Polarization direct current electric field

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... 

A phenomenon called dielectrophoresis has been used to pattern a variety of cell types on 2D substratesand more recently in 3D culture constructs. Unlike electrophoresis where charged species move in an applied electric field due to Coulombic forces (F = qE), dielectrophoresis capitalizes on the ability of a cell to become polarized when placed in an electric field. Dielectrophoresis is most often used in conjunction with dtemating current (AC) electric fields since AC fields eliminate electrophoretic movement, and have less physiological impact on cells than direct current (DC) fields. When a cell is placed in an AC field, the magnitude and polarity of the induced dipole depend on the frequency of the applied field and the conductivities of the cell and the surrounding medium, described by the equation... 

Displacement of mobile charges is often spatially limited due to heterogeneous stmcture of the polymer matericds or blocking electrodes therefore, the dc conductance may be not observable, but alternative current (ac) conductivity (cfac) can be found at sufficiently high frequencies. If the limitation of carrier mobility occurs at the phase boundary, the interfacial polarization occurs. The polarization may also be induced by the reorientation of the dipoles present in the stmcture of maciomolecules in the direction of electrical field. The polarization effects are related to dielectric properties characterized by complex dielectric permittivity which is frequency... 

If a piezoelectric plate (Fig. 6.1), polarized in the direction indicated by P, carries electrodes over its two flat faces, then a compressive stress causes a transient current to flow in the external circuit a tensile stress produces current in the opposite sense (Fig. 6.1(a)). Conversely, the application of an electric field produces strain in the crystal, say a negative strain reversal of the field causes a positive strain (Fig. 6.1(b)). The changes in polarization which accompany the direct piezoelectric effect manifest themselves in the appearance of charges on the crystal surface (see Eq. (2.71)) and, in the case of a closed circuit, in a current. 

Thus let the electric field that is applied to the film be oriented along the z axis. The polarization current appears as a result of rotation of the dipoles in the direction of the field that is, the dipoles periodically change the orientation and are now straight and in parallel direction and antiparallel direction in relation to the z axis. These positions of the dipoles in Ising s model are linked with two projections of the zth component of the pseudospin (Sz) 1/2 and —1/2, respectively. Since the water molecules are situated on the surface of the film, we should include in the consideration the interaction with the film, which in turns introduces the interaction with the film phonons. 

When an alternating electric field (a.c.) is applied across an insulator, a time dependent polarization current flow is induced. This is because the electrical charges present in the atoms and molecules in the material respond to the changing directions of the field. This is also referred to as dielectric response of the material. When the frequency of the applied field is well below the phonon frequencies, the dielectric polarization of the bound charges is instantaneous. Therefore, the dielectric constant, e (oo), characterizing the bound charge response, is frequency independent. The frequency dependent part of dielectric constant is by definition related to the frequency dependent conductivity, CT (co) as... 

Polar molecules display the property that they can be oriented along an electric field dipolar polarization phenomenon). In the absence of this phenomenon, dipoles are orientated at random and molecules submitted to Brownian movement only. In the presence of a continuous electric current, aU the dipoles are lined up together in the same direction. If submitted to an alternating current, the electric field is inversed at each alternance with a subsequent tendancy for dipoles to move together to follow the field. Such a characteristic induces stirring and friction of molecules which dissipates as internal homogeneous heating (Scheme 21). 

The sign of the Kerr ellipticity eK is defined as positive if the electric field of the reflected beam rotates in the same direction as the electric current that can be associated with the sample magnetization M, i.e. eK is positive if the rotation vector that can be associated with the handedness of the reflected polarization is parallel to M. The sign of the Kerr rotation polarization axis after reflection is parallel to M, too. 

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