Slow relaxation processes in nematic liquid crystals at weak surface anchoring

Photoinduced gliding of the surface director in azo‐dye doped nematic liquid crystals

“…Both azimuthal (in-plane) and zenithal (outof-plane) gliding have been observed in the presence of externally applied electric [1,2,3,4] or magnetic fields [5,6]. Various workers have suggested that gliding can be the result of adsorption and desorption of the liquid crystal on the polymer surface [7], of a mutual reorienting of the liquid crystal and polymer network [2,5,6] or of sublayers with different physical properties near the polymer surface [7,8]. It has also been demonstrated that gliding is not restricted to polymers with weak anchoring energy, so-called 'soft' polymers [3,4].…”

Experimental Determination of Elastic and Viscous Parameters of Liquid Crystals

“…Some phenomenological description of surface dynamics was proposed by a number of authors [143,153,[156][157][158][159][160][161]. In particular, they explain some features of extremely slow field-induced motion (gliding) of an easy axis n e defining the preferable surface direction of a liquid crystal.…”

“…As an example of a phenomenological model, incorporating both fast and slow surface dynamics, we can consider a sublayer model proposed to explain experimental results of azimuthal gliding at a weak surface anchoring, induced by strong in-plane electric field [159] or by a combined action of electric field and light [161].…”

“…These processes are described by two coupled torque balance equations (5.136) and (5.137) corresponding to torques acting on the surface director (n s ) and on the easy axes (n e ). In a Mauguin regime, which takes place after turning the electric field off, motion of the easy axis can be easily detected by measuring the intensity of light I passing through the cell placed between crossed polarizers expressed as I ¼ I 0 sin 2 j e ¼ I 0 f½tg 2 j 1 e expðÀ2t=t 0 e Þ=½1 þ tg 2 j 1 e expðÀ2t=t 0 e Þg: ð5:142Þ In the work [159], both the direct measurements of intensity of the laser light, passing through the narrow (about 100 mm) intraelectrode gap, and the digital snapshots processing were used to extract I(t) dependences. The characteristic lengths L s and L e are connected (Equation 5.126) with anchoring strengths W s and W e , Figure 5.57 Modified phenomenological model of electrically induced gliding: n bbulk director, n ssurface director, n eeasy axis, n 0unit vector of the initial surface direction produced by preliminary UV treatment, and n 2new preferable surface direction induced by both electric field and polarized light at combined effect under consideration.…”

“…Only a slight modification of the original phenomenological model [159] was required to describe the main features of a very complicated surface dynamics under the combined action of field and light. Namely, the additional preferred surface direction n 2 ( Figure 5.57) with a corresponding anchoring strength W 2 has to be introduced to account for the dependence of the slow relaxation time t e on control parameters.…”