Flow-reorientation effect

R. J. Mansfield, P. LoPresti, Dynamics of picosecond laser-induced density, temperature, and flow-reorientation effects in the mesophases of liquid crystals, J. Appl. Phys. 1991, 29, 2972-2976. 

Figure 9.17. Oscilloscope trace of the probe diffraction showing the observed flow-reorientational effect (time scale 20 ms/div). The initial spike is the density and thermal components (see Figs. 9.7 and 9.8). The slowly varying component in the milliseconds time scale is the flow-reorientational effect (after Ref 14).

In materials of positive dielectric anisotropy, most electro-optic phenomena are frequency independent field effects hydrodynamic effects, occurring with certain boundary conditions result in stable (laminar) flow, and no turbulent-flow reorientation is observed. ... 

The dynamics of the other two types of orientation distortions, bend and splay deformations, are more complicated because such distortions are necessarily accompanied by flow (i.e., physical translational motion of the liquid crystal) this phenomenon is sometimes called the backflow effect, which may be regarded as the reverse effect of the flow-induced reorientation effect discussed in Chapter 3. A quantitative analysis of these processes shows that the half-widths of the spectra associated with pure splay and pure bend deformations are given, respectively, by... 

Figure 8.2. An example of the flow-induced director axis reorientation effect in nematics, v is the flow velocity.

From now on, the permeation in (16) is neglected as it is several orders of magnitude smaller than the advection due to the radial component of the velocity vr (now playing the role of vz in the planar case). As far as the velocity perturbation is concerned, our aim is to describe its principal effect-the radial motion of smectic layers, i.e., instead of diffusion (permeation) we now have advective transport. In this spirit we make several simplifications to keep the model tractable. The backflow-flow generation due to director reorientation-is neglected, as well as the effect of anisotropic viscosity (third and fourth line of (19)). Thereby (19) is reduced to the Navier-Stokes equation for the velocity perturbation, which upon linearization takes the form... 

The use of atomic force microscopy (AFM) and flow injection QCM in tandem provided important information about the surface coverage and orientation on gold of a thiolated DNA probe, as reported by Zhou and co-workers [58]. The effect of using a different alkanethiol to reorient the... 

Because of the dielectric anisotropy property of LCs, the LC molecules can align either parallel or perpendicular to the electric field, theoretically, according to their values of dielectric anisotropy [44]. However, under certain conditions, the uniform director reorientation in an a-c electric field is unfavorable the domain structure corresponding to a minimum free energy is formed. The domain patterns can be classified into two main types orientational domains with pure director rotation without fluid motion and the electrohydrodynamic domains caused by the combined effects of the periodic director reorientation and regular vortices of material moving [44]. This kind of movement of LC materials is called hydrodynamic flow, mainly resulting from the effects of conductivity anisotropy of LC molecules and ionic electric current. 

The electro-optic effects described above all refer to director reorientation induced through the dielectric properties (Af) of the polymer. If, however, a change in the optical properties is induced by a current flow resulting from the anisotropic electrical conductivity (A a) of the material, then the so-called electro-hydrodynamic effects are observed. ... 

We use here the Ericksen-Leslie continuum theory to describe the effect. The rotational motion of the director (i.e., molecular reorientation) is driven by the pump laser pulse, but it is also coupled with the translation motion (flow) of the fluid through viscosity. Thus, with a finite pump beam, a rigorous theoretical calculation would require the solution of a set of coupled three-dimensional nonlinear partial differential equations for the angle of... 

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