Transiciones de orden inducidas en sistemas mesomórficos

Abstract

Experimentos realizados en celdas electro-ópticas sometidas a la aplicación de tensiones alternas (AC) y continuas (DC), conducen a resultados escasamente explorados en la literatura y jamás explicados de manera consistente. Particularmente, se observan patrones celulares que, bajo ciertas condiciones, evolucionan a patrones hexagonales. Teniendo en cuenta que la aplicación de tensiones alternas (AC) y continuas (DC), dan lugar a efectos que pueden ser relevantes en la formación de estas estructuras: inyección de cargas desde los electrodos, separación de cargas, interacciones flexoeléctricas, inestabilidades convectivas, etc, estos efectos fueron analizados a fin de determinar su importancia en la formación de las mencionadas estructuras. También se destaca la importancia de la presencia de un campo DC en la formación de las estructuras observadas, sugiriendo que posibles efectos flexo-eléctricos también deben tomarse en cuenta al momento de analizar las estructuras conseguidas.

Recent nuclear magnetic resonance (NMR) experiments suggest the possibility of order transitions in a nematic liquid crystal (5CB), as induced by the simultaneous action of an acoustic and a magnetic elds. The order tansition manifests as a structure, pattern or a defect network within the volume. An inherent problem of the NMR set-up is related to the di culty in observing, by optical methods, the changes induced in the sample volume during the experiment. In order to overcome this limitation, the problem was tackled within an electro-optical cell, thus having full access to optical microscopy techniques. This approach allows a direct observation of structures, even at a three-dimensional environment. In this context and, by considering the equivalence among the elastic, electric and acoustic torques, a series of experiments were performed using electro-optic cells with surface treatment subjected to electric excitation, gradually increasing their thickness in order to scan from a two-dimensional regime (2D) to a three-dimensional one (3D).The study revealed, especially in a 3D environment, the importance of the joint action of an alternate (AC) and continuous (DC) excitation, a case barely mentioned in the literature. Cellular structures were obtained in 5CB, that under certain particular conditions, evolve into a hexagonal pattern. Based on the experimental design and the observed behavior, we conclude that these structures are not dissipative. In 2D environment, a pattern formation possible due to exoelectric interaction was observed in 5CB.