Electroosmosis electric double layer

The physical separation of charge represented allows externally apphed electric field forces to act on the solution in the diffuse layer. There are two phenomena associated with the electric double layer that are relevant electrophoresis when a particle is moved by an electric field relative to the bulk and electroosmosis, sometimes called electroendosmosis, when bulk fluid migrates with respect to an immobilized charged surface. 

Figure 9.2 Formation of an electrical double layer responsible for electroendosmotic flow in an uncoated fused-silica capillary. The negative charges on the surface of the capillary are neutralized by positive charges of cations present in the buffer, which form an electrical layer near the surface of the capillary. When the electric held is apphed, the positive charges migrate toward the negative electrode, generating a bulk flow of the solution contained within the column. Electroosmosis exhibits a flat prohle, in contrast to hydraulic flow, which is parabolic.

Electroosmosis (p. 167) plays a very significant role in hpce because the interior surface of a quartz capillary develops a negative charge when in contact with aqueous solutions due to the ionization of surface silanol groups (Si-OH) above pH 4 and the adsorption of anions. As a result, a layer of cations from the bulk solution builds up close to the wall to maintain a charge balance by forming an electrical double-layer . The high fields employed... 

The spatial charge distribution in the electrical double layer is exactly what causes the electrokinetic phenomena, namely the mutual displacement of the phases in contact in an applied external electric field (electrophoresis and electroosmosis) or the charge transfer that occurs upon the mutual motion of phases (streaming and sedimentation potentials and currents). The following consideration, the simplest consistent with the Helmholtz model, establishes the relationship between the rate of the phase shift, e.g. that of electroosmosis, and the strength of the external electric field, E, directed along the surface3. 

We consider the velocity field bounded by a cylindrical slip surface, which excludes a thin electric double layer (EDL) [16]. The velocity scale Ut includes the effects of local pressure gradients, electroosmosis, and electrophoresis and can be expressed as follows ... 

There are four electrokinetic phenomena (electrophoresis, electroosmosis, streaming potential, and sedimentation potential), all of which involve both the theory of the electric double layer and that of liquid flow. Among them electrophoresis has the greatest practical applicability to the study of biomolecules and biocell surface porperties. In this section, the relation between electrophoretic mobility and its related electrokinetic potential C will be discussed. 

Electrokinetic flow covers in principle the transport of liquids (electroosmosis) and samples (electrophoresis) in respraise to an electric field. Both motions are associated with the electric double layer that is formed spraitaneously at the solid-liquid interface in which there is a net charge density. Compared to the traditional pressure-driven flow, electrokinetic flow is more suited to miniaturization due to its nearly plug-like velocity profile and much lower flow resistance. However, Joule heating is a ubiquitous phenomenon in electrokinetic flow that will affect the transport of both liquids and samples via temperature-sensitive material properties. 

The electrokinetics are a class of several different interfacial effects that become important in micron and submicron dimensions. The most important and widespread categories of the electrokinetic effects are the electroosmosis and the electrophoresis. When the ionized liquids are in contact with stationary charged surfaces, counterions accumulate near the surface and buUd a layer that is called the electric double layer (EDL). The presence of an external electric field moves this layer and consequently generates the bulk flow field in the channels. This effect is named as electroosmosis and the generated flow is electroosmotic or electrokinetic flow. The external electric field also moves charged species and macromolecules in the micro- and nanochaimels which is usually referred to electrophoresis or electrophoretic effect. 

As described in the article on nonlinear elec-trokinetic phenomena, electroosmosis of the second arises when the bulk salt concentration goes to zero at a surface passing a diffusion-limited current. Under conditions of super-limiting current, the density of counterions in the electric double layer loses its classical quasiequilibrium profile, and a region of dilute space charge extends into the solution to the... 

Electroosmotic flow is the bulk liquid motion that results when an externally applied electric field interacts with the net surplus of charged ions in the diffuse part of an electrical double layer. The term electroosmotic flow and electroosmosis are generally used interchangeably in the context of micro- and nanofluidics. Alternating current electroosmosis in which bulk flow is generated using... 

Electrophoresis. The movement of a charged interface (usually colloidal particles or macromolecules) plus its electrical double layer relative to a stationary liquid, under the influence of an applied field. Electrophoresis is, of course, the complement of electroosmosis. 

It may be appreciated that electrokinetic phenomena are determined by electric properties at the plane of shear rather than at the real surface. In the following sections of this chapter, the relation between the measured property and is further analyzed. This is done for electroosmosis, electrophoresis, streaming current, and streaming potential. The sedimentation potential will not be discussed any further, because in practice this phenomenon does not play an important role. The electrokinetic charge density may then be derived from using the theory for the diffuse electrical double layer. 

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