Conoscopic observations of shear-induced rotations in nematic liquid crystals

Conoscopic observation of director reorientation during Poiseuille flow of a nematic liquid crystal

“…As in flow experiments, the director profile will be graduated from the top plate to the bottom plate due to wall effects at the surface and a distortion toward the Leslie angle in the bulk of the sample. 7 As such, the conoscopic interference figure gives an average and smaller value then the magnitude of director distortion at the midplane of the sample, as described by Boudreau et al ͑Ref. 7͒.…”

“…It is expected that as the initial alignment angle increases to 90°͑normal to flow direction͒ a hydrodynamic analog of the Freedericksz transition will be seen as has been previously shown for shear flow. 6 Translational distortion of the conoscopic figure observed in shear flow experiments 7,6 is not present under Poiseuille flow regimes due to the symmetry of the velocity distribution about d / 2. As discussed earlier, the conoscopic interference figure is an averaging through the depth of the entire cell, and such a symmetric velocity distribution results in a splayed state and an average distortion out of the shear plane of =0.…”

“…3 To date, much work has been carried out in examining director reorientation and measurement of various viscoelastic properties of nematic liquid crystals under shear induced ͑Couette͒ flow 4,5 including work involving the shearing of nematic liquid crystals under varying initial azimuthal ͑͒ alignment conditions. 6,7 Such shear flow experiments regularly involve the flowing of a liquid crystal sample between two glass plates by way of dragging one plate relative to the other at a constant velocity, producing a linear velocity distribution across the depth of the cell. In the Poiseuille flow study presented here, a distinctly different flow distribution is observed ͑particularly at low rates͒, 8 a parabolic distribution of flow speeds symmetrical about d / 2 ͓see Fig.…”

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

“…In general, liquid crystal flow experiments have involved shearing one boundary plate of an aligned cell relative to the other at a constant velocity, producing a linear velocity distribution across the depth of the cell [12][13][14]. Under pressure-driven flow, a distinctly different, often symmetric about the cell midplane, velocity distribution is observed [15] (see Fig.…”

“…Azimuthal distortion is revealed by symmetry of the conoscopic figure without accounting for the effect of tilt distortion on total birefringence. The conoscopic interference figure gives an average but smaller value of the magnitude of director distortion at the midplane of the sample, as described by Boudreau et al [13]. The azimuthal angle of the measured conoscopic figure rotation is compared with figures computed from the modeled director profiles by Berreman's 4 Â 4 matrix method [23,24].…”