Electric field-induced phenomena

Concluding remarks and recapitulation. Rdated electric field-induced phenomena non-linear polarizabilities of atoms and molecules radial and angular correlations fluctuations of molecular fields. 

In this section we discuss electrohydrodynamic (EHD) instabilities, that is electric-field-induced phenomena that are caused by the flow of a liquid crystal (see also [8,219]. The reason for the flow is electrical conductivity, which has been disregarded in previous sections. The flow may arise either independently of the anisotropic properties of substance, as in isotropic liquids (isotropic modes of the electrohydrodynamic instability), or may be driven by the conductivity anisotropy, as in liquid crystals (anisotropic modes). The threshold for EHD instabilities depends on many parameters, such as the electrical and viscoelastic properties of substance, the temperature, and the applied field frequency. Due to flow distortion of the director alignment, the instability is usually accompanied by a characteristic optical... 

Electrostriction. As distinct from inverse piezoelectric effect, electrostriction is a phenomenon in which the strain and the electrical field inducing the strain are related by Sy = where My are electrostriction coefficients. Several relaxor... 

A very useful type of phenomenon in the study of colloidal particles is the electrokinetic phenomenon that results from the movement of a solid phase with surface charge relative to an electrolyte-containing liquid phase. An applied electric field induces movement or, conversely, movement induces an electric field. The phenomena can be divided into two types ... 

The dynamic yield stress (extrapolated to zero shear rates, Figure 8.15) becomes greater with stronger field, indicating the increase of attractive forces between the polarized particles with applied electric field. This phenomenon is attributed to columnar or fibrillar structure formed by the particles as a response to electrostatic interactions induced by electric field. The stronger the field, the larger shear rate is needed to destroy the structure. 

Consequently, we can understand that sound waves change the refractive index of a medium according to Eq. (4.31). In a piezoelectric medium sound waves can induce a new electric field. This phenomenon is very complex because the induced electric field relates to a change in the refractive index of the medium. In general, the acousto-optic effects are classified into two phenomena depending on the frequency of the sound waves, the Raman-Nath diffraction and the Bragg diffraction (see Table 4.10). 

Flow movement also has a relationship with the electrokinetic phenomenon, which can promote or retard the motion of the fluid constituents. Electrokinetic effects can be described as when an electrical double layer exists at an interface between a mobile phase and a stationary phase. A relative movement of the two phases can be induced by applying an electric field and, conversely, an induced relative movement of the two will give rise to a measurable potential difference.33... 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

Before we move on to consider the interaction between macroscopic bodies, let us look briefly at the phenomenon of retardation . The electric field emitted by an instantaneously polarized neutral molecule takes a finite time to travel to another, neighbouring molecule. If the molecules are not too far apart the field produced by the induced dipole will reach the first molecule before it has time to disappear, or perhaps form a dipole in the opposite direction. The latter effect does, however, occur at larger separations (>5 nm) and effectively strengthens the rate of decay with distance, producing a dependence of 1/R instead of 1/R . 

The phenomenon of two-photon absorption (2 PA) can be presented as a process of simultaneous absorption of two photons under high intensity irradiation, resulting in one excited molecule [Ij. The investigations of 2PA previously undertaken are of great interest for a wide variety of emerging applications [3,36-42]. The nature of 2PA can be described based on the interaction of molecular electrons with an optical field. On a microscopic level the displacement of molecular electronic charge under the electric field, E, is related to the induced molecular dipole moment fi ... 

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