Separate measurements of the flexoelectric and surface polarization in a model nematic liquid crystal p-methoxybenzylidene-p´-butylaniline : Validity of the quadrupolar approach

The temperature dependences of the surface polarization have been measured at the interface of a conductive glass with both the homogeneously and homeotropically oriented nematic liquid crystal p-methoxybenzylidene-p´-butylaniline. The polarization was found in the field-off regime from the pyroelectric response of a cell to a short laser pulse, absorbed in the bulk of the liquid crystal. The temperature increment was calculated from the measurements of the birefringence induced by the same light pulse. It has been shown that the surface polarization at the homeotropic (mh) and planar (mp) interfaces is directed from an interface into the bulk and from the bulk to an interface, respectively (with a magnitude mh~— 0.3 pC/m and mp' ≈ 0.2 pC/m at 25℃). The experimental data may be explained in terms of the quadrupole model of the order-electric polarization with account of some additional contribution from molecular dipoles. The same technique also allows for the measurements of the z component of the flexoelectric polarization using a pyroelectric response of a hybrid (homeoplanar) aligned nematic cell and proper subtracting of the surface contributions. The flexoelectric polarization has been shown to be opposite to the sum of the surface terms mh + mp and directed from the planar to homeotropic interface. This means that the sum of the flexoelectric coefficients e=(e1 + e3) is positive (e ≅ 1.7 pC/m at 28℃). The temperature dependence of e has been shown to involve a combination of both the quadrupolar and dipolar contributions

Study of the vertically aligned in-plane switching liquid crystal mode in microscale periodic electric fields

The ongoing interest in fast liquid crystal (LC) modes stimulated by display technology and new applications has motivated us to study in detail the in-plane switching (IPS) vertically aligned (VA) mode. We have studied how the decrease of the period of the interdigitated electrodes (down to sub-micrometer scale) influences the switching speed, especially the LC relaxation to the initial homeotropic state. We have found that there are two types of the relaxation: a fast relaxation caused by the surface LC sub-layer deformed in the vicinity of the electrodes and the slower relaxation of the bulk LC. The speed of the fast (surface) mode is defined by half of a period of the electrode grating, while the relaxation time of the bulk depends on the LC layer thickness and the length of the driving electric pulses. Thus, the use of the surface mode and the reduction of the electrode grating period can result in significant increase of switching speed compared to the traditional LC modes, where the bulk relaxation dominates in electrooptical response. We have studied thoroughly the conditions defining the surface mode applicability. The numerical simulations are in good agreement with experimental measurements