Kickback effect

Homeotropic to planar transition backflow and kickback effects The other two geometries used in the Freedericksz experiment are more interesting as they result in a new effect, namely, hydrodynamic flow induced by orientational deformation. This is the inverse of the more familiar property of flow alignment that has been discussed at length in previous sections. 

The switch-off time is also called the decay time or relaxation time by some authors. The analogous result for electric fields yields the same value for the full solution z, t) for t > 0 is of course modified and is obtained via (5.416), (5.417) and (5.423) by replacing 0o with the analogous static solution for the electric field. In many practical situations, such as that to be encoimtered below when we discuss the kickback effect, it is often convenient and appropriate to approximate z) by a fixed constant when H He- The form in equation (5.424) for Toff is common for many relaxation processes in nematic liquid crystals when electromagnetic fields are set to zero. For example, the following values... 

Before progressing to a discussion of the homeotropic to planar transition and further comments on backflow, we reconsider the current planar to homeotropic problem when a high magnitude magnetic field is suddenly removed this leads us to a discussion of the kickback effect. 

Figure 5.11 Backflow and the kickback effect for the nematic MBBA using the physical parameters listed in Table D.3. A sample depth d of 200 microns was chosen.

The kickback effect can have serious implications in twisted nematic devices (cf. Section 3.7 above). Examples of such complications in connection with experiments and displays have been reported and can be found in the brief article by Gerritsma, van Doom and van Zanten [111]. Numerical studies showing the influence of back-flow in the twisted nematic cell under the application of a strong electric field being switched on or off have been carried out by van Doom [64]. 

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