Security devices based on liquid crystals doped with a colour dye

Liquid crystal properties make them useful for the development of security devices in applications of authentication and detection of fakes. Induced orientation of liquid crystal molecules and birefringence are the two main properties used in security devices. Employing liquid crystal and dichroic colorants, we have developed devices that show, with the aid of a polarizer, multiple images on each side of the device. Rubbed polyimide is used as alignment layer on each substrate of the LC cell. By rubbing the polyimide in different directions in each substrate it is possible to create any kind of symbols, drawings or motifs with a greyscale; the more complex the created device is, the more difficult is to fake it. To identify the motifs it is necessary to use polarized light. Depending on whether the polarizer is located in front of the LC cell or behind it, different motifs from one or the other substrate are shown. The effect arises from the dopant colour dye added to the liquid crystal, the induced orientation and the twist structure. In practice, a grazing reflection on a dielectric surface is polarized enough to see the effect. Any LC flat panel display can obviously be used as backlight as well

Measuring Thickness and Pretilt in Reflective Vertically Aligned Nematic Liquid Crystal Displays

Pretilt angle is a parameter of the utmost importance in the ultimate performance of vertically-aligned negative nematic LC displays. When these devices work in reflective mode, as is the LCOS microdisplays, accurate measurement of pretilt angles becomes a difficult problem, since usual experimental setups based on retardation of the polarization components of the impinging light are proportional to the product effective birefringence (neff - no) times thickness, and any attempt to separate these variables is cancelled out by symmetry. This work shows a relatively simple method capable of separating both variables. An experimental setup specifically aimed at vertically aligned reflective cells has been prepared. At the same time, a simulation model has been developed taking into account the properties of actual reflective displays. Comparison between experimental and theoretical results shows some discrepancies that can be explained assuming that the LC profile contains a residual twist. Including that twist in the model, an excellent agreement between theory and experiment has been achieved. Matching of simulations and measurements yields to the separate determination of pretilt angle and thickness and gives good estimates for the residual twist angle

Neoplasias orales en el perro y en el gato

En el presente trabajo se revisan las principales neoplasias orales del perro y del gato, de acuerdo con la experiencia clínica de los autores. Para cada neoplasia se discute la etiología y el cuadro clínico, y los autores recomiendan unas determinadas pautas de diagnóstico y tratamiento.In this paper the most frequent neoplasias of the oral cavity of the dog and cat are described and discussed according to the clinical experience of the autbors. For each neoplasia a diagnostic and therapeutic approach is proposed

V-Shape Liquid Crystal-Based Retromodulator Air to Ground Optical Communications

This paper describes the use of a 2D liquid crystal retro-modulator as a free space, wireless, optical link. The retro-modulator is made up of a retro-reflecting cornercube onto which 2 cascaded V-shape smectics liquid crystal modulators are mounted. The communication link differs with respect to more conventional optical links in not using amplitude (nor frequency) modulation, but instead state-of-polarisation (SOP) modulation known as Polarisation Shift Keying (PolSK). PolSK has the advantage over amplitude modulation, that it is less sensitive to changes in the visibility of the atmosphere, and increases inherently the bandwidth of the link. The implementation of PolSK both in liquid crystal based and in retro-modulated communication are novelties

Caracterización de la membrana NF-97 mediante disoluciones salinas. Aplicacióndel modelo de Spiegler-Kedem-Kachalsky

Los procesos fÍsicos basados en las tecnologÍas de membrana se muestran como una clara alternativa en el campo de la desalinización y tratamiento de aguas. Un paso previo en el diseño de estos procesos consiste en la selección y caracterización de las membranas que van a ser utilizadas, siendo estos ensayos de gran utilidad para predecir el comportamiento de las mismas durante su posterior empleo. En este trabajo se ha caracterizado, mediante el uso de dos disoluciones salinas distintas, la membrana de nanofiltración de poliamida (NF-97), al objeto de obtener sus condiciones óptimas de funcionamiento. Los resultados obtenidos se han ajustado al modelo de transporte de flujo a través de la membrana de Spiegler-Kedem-Katchalsky, determinando, mediante resolución gráfica, las constantes del mismo. Se ha comprobado que el modelo es adecuado para describir el proceso de eliminación de las sales mediante nanofiltración, lo que permite predecir el comportamiento de la membrana en otras condiciones de operación

El maltrato psicológico. Causas, consecuencias y criterios jurisprudenciales. El problema probatorio

Tradicionalmente, la violencia psicológica ha sido considerada como un tipo de violencia «invisible», ya que no se expresa a través de agresiones físicas. Es un hecho admitido que el maltrato psicológico, en sentido estricto, implica siempre conductas dirigidas a causar un daño en la víctima de muy difícil prueba porque, al no tratarse de menoscabos o lesiones físicas, no quedan huellas visibles en la mujer maltratada. Sin embargo, los comportamientos a través de los cuales se manifiesta, son susceptibles de ser identificados, si contamos con las herramientas conceptuales adecuadas para sacar a la superficie los elementos necesarios para su acreditación, lo que además nos permitirá advertir acerca de las consecuencias nocivas que el maltrato psicológico produce en las mujeres afectadas, consecuencias que han permanecido prácticamente desapercibidas, hasta hace relativamente poco tiempo. Tanto es así, que hace veinticinco años, aproximadamente, se consideraba muy difícil de trascender y objetivar la fundamental cuestión de la prueba de las conductas implicadas en la violencia psicológica perpetrada en el ámbito de las relaciones de pareja

Synthesis and Properties of High Tilted Antiferroelectric Esters with Partially Fluorinated Alkoxyalkoxy Terminal Chains

Novel chiral esters with partially fluorinated alkoxyalkoxy terminal chains are described. Their phase transition temperatures, enthalpies, and electrooptical properties are reported. A helical pitch in pure compounds and their mixtures based on selective reflection of light is also characterized