Conventional nematic liquid crystal cells are fabricated with small surface pretilt of the director induced by rubbed polymer alignment. Depending on the orientation of the bounding surfaces, this may lead to two slightly different untwisted director configurations, splay and parallel. This small difference leads to remarkably different director profiles during pressure-driven flow, observed here using optical conoscopy. Data show excellent agreement with numerical modeling from Leslie-Ericksen-Parodi theory.

Director reorientation under pressure driven ͑Poiseuille͒ flow is observed conoscopically for the liquid crystal 5CB aligned at an azimuthal angle of 45°to the direction of flow. A polyimide surface treatment ͑AL 1254͒ is used to promote planar homogeneous alignment and rubbed to produce an initial azimuthal alignment angle 0 . Conoscopic interference figure rotation is documented as a function of flow rate and compared to that produced from numerical models using Leslie-EricksenParodi theory. Model and data show excellent agreement. © 2009 American Institute of Physics. ͓doi:10.1063/1.3251792͔The viscodynamics of pressure induced ͑Poiseuille͒ flow in nematic liquid crystals are important in many technological applications. These range from the use of low molar mass liquid crystals in the manufacture of liquid crystal displays to liquid crystal polymer ͑LCP͒ injection molding used to spin textile fibers.1 Director orientation and flow channel dimensions for fabrication techniques such as LCP injection molding can often play an important role in the order parameter 2 and the number of defects in finished products. Other studies have examined nematic flow alignment in samples with colloidal dispersions and the topological defects associated with them, as commonly found in foods, paint and drugs. 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. 1͑a͔͒, characteristic of any classical pressure gradient flow regime. Such a symmetric flow speed distribution results in differing azimuthal ͑-rotation͒ and tilt ͑-rotation͒ ͓see Fig. 1͑b͔͒ distortions through the depth of the sample compared to that of the linear velocity distribution produced from shear flow.The dynamics of anisotropic fluid flow are inherently complex. This is due to the need for consideration of both the translational and orientational order of the constituent molecules under flow. Liquid crystals undergoing flow are well described by the continuum theory proposed by that of Leslie 9 and Ericksen, 10 with five independent ͑Parodi 11 ͒ viscosity coefficients ͑␣ 1 ...␣ 5 ͒ determining director orientation under perturbation from an external source. Such a theory ͑for the ideal case of no boundary effects͒ proposes that flow aligning liquid crystals ͑␣ 3 / ␣ 2 Ͼ 0͒ rotate to achieve a constant angle L ͑Leslie angle͒ out of the shear plane, a...

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