Field-induced reorientation

Obviously, the model is crude and does not take into account many of the factors operating in a real molecular stack. Lack of symmetry with respect to the polar axis and the fact that dipoles may not necessarily be situated in one plane represent additional complications. The angle a could also be field dependent which is ignored in the model. The model also requires that interactions between molecules in adjacent stacks be very weak in order for fields of 10 to 20KV/cm to overcome barriers for field induced reorientation. The cores are then presumably composed of a more or less ordered assembly of stacks with a structure similar to smectic liquid crystals. 

Kitzerow et al. recently demonstrated that temperature-induced phase transitions (Iso-N) and electric field-induced reorientation of a nematic liquid crystal (5CB in this case) can be used to tune photonic modes of a microdisc resonator, in which embedded InAs quantum dots serve as emitters feeding the optical modes of the GaAs-based photonic cavity [332],... 

Jan. 26, 1927, Farnborough, Great Britain - July 9, 2005, Ottawa, Canada) Canadian electrochemist, 1946-1949 Imperial College, London University, thesis on -> electrocatalysis and corrosion inhibitors (supervisor J.O M. Bockris), 1949-1954 Chester-Beatty Cancer Research Institute with J.A.V. Butler on DNA, 1954-1955 post-doc at University of Pennsylvania with J.O M. Bockris (among other subjects -> proton -+ mobility, the effect of field-induced reorientation of the water molecule), since 1956 professor at the University of Ottawa (Canada), more than 400 publications on physical electrochemistry, electrode kinetics and mechanisms, - electrochemical capacitors. 

This process is called field-induced reorientation of the water molecules. 

Fig. 4. 133. Proton field-induced reorientation of a water molecule. (After Lyons, 1996.)...

At T = 1.3 K, the sir rate is exclusively determined by the direct process, as will be shown explicitlybelow. By use of Eq. (15) and with the values of (1.3 K) = 1.39 X 10 s and AEn j = 7 cm and under assumption of reasonable values for the mass density (p = 0.9 g/cm , as determined from the crystal structure of n-octane [85]) and the velocity of sound (v = 1.3 km/s for a similar compound [109,158]) one can estimate the size of the matrix element (II V I) to 1.1 cm [65]. This interaction energy expresses the size of coupling of the triplet substates II and I induced by the phonon perturbation. Interestingly, application of a magnetic field of B = 10 T reduces this matrix element to about 0.7 cm k In Ref. [24], it has been proposed that this reduction could be related to a field-induced reorientation of the spin system from the molecular frame towards the external magnetic field axis. 

Nikolov O, Hall I, Barilo SN, Mukhin AA (1996) Field-induced reorientations in TbFeOs at 4.2 K. J Magn Magn Mater 152 75-85... 

In addition to its hquid crystalline phase behavior, graphene has been demonstrated as a transparent electrode in LCD devices and a potential substitute for metal oxide electrodes [132]. It has been found that the electrooptical characteristics of such devices are superior in nature. In a similar fashion GO has been utilized in its reduced form in LC cells to study the field-induced reorientation of a nematic LC [133, 134]. These preliminary results are very encouraging for future LC devices. The advantages of graphene compared to conventionally used metal oxide electrodes in terms of low resistivity, high transparency and chemical stability hold great promise for large scale exploitation as transparent conductive electrodes. 

The result of the investigations described above fit well into the interpretation of the laser field induced reorientation of nematics, given in our earlier papers. The possibility of controlling the beam divergence by an external field may find some practical applications. 

The response time r given by Eq. (12) is a function of the pump intensity, but in our case, r (of the order of 10—ICX) fistc during the pump pulse) is always much longer than the pump pulse width. The solution of Eq. (14) is then trivial. Immediately after the pump pulse is over, the optical-field-induced reorientation is simply... 

The field-induced reorientation of nematic polymers caused by the coupling of the electric field with their dielectric anisotropy was studied in a variety of papers [229, 231-238]. Unfortunately, only in a few papers (e.g., [237]) is a certain preliminary orientation of a polymer specified and we can speak of the true Frederiks transition with a well-defined threshold voltage. Nevertheless, the general opinion is that the Prank elastic moduli of both comb-like [232-235, 238] and linear-chain [236, 237] nematic polymers are of the same order of magnitude as of their low-molecular mass counterparts. 

The storage effects could also be realized in polymer liquid crystals. On cooling, ferroelectric liquid crystal polymers with the electric field applied, the macroscopic polarization is frozen in the glassy state [74]. Thus, the polymer film becomes a pyroelectric and a piezoelectric. Unfortunately, the glassy state is too viscous to allow the field-induced reorientation of the polarization and the film cannot be considered to be a ferroelectric. 

Field-induced reorientation of the director with attendant optical changes has recently been used in a novel application with the potential for large-area LCDs polymer dispersed LCs (PDLCs). A PDLC is a microemulsion of MLC dispersed in a conventional transparent polymer film. In the off state there is a mismatch between the refractive index of the MLC and that of the host polymer film. Hence the dispersion of MLC droplets scatters light very effectively, giving an optically opaque film (Fig. 5.14, left-hand side). On application of an external electric field (across a capacitor-like transparent coating of tin oxide on both sides of the polymer film), the director assumes the same orientation in all of the microdroplets. If the... 

Very often, the applied electric field induces reorientation of anisotropic particles and buildup of aligned particulates. This was elegantly demonstrated by Walcarius et al. [45] for the electrodeposition of perpendicular orientation of mesoporous electrodeposit on electrodes. 

As the field-induced reorientation of the director occurs at rather weak fields, the geometries worth discussing here for nonpolar mesophases are (i) the field parallel to the director and a>0 and (ii) the field perpendicular to the director and a<0. In the first case, uniaxial symmetry is conserved and the field stabilizes thermal fluctuations and increases the orientational order parameter in both the nematic and smectic A phase [26]. A qualitative picture is... 

The process of the field-induced reorientation of the director, called the Frederiks transition [60, 61], is usually discussed for the three typical geometries shown in Fig. 10. 

Under certain conditions the field-induced reorientation caused by the dielectric torque may result not in a uniform distribution of the director in the plane of a liquid crystal layer but in a spatially periodic distortion, either steady-state or transient. 

Field-Induced Reorientation without Flow Coupling ... 

The following example demonstrates how the viscosity coefficient Yi comes into play in field-induced reorientational effects. Consider pure twist deformation caused by an externally applied field 77 on a planar sample as depicted in Figure 3.12. Let 9 denote the angle of deformation. The director axis h is thus given by n = (cos 0, sin 0, 0). 

Although by far nematics are the most extensively used ones, other phases (smectic, cholesteric, etc.) of hquid crystals and mixed systems such as polymer-dispersed liquid crystals capable of field-induced reorientation have also been employed for electro-optical studies and applications. They are basically based on the same basic mechanism of field-induced director axis reorientation similar to nematic hquid crystals i.e., the response is Kerr like in that it is independent of the direction of the electric field. In general, nematic liquid crystal electro-optics devices switch at a rate of several terrs of hertz, corresponding to response times from a few to tens of microsecorrds. 

In the nematic phase field-induced reorientation of the director axis arises as a resirlt of the tendency of the total system to assirme a new configuration with the... 

For P = 0, there is a threshold intensity for finite reorientation to occur. The threshold intensity depends on both the thickness of the film and the beam size coq- For coq. the threshold intensity increases dramatically (compared with the value for a plane wave). There is no threshold intensity for field-induced reorientation in the p A 0 case. 

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