Fluorescence depolarization and contact angle investigation of dynamic and static interfacial tension of liquid crystal display materials

Abstract

Interfacial interactions control two processes empirically known to be critical for molecular anchoring in twisted nematic liquid crystal displays technology (TN-LCDs): surface treatment and filling procedure. Static and dynamical interfacial tensions (ΓSL) between liquids and several substrates with similar roughness were observed respectively by contact angle (θc) of sessile drops and by fluorescence depolarization of thin liquid films flowing at high velocity. ΓSL decreased when glass was coated with tin dioxide and increased with polyvinyl alcohol (PVA) deposition. Drops were circular for all substrates except rubbed PVA, where they flowed spontaneously along the rubbing direction, reaching an oblong form that had θc parallel and perpendicular to the rubbing direction respectively greater and smaller than θc for nonrubbed PVA. This is attributed to polar group alignment generating an asymmetric ΓSL distribution with nanometric preferential direction, inducing a capillary-like flow. Polarization and anisotropy maps for high-velocity flow parallel to the PVA rubbing direction showed an increase in the net alignment of molecular domains and a widening of the region where it occurred. This is attributed to preferential anchoring in the downstream direction, instead of in several directions, as for nonrubbed PVA. This explains why filling direction is crucial for TN-LCDs homogeneous behavior.