Academic literature on the topic 'Active Cholesterics'

L’objectif de cette thèse est de réaliser une peau active pour le camouflage de véhicule terrestre à base de la technologie cristal liquide cholestérique. Après analyse de l’environnement, une texture est appliquée sur les écrans cristaux liquides afin de réduire la signature visuelle et thermique du véhicule. Les travaux ont consisté à proposer un dispositif trichrome cristal liquide permettant d’obtenir la colorimétrie définie par le cahier des charges. Les problèmes de tenue en température, de réflexion spéculaire, de niveaux de gris, niveaux de réflectivité, de réduction des tensions appliquées ont été traités au cours de cette thèse. La technologie proposée a permis la réalisation d’un démonstrateur sur un robot télécommandé qui a été présenté au salon Eurosatory 2018 sur le stand du ministère des armées. Des travaux ont été également engagés pour le développement de modulateur optique dans l’infrarouge, à base de cristaux liquides cholestérique, afin de moduler la réflectivité et donc de pouvoir limiter la signature thermique du véhicule. Plusieurs pistes ont été testées et proposées<br>The objective of this thesis is to achieve an active skin for the camouflage of a land vehicle based on cholesteric liquid crystal technology. After analysis of the environment, a texture is applied on the LCDs to reduce the visual and thermal signature of the vehicle. The work consisted of proposing a trichrome liquid crystal device to obtain the colorimetry defined by the specifications. The problems of temperature resistance, specular reflection, gray scale, reflectivity levels, reduction of applied voltages were discussed during this thesis. The proposed technology enabled to realization of a demonstrator on a remote controlledrobot that was presented at Eurosatory 2018 on the stand of the Ministry of the Armed Forces. Work has also been started on the development of optical modulators in the infrared, based on cholesteric liquid crystals, in order to modulate the reflectivity and thus to be able to limit the thermal signature of the vehicle. Several tracks have been tested and proposed

This thesis consists of research work in the broad area of soft condensed matter theory with a focus on active matter. The study of long wavelength, low frequency collective behavior of active particles (bacterial suspensions, fish schools, motor-microtubule extracts, active gels) forms an interesting modification to liquid-crystal hydrodynamics, in which the constituent particles carry permanent stresses that stir the fluid. Activity introduces novel instabilities and many novel aspects emerge. Our works focus on the dynamics, order, fluctuations and instabilities in these systems. In particular, we investigated the dynamics, order and fluctuation properties emerging from effective hydrodynamic descriptions of translationally ordered active matter and also studied those in microwave-driven quantum Hall nematics. We also investigated the rheological properties of active suspensions subjected to an applied orienting field. A summary of the works carried out is as follows. Translationally ordered active phases – active smectics and active cholesterics: Active or self-propelled particles consume and dissipate energy generating permanent stresses that stir the fluid around them. The collective behavior of systems of active particles, in systems with translational order, pose interesting questions and possibilities of new physics that differ strikingly from those in systems at thermal equilibrium with the same spatial symmetry. We developed the hydrodynamic equations of motion for (a) an active system with spontaneously broken translational symmetry in one direction, i.e., smectic and (b) the simplest uniaxially ordered phase of active chiral objects, namely, an active cholesteric. We analyze the fluctuation properties as well as the nature of characteristic instabilities that these systems can display and make a number of predictions. For example, in the case of an active smectic, we show that active stresses generate an effective active layer tension which, if positive, sup-presses the Landau-Peierls effect, leading to long-range smectic order in dimension d =3 and quasi-long-range in d =2, in sharp contrast with thermal equilibrium systems. Negative active layer tension in bulk systems, however, lead to a spontaneous Helfrich-Hurault undulation instability of the layers, accompanied by spontaneous flow. Also, active smectics, unlike orientationally ordered active systems, normally have finite concentration fluctuations. Similarly, for the case of cholesterics we show that cholesteric elasticity intervenes to suppress some of the instabilities present in active nematics. xi Numerical simulation of active smectics: We present results from a Brownian Dynamics simulation, with no hydrodynamic interaction, of a system of apolar active particles form-ing translational liquid-crystalline order in a suspension. The particles interact through a prolate-ellipsoidal Gay-Berne potential. We model activity minimally through different noise temperatures for movement along and normal to the orientation axis of each particle. We present preliminary results on the disruptive effect of activity on smectic order for the parameter values investigated. Future work will test the predictions of our theory [1] on active smectics. Rheology of active suspensions near field-induced critical points : Shear induces orientation of active stresses in a suspension, through flow alignment. Depending on the sign, activity then either enhances or reduces the viscosity. The change in viscosity, in the zero frequency limit, is proportional to the product of the magnitude of active stress and the system relaxation time. A strong enough orienting field can make the system approach a critical point and the relaxation time diverges. We show that, this results in the divergence of viscosity at zero frequency making the system strongly viscoelastic. Depending on the sign, activity strengthens or reduces the effect of the field. We also investigate the rheological property of an active suspension with mixed polar and nematic oreder. Active quantum Hall systems: We construct the hydrodynamic theory for a 2d charged active nematic with 3d electrostatics. We have investigated the interplay of the Coulomb interaction and activity in these systems. We show that activity competes to enhance the charge density fluctuations normally suppressed by long-ranged Coulomb interactions. The charge structure factor Sq of the corresponding passive charged nematic goes to zero as q, whereas in charged active nematics, activity leads to a nonvanishing charge structure factor at small wavenumber. We also show how the effect of an applied magnetic field can be incorporated into the dynamics of the system and leave scope for further studies on these effects.

This thesis consists of research work in the broad area of soft condensed matter theory with a focus on active matter. The study of long wavelength, low frequency collective behavior of active particles (bacterial suspensions, fish schools, motor-microtubule extracts, active gels) forms an interesting modification to liquid-crystal hydrodynamics, in which the constituent particles carry permanent stresses that stir the fluid. Activity introduces novel instabilities and many novel aspects emerge. Our works focus on the dynamics, order, fluctuations and instabilities in these systems. In particular, we investigated the dynamics, order and fluctuation properties emerging from effective hydrodynamic descriptions of translationally ordered active matter and also studied those in microwave-driven quantum Hall nematics. We also investigated the rheological properties of active suspensions subjected to an applied orienting field. A summary of the works carried out is as follows. Translationally ordered active phases – active smectics and active cholesterics: Active or self-propelled particles consume and dissipate energy generating permanent stresses that stir the fluid around them. The collective behavior of systems of active particles, in systems with translational order, pose interesting questions and possibilities of new physics that differ strikingly from those in systems at thermal equilibrium with the same spatial symmetry. We developed the hydrodynamic equations of motion for (a) an active system with spontaneously broken translational symmetry in one direction, i.e., smectic and (b) the simplest uniaxially ordered phase of active chiral objects, namely, an active cholesteric. We analyze the fluctuation properties as well as the nature of characteristic instabilities that these systems can display and make a number of predictions. For example, in the case of an active smectic, we show that active stresses generate an effective active layer tension which, if positive, sup-presses the Landau-Peierls effect, leading to long-range smectic order in dimension d =3 and quasi-long-range in d =2, in sharp contrast with thermal equilibrium systems. Negative active layer tension in bulk systems, however, lead to a spontaneous Helfrich-Hurault undulation instability of the layers, accompanied by spontaneous flow. Also, active smectics, unlike orientationally ordered active systems, normally have finite concentration fluctuations. Similarly, for the case of cholesterics we show that cholesteric elasticity intervenes to suppress some of the instabilities present in active nematics. xi Numerical simulation of active smectics: We present results from a Brownian Dynamics simulation, with no hydrodynamic interaction, of a system of apolar active particles form-ing translational liquid-crystalline order in a suspension. The particles interact through a prolate-ellipsoidal Gay-Berne potential. We model activity minimally through different noise temperatures for movement along and normal to the orientation axis of each particle. We present preliminary results on the disruptive effect of activity on smectic order for the parameter values investigated. Future work will test the predictions of our theory [1] on active smectics. Rheology of active suspensions near field-induced critical points : Shear induces orientation of active stresses in a suspension, through flow alignment. Depending on the sign, activity then either enhances or reduces the viscosity. The change in viscosity, in the zero frequency limit, is proportional to the product of the magnitude of active stress and the system relaxation time. A strong enough orienting field can make the system approach a critical point and the relaxation time diverges. We show that, this results in the divergence of viscosity at zero frequency making the system strongly viscoelastic. Depending on the sign, activity strengthens or reduces the effect of the field. We also investigate the rheological property of an active suspension with mixed polar and nematic oreder. Active quantum Hall systems: We construct the hydrodynamic theory for a 2d charged active nematic with 3d electrostatics. We have investigated the interplay of the Coulomb interaction and activity in these systems. We show that activity competes to enhance the charge density fluctuations normally suppressed by long-ranged Coulomb interactions. The charge structure factor Sq of the corresponding passive charged nematic goes to zero as q, whereas in charged active nematics, activity leads to a nonvanishing charge structure factor at small wavenumber. We also show how the effect of an applied magnetic field can be incorporated into the dynamics of the system and leave scope for further studies on these effects.

L'objectif de cette thèse est de fabriquer différents dispositifs optiques basés sur la structuration des cristaux liquides. Nous avons tout d'abord présenté différentes méthodes pour aligner les molécules de cristaux liquides et détaillé celles que nous avons utilisées au cours de ce travail. L'alignement et certaines propriétés physiques des cristaux liquides ont permis de fabriquer des dispositifs optiques. Ces dispositifs se divisent généralement en trois catégories : les filtres optiques, les modulateurs spatiaux et les guides d'ondes optiques. Ils sont présents dans divers secteurs et particulièrement dans le domaine des télécommunications. La structure des cristaux liquides cholestériques a une biréfringence périodique qui donne lieu à une réflexion sélective de la polarisation circulaire de la lumière. Nous avons tiré profit de cette propriété en fabricant un miroir de Bragg commutable. Ce miroir nous a permis de fabriquer un filtre de Fabry-Pérot commutable et accordable. Un réseau de polymère a été utilisé pour stabiliser le cristal liquide cholestérique, afin d'apporter résistance mécanique et durabilité aux champs électriques. Les cristaux liquides ferroélectriques présentent des propriétés électro-optiques efficaces, en particulier un temps de réponse élevé. Cette propriété a été exploitée pour fabriquer des obturateurs optiques pour lunettes 3D actives basées sur des cristaux liquides ferroélectriques. Malgré un temps de réponse élevé, les cristaux liquides ferroélectriques présentent certains défauts structurels. Nous avons proposé une nouvelle technique pour pallier ces défauts. Enfin, nous avons fabriqué des guides d'ondes gravés dans le polymère à cristaux liquides. Nous avons fabriqué ce polymère à cristaux liquides de manière à obtenir deux phases de cristal liquide différentes : isotrope et anisotrope, sur le même substrat. Le substrat a ensuite été gravé afin de créer un séparateur de polarisation séparant le mode TE et TM dans les deux branches du guide.