Electrokinetic effects electro-osmosis

If a liquid moves tangential to a charged surface, then so-called electrokinetic phenomena arise [101]. Electrokinetic phenomena can be divided into four categories Electrophoresis, electro-osmosis, streaming potential, and sedimentation potential [102], In all these phenomena the zeta potential plays a crucial role. The classic theory of electrokinetic effects was proposed by Smoluchowski2 [103],... 

For well-dispersed colloid systems, particle electrophoresis has been the classic method of characterization with respect to electrostatic interactions. However, outside the colloidal realm, i.e., in the rest of the known world, the measurement of other electrokinetic phenomena must be used to characterize surfaces in this respect. The term electrokinetic refers to a number of effects induced by externally applied forces at a charged interface. These effects include electrophoresis, streaming potential, and electro-osmosis. 

To characterize a surface electrokinetically involves the measurement of one of the above electrokinetic effects. With disperse colloidal systems it is practical to measure the particle electrophoretic mobility (induced particle velocity per unit applied electric field strength). However, for a nondisperse system one must measure either an induced streaming potential or an electro-osmosis fluid flow about the surface. 

The potential development of electrokinetic geotextUes offers an opportunity to increase the redevelopment of derelict land as a means of reducing the pressure for construction within the natural environment. This electrically conductive material enhances the filtration and drainage functionalities by electro-osmosis and ion migration. Various types of conductive component can be incorporated in the geotextile structure to achieve this effect, but the possible use of activated carbon fibre or similar additives with pollution sorptive surface properties can aid land recovery of poUuted areas. 

W By electro-osmosis In 1808 Reuss first observed the flow of water through a membrane of powdered quartz when a potential difference was applied. Based on this early experiment, theoretical studies in electrokinetics were made and several techniques have been devised to study effectively the behaviour of macromolecules in solution. 

The possibility of nonlinear electro-osmotic flow, varying as tt a E, seems to have been first described by Murtsovkin [1, 16], who showed that an alternating electric field can drive steady quadrupolar flow around a polarizable particle (Fig. la). This effect has recently been unified with other nonlinear electrokinetic phenomena in microfluidics [2], such as AC electro-osmotic flow (ACEO) at microelectrodes [4, 11,12] (Fig. lb), DC electrokinetic jets at dielectric comers [5] (Fig. Ic), and nonlinear flows around metal posts [3] (Fig. Id-e). These are all cases of induced-charge electro-osmosis (ICEO) -the nonlinear electro-osmotic flow resulting from the action of an electric field on its own induced diffuse charge near a polarizable surface. 

Another effect which is important in a discussion of the conductivity of colloidal systems is the surface conduction, i.e. a conductivity contribution from the double-layers. This contribution is important when the electrolyte content is relatively low in the bulk phase. The surface conductance is also important when measurements of electrokinetic phenomena (electrophoresis, electro-osmosis, etc.) need to be evaluated. Recently, it... 

The four possible types of electrokinetic phenomena are streaming (current) potential (electric potential generated by fluid movement relative to another phase), sedimentation potential or Dorn phenomenon or Dom effect (due to dispersed particles motion relative to the fluid caused by sedimentation) and electrophoresis and electro-osmosis (movement of two phases is caused by an external potential difference). 

Electrokinetic phenomena, namely electrophoresis, electro-osmosis and streaming potential are discussed in Vol. 1 at a fundamental level. These effects arise because of charge separation at the interface that is induced for example by application of an electric field. The plane at which the liquid starts to move is defined as the shear plane and the potential at this plane is defined as the electrokinetic or zeta potential. A schematic picture is given that describes the shear plane and zeta potential. The latter is mostly assumed to be equal to the Stern potential and in the absence of specific adsorption it can be equated to the surface potential, which is the parameter... 

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. 

Electro-osmosis is one of the electrokinetic effects (q.v.). If a potential difference is applied between the ends of a capillary tube containing electrolyte, or across a plug of finely divided material (which can be regarded as a bundle of capillaries), a movement of the liquid is observed. This is the reverse effect of electrophoresis (q.v.), where particles move through a liquid which is stationary. The effect can be studied in an apparatus such as that sketched in figure E.IO. A plug of the finely divided material is in the centre of the tube, which is completely filled with liquid, and a potential of, say, 200 V is applied between the two calomel electrodes. An air-bubble trapped in the capillary measures the rate of movement of the liquid. 

---