Electrokinetic phenomena dielectrophoresis

Observation of this electrokinetic phenomenon involves the application of a spatially nonhomoge-neous electric field to the suspension this provoking the migration (or dielectrophoresis) of the polarized particles [48,49]. The particles will move toward the high-field region if they are more polarizable than the dispersion medium, or will be repelled toward the low-field region otherwise. 

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]. 

An alternative method is electrokinetic focusing, which uses a direct current electric field (at high voltages of 1 kV) to focus particles and Uquids into a narrow stream. Typically, the sample fluid stream is driven along the central channel of a cross-shaped channel. As the sample enters the intersection, three fluid streams meet and the sample stream is focused into narrow stream. Dielectrophoresis (DEP) can also be used for sample focusing. DEP is the movement of polarized particles in a nonuniform electric field. This phenomenon is explained further in the next section. The main advantage of this... 

Since all materials are polarizable to some degree, the surface charge is generally not fixed. This leads to a broad class of nonlinear electrokinetic phenomena, where bulk electric fields interact with induced diffuse charge in solution to produce nonlinear electrophoretic motion, U =f(E). In electrolytes, such effects of induced-charge electrophoresis (ICEP) occur in addition to the purely electrostatic effect of dielectrophoresis (DEP) in low-frequency AC fields (< 100 kHz), where there is enough time for diffuse-charge relaxation around the particle within each period. ICEP is a complex phenomenon, which can lead not only to nonlinear mobility (in the field direction) but also to rotation and motion in arbitrary directions, even in uniform fields. 

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