Field-driven dynamics of nematic microcapillaries

Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of study for their unique electro-optical properties which have resulted in various applications such as switchable (transparent/translucent) windows. These composites are manufactured using desirable "bottom-up" techniques, such as phase separation of a liquid crystal/polymer mixture, which enable production of PDLC films at very large scales. LC domains within PDLCs are typically spheroidal, as opposed to rectangular for an LCD panel, and thus exhibit substantially different behaviour in the presence of an external field. The fundamental difference between spheroidal and rectangular nematic domains is that the former results in the presence of nanoscale orientational defects in LC order while the latter does not. Progress in the development and optimization of PDLC electro-optical properties has progressed at a relatively slow pace due to this increased complexity. In this work, continuum simulations are performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. Using a simplified elliptic cylinder (microcapillary) geometry as an approximation of spheroidal PDLC domains, the effects of geometry (aspect ratio), surface anchoring, and external field strength are studied through the use of the Landau--de Gennes model of the nematic LC phase.Comment: 22 pages, 9 figures, Physical Review

Formation and Field-switching Dynamics of Nematic Droplets

Liquid crystals (LCs) refer to a class of materials which have anisotropic properties. They are used in many technological applications ranging from displays to biological sensors. One example of a category of technologically relevant LC applications is optical functional materials, which include polymer-dispersed liquid crystal (PDLC) films. In these films, non-deformable micron-scale LC droplets are dispersed in a solid polymer matrix. Application of an electric field through the thickness of a PDLC film results in "switching" between a transparent "on" state (field-on) and a translucent "off" state (field-off). Thus the main application of these films are as switchable windows. Key to this mechanism is the ability for external fields to reorient the direction LC molecules within the droplets. In this work, the LC phase formation and external electric field-switching dynamics of orientationally-ordered LC droplets are studied using the continuum Landau--de Gennes model. The model is able to capture phase transition and reorientation dynamics on device-relevant length and time scales when combined with numerical methods such as the finite element method. Formation dynamics correspond to transitioning from a high-temperature disordered liquid phase to an orientationally-ordered phase referred to as a nematic LC. Field-switching dynamics correspond to the imposition and release of an external (electric) field. Particular emphasis is placed on non-spherical droplets, which may form naturally or intentionally under controlled conditions in the manufacturing of PDLC films. The interactions between shape, LC/polymer interfacial effects, electric field strength, and other parameters are first investigated for capillary geometries using a simplified model, which is then followed by fully three-dimensional droplet simulations. Finally, simulation results predicting a symmetry-breaking phase transformation process for spherical droplet domain are presented. This observation is found to be predicted when using physically-realistic material parameters approximating the LC compound pentyl-cyanobiphenyl (5CB), but not for simulations with a typical simplification of nematic elasticity known as the single elastic constant approximation

Anisotropy Across Fields and Scales

This open access book focuses on processing, modeling, and visualization of anisotropy information, which are often addressed by employing sophisticated mathematical constructs such as tensors and other higher-order descriptors. It also discusses adaptations of such constructs to problems encountered in seemingly dissimilar areas of medical imaging, physical sciences, and engineering. Featuring original research contributions as well as insightful reviews for scientists interested in handling anisotropy information, it covers topics such as pertinent geometric and algebraic properties of tensors and tensor fields, challenges faced in processing and visualizing different types of data, statistical techniques for data processing, and specific applications like mapping white-matter fiber tracts in the brain. The book helps readers grasp the current challenges in the field and provides information on the techniques devised to address them. Further, it facilitates the transfer of knowledge between different disciplines in order to advance the research frontiers in these areas. This multidisciplinary book presents, in part, the outcomes of the seventh in a series of Dagstuhl seminars devoted to visualization and processing of tensor fields and higher-order descriptors, which was held in Dagstuhl, Germany, on October 28–November 2, 2018

Anisotropy Across Fields and Scales

This open access book focuses on processing, modeling, and visualization of anisotropy information, which are often addressed by employing sophisticated mathematical constructs such as tensors and other higher-order descriptors. It also discusses adaptations of such constructs to problems encountered in seemingly dissimilar areas of medical imaging, physical sciences, and engineering. Featuring original research contributions as well as insightful reviews for scientists interested in handling anisotropy information, it covers topics such as pertinent geometric and algebraic properties of tensors and tensor fields, challenges faced in processing and visualizing different types of data, statistical techniques for data processing, and specific applications like mapping white-matter fiber tracts in the brain. The book helps readers grasp the current challenges in the field and provides information on the techniques devised to address them. Further, it facilitates the transfer of knowledge between different disciplines in order to advance the research frontiers in these areas. This multidisciplinary book presents, in part, the outcomes of the seventh in a series of Dagstuhl seminars devoted to visualization and processing of tensor fields and higher-order descriptors, which was held in Dagstuhl, Germany, on October 28–November 2, 2018