decoupled. The other reason is that protonic charge carriers are produced by self-dissociation of the proton solvent (phoshoric acid), that is the number of positively and negatively... 

We next calculate the stationary level and observed particle velocity in a cylindrical capillary of radius assuming Aq/a < 1. The electroosmotic effect gives rise to a velocity across the cross section of the tube toward the electrode of the same polarity as the charge on the cell wall. The liquid velocity across the tube according to the description above, is the vector sum of the electroosmotic velocity and the reverse Poiseuille flow ... 

Water permeation to the air cathode in liquid methanol solution-fed DMFCs creates a barrier for air diffusion to active sites in the cathode catalyst layer by flooding the electrode. The water transport mechanisms from the aqueous anode to the gaseous cathode are electroosmotic drag and diffusion. The Naflon membrane is saturated with water in the case of DMFCs, which gives rise to high electroosmotic drag coefficients in comparison to partially saturated membranes. 

As the usual parabolic velocity profile of pressure-driven Poiseuille flow leads to the flow rate scaling as R whereas the flat velocity profile obtained in pure electroosmotic flow gives rise to a flow rate that scales as R, it can be seen that it is more efficient to drive microchannel flows where R becomes very small using electrokinetic flows as opposed to pressure-driven flow. However, there are some design issues to be considered in electrokinetic bubble transport. 

A myriad of conditions could give rise to nonuniform charge distributions in the fluid. For example, electroosmotic flows or solutions of electrolytes are often used to manipulate microfluidic flows. In these situations, the macromolecule may be subject to Coulomb forces. There is no general expression to account for these forces, but they will depend on the location and strength of the charge distribution in the fluid as well as along the molecule. 

Fluid movement through a charged material (like earth) gives rise to electroosmotic pressure. This arises from asymmetrical charge distribution at the liquid-solid interface, which depends on the magnitude of the surface potential. 

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