Electrokinetics flow-generated potentials

Electro-osmosis is another electrokinetic phenomenon-in which an electric field is applied across a charged porous membrane or a slit of two charged nonporous membranes (see figure IV - 31). Due to the applied potential difference an electric current will flow and water molecules will flow with the ions (electro-osmotic flow) generating a pressure difference. As can be derived from nonequilibrium thermodynamics (sec chapter V) the following equation can be obtained indicating that both phenomena, electro-osmose and streaming potential, are similar... 

Several experiments have shown that electrokinetically mediated iron mineralization is, in essence, a self-limiting process in which the current falls as an essentially impermeable (1 x 10" m/s or less) iron-rich lithilied soil mass develops (e.g. Hopkinson and Cundy, 2003 Cundy and Hopkinson, 2005). In the context of Cr(VI) stabilization, the generation of low-permeability zones, in which chemically stabilized Cr(III) is locked in iron-rich mineral phase(s), means that stabilized zones are likely to be resilient to reactive flow, with the low permeability also serving to limit potential loss of adsorbed Cr(VI) residue on the iron mineral phase(s). 

Another aspect of electrokinetics in the biological sciences deals with the generation of potentials due to the flow of blood in vivo. It was recently demonstrated that the ECG is due, at least partially, to electrokinetic rather than myoneural potentials. It was also shown that electrokinetic potentials are... 

Zeta potential is an electric potential on this surface that is usually assumed as an electrical boundary condition at the walls of the microchannels. The interaction of an externally applied electric field and the ions of the EDL causes the net ion movement toward the oppositely charged electrode. It drags the viscous fluid and generates the bulk flow field in the channel, which is called the electrokinetic or the electroosmotic flow. 

Briefly, one could say that they involve the generation of a liquid flow (or a particle motion) when an external field is applied to the solid-liquid interface, or, conversely, the generation of an electric potential when a relative solid-liquid motion is provoked by, for instance, a pressure gradient or a gravitational field. In this context, electrokinetic phenomena and the techniques associated with them demonstrate their importance. They are manifestations of the electrical properties of the interface, and hence deserve attention by themselves. But, furthermore, they are a valuable (unique, in many cases) source of information on those electrical properties, because of the possibility of being experimentally determined [4-6]. 

In 1808, Rous, a colloid chemist, observed that imposing an electrical potential difference across a porous wet clay led not only to the expected flow of electricity but also to a flow of water. He later applied hydrostatic pressure to the clay and observed a flow of electricity. This experiment displayed the electrokinetic effect and demonstrated the existence of coupled phenomena where a flow may be induced by forces other that its own driving force. Therefore, the electric current is evidently caused by the electromotive force, but it may also be induced by the hydrostatic pressure. When two chambers containing electrolytes are separated by a porous wall, an applied potential generates a pressure difference called the electroosmotic pressure. Also, mass flow may generate an electric current called the streaming current. 

Figure 16.4 Electrokinetic phenomena. Two chambers containing electrolytes are separated by a porous wall or capillary, (a) An applied potential V generates a pressure difference Ap, called the electroosmotic pressure, (b) If the fluid is made to flow from one chamber to another by a piston, it generates an electrical current I, called the streaming current...

Two other electrokinetic effects result from the flow of liquid past a stationary charge. In electro-osmosis, the flow of Uquid past a stationary charged surface (for example the wall of a capillary tube) is induced by an applied electric field. The pressure necessary to counterbalance this flow is called the electro-osmotic pressure. In the reverse effect, the electric field generated by charged particles flowing relative to a stationary liquid is termed the sedimentation potential. 

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