Electrokinetic phenomena techniques

Let us mention that dielectrophoresis has also found wide application in manipulation and sorting of particles and biological cells. Together with standard electrophoresis, it is perhaps the most often used electrokinetic phenomenon with practical applications in mind. Even particle separation can be achieved by using microelectrode arrays [55]. Based on the dielectrophoresis phenomenon, a new technique has recently become available for particle or cell separation, namely the dielectrophoresis/gravitational field-flow fractionation (DEP/G-FFF). In DEP/ G-FFF, the relative positions and velocities of unequal particles or cells are controlled by the dielectric properties of the colloid and the frequency of the applied field. The method has been applied to model polystyrene beads, but, most interestingly, to suspensions of different biological cells [56]. 

Another very important issue in this respect is the way to account for the surface conductivity. The formula of Bikerman (Equation 5.359), the correction factor to the electrophoretic mobility of Henry 3 (Equation 5.368), and the formula of O Brien and Hunter (Equation 5.371), quoted above are derived under the assumption that only the ions in the movable part (x > x Figure 5.67) of the EDL contribute to the surface conductivity, Xs- Moreover, the ions in the EDL are taken to have the same mobility as that in the bulk electrolyte solution. A variety of experimental data ° suggest, however, that the ions behind the shear plane (x < x ) and even those adsorbed in the Stem layer may contribute to Xs- Th term anomalous surface conductance was coined for this phenomenon. Such an effect can be taken into account theoretically, but new parameters (such as the ion mobility in the Stem layer) must be included in the consideration. Hence, the interpretation of data by these more complex models usually requires the application of two or more electrokinetic techniques which provide complementary information. Dukhin and van de Ven specify three major (and relatively simple) types of models as being most suitable for data interpretation. These models differ in the way they consider the surface conductivity and the connection between i and "Q. 

Acoustics has a related field that is usually referred to as electroacoustics (8). Electroacoustics can provide particle size distribution as well as zeta potential. This relatively new technique is more complex than acoustics because an additional electric field is involved. As a result, both hardware and theory become more complicated. There are even two different versions of electroacoustics depending on what field is used as a driving force. Electrokinetic sonic amplitude (ESA) involves the generation of sound energy caused by the driving force of an applied electric field. Colloid vibration current (CVC) is the phenomenon where sound energy is applied to a system and a resultant eleetrie field or eurrent is created by the vibration of the colloid electric double layers. 

The potential differences caused by the movement of particles through a liquid have only been investigated very rarely and rather superficially. The technique is difficult and it is not to be expected that the experiments will give additional information to the results of the other electrokinetic phenomena. We mtntion here the experiments of Dorn S Billxter Maxelt and Stock the last of which could confirm Von Smolu-CHOWSKi s equation for this phenomenon. 

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