A patterned surface defect of strength m = +1 and its associated disclination lines can decompose... more A patterned surface defect of strength m = +1 and its associated disclination lines can decompose into a pair of surface defects and disclination lines of strength m = +½. For a negative dielectric anisotropy liquid crystal subjected to an applied ac electric field E, these half-integer defects are observed to wobble azimuthally for E > than some threshold field and, for sufficiently large fields, to co-revolve antipodally around a central point approximately midway between the two defects. This behavior is elucidated experimentally as a function of applied field strength E and frequency ν, where the threshold field for full co-revolution scales as ν 1/2 . Concurrently, nematic electrohydrodynamic instabilities were investigated. A complete field vs. frequency "phase diagram" compellingly suggests that the induced fluctuations and eventual co-revolutions of the ordinarily static defects are coupled strongly to -and driven by -the presence of the hydrodynamic instability. The observed behaviour suggests a Lehmann-like mechanism that drives the co-revolution.
Electrically switchable polymer membranes with photo-aligned nematic structures for photonic applications
Optical Materials, Nov 1, 2020
Abstract Composite materials containing liquid crystalline microstructures, providing fast stimul... more Abstract Composite materials containing liquid crystalline microstructures, providing fast stimuli responsive variations of birefringence at low consumption power, have great potential applications in photonics. Here, a composite material based on polymer membranes with photo-aligned nematic microstructures is proposed for the first time. For this purpose, porous polyethylene terephthalate (PET) films were covered with an azo-dye and subjected to a modified photoalignment technique to control the azimuthal orientation and anchoring strength of nematic liquid crystals (LCs) at the pore walls. Nematic internal ordering was identified using polarized optical microscopic observations assisted by theoretical simulations. The proposed approach provides spatial distribution of nematic microstructures characterized by specific internal ordering and fingerprint in near infrared spectra at an electro-optical response. These composites can be used as flexible photonic LC elements such as spatial light modulators, optical filters and lenses.
We study decomposition of geometrically enforced nematic topological defects bearing relatively l... more We study decomposition of geometrically enforced nematic topological defects bearing relatively large defect strengths m in effectively two-dimensional planar systems. Theoretically, defect cores are analyzed within the mesoscopic Landau -De Gennes approach in terms of the tensor nematic order parameter. We demonstrate a robust tendency of defect decomposition into elementary units where two qualitatively different scenarios imposing total defect strengths to a nematic region are employed. Some theoretical predictions are verified experimentally, where arrays of defects bearing charges m = ±1, and even m = ±2, are enforced within a plane-parallel nematic cell using an AFM scribing method.
The core structure of a laboratory-made dust devil-like vortex and its condensed matter analogs
Journal of Molecular Structure, Aug 1, 2021
Abstract We consider the defect core structures of topological line defects characterized by a wi... more Abstract We consider the defect core structures of topological line defects characterized by a winding number k=1. In the experimental part of the work, we illustrate a simple and robust method of creating a laboratory-scale air-flow vortex, simulating a dust devil. We stabilized an escaped-type non-singular vortex-type configuration in the small-scale confined convective system with a converging inflow with imposed shear but lack of horizontal vorticity. Furthermore, we show that analogous defect core structures could be stabilized in simple condensed matter systems. In particular, different liquid crystalline phases represent an ideal testing ground to study different topological defects due to their unique combination of fluidity, softness, optical anisotropy, and rich diversity of defect configurations. In the theoretical and numerical study, we focused on the most common static k=1 line defect core structures in vector and nematic ordering. Namely, these two cases exhibit two different symmetries and can consequently exhibit qualitatively different defect core structures. We discuss conditions stabilizing either singular or non-singular core structures.
Topological Defects in Nematic Liquid Crystals: Laboratory of Fundamental Physics
Physica Status Solidi A-applications and Materials Science, Feb 1, 2021
Topological defects (TDs) appear in all branches of physics due to the simplicity of generic mech... more Topological defects (TDs) appear in all branches of physics due to the simplicity of generic mechanisms: The necessary condition for their existence is spontaneous symmetry breaking in a relevant physical field. Nematic liquid crystals (NLCs) represent an ideal testbed for their study and they can display point, line, textures, and in favorable conditions also wall defects. TDs in NLCs can be relatively easily created, controlled, and experimentally observed. This enables a detailed and controlled analysis of their physical properties, leading to the cross‐fertilization of knowledge among different areas of physics, including condensed matter, particle physics, and cosmology. Furthermore, TDs in NLCs could be exploited in diverse applications. Herein, some salient features of most common TDs in NLCs, their generic mechanisms, their importance for fundamental science, and possible applications based on them are illustrated.
Mixtures of Nanoparticles and Liquid Crystal Phases Exhibiting Topological Defects
Molecular Crystals and Liquid Crystals, Aug 3, 2012
We study theoretically mixtures of liquid crystals (LCs) and nanoparticles (NPs) at a mesoscopic ... more We study theoretically mixtures of liquid crystals (LCs) and nanoparticles (NPs) at a mesoscopic level. Orientational LC ordering is given in terms of a tensor order parameter. We estimate conditions which favor or disfavor phase separation. We show under which conditions topological defect could be exploited as trapping centers for nanoparticles. Influence of Defect Core Replacement mechanism on phase stability of a structure possessing topological defect is demonstrated.
Interference textures of defects in a thin nematic film: an applet presentation
European Journal of Physics, May 5, 2004
We have developed an applet presentation showing different structures of a nematic liquid crystal... more We have developed an applet presentation showing different structures of a nematic liquid crystal confined to a thin transparent plan-parallel cell. The nematic structures possessing single defects or their pairs may be studied. The corresponding interference textures are shown simulating the polarization optic microscopy experiment. Several parameters defining the experimental set-up can be interactively varied enabling observation of subsequent changes
Sunlight sterilized, recyclable and super hydrophobic anti-COVID laser-induced graphene mask formulation for indelible usability
Journal of Molecular Structure, Jun 1, 2021
The uncontrollable outbreak of the novel coronavirus (COVID-19) rapidly affected almost 230 count... more The uncontrollable outbreak of the novel coronavirus (COVID-19) rapidly affected almost 230 countries and territories and its transmission mainly due to respiratory droplets. To fight and protect against micron dimension (∼1.4µm) corona virus the usage of disposable medical masks is one and only trivial option for patients, doctors, health employers and in fact mandatory for kids to senior citizens, as well as public places in a risky environment. Ordinary medical masks unable to self-sterilize in order to recycle for other appliances resulting further destroying impact of societies high economic and environmental costs. To minimize this global pandemic issue this proposal explores novel mechanism for further commercialization of surgical mask of photo-thermal and self-cleaning functionalization. Indeed, depositing few layer ultra-thin graphene coating onto low-melting temperature non-woven mask by tempering a dual mode laser induced mechanism. Incoming aqueous droplets are bounced off due the super-hydrophobic states were treated on the mask surface. Superficial hydrophobic surface yields an advanced safety towards approaching respiratory droplets. Due to the huge absorption coefficient capability of the sunrays activated laser-induced mask may rapidly boost temperature exceeds 85ºC under sunlight illumination, causes making the mask reusable after sunlight distillation. For SARS/coronavirus/ aerosolized bacteria, laser induced graphene mask is a recent breakthrough in superior antibacterial capacity. Furthermore, cost-effective ultra-thin layered mask formulation recycled directly utilizes solar-driven desalination with remarkable self-exclusion performance for indelible usability. Featured article, deals with remarkable achievements from forthcoming experimentation which may be inspired with layered mask designing by more progressive materials.
We study numerically the reconfiguration process of colliding |m| = 1/2 strength disclinations in... more We study numerically the reconfiguration process of colliding |m| = 1/2 strength disclinations in an achiral nematic liquid crystal (NLC). A Landau-de Gennes approach in terms of tensor nematic-order parameters is used. Initially, different pairs {m 1 , m 2 } of parallel wedge disclination lines connecting opposite substrates confining the NLC in a plane-parallel cell of a thickness h are imposed: {1/2,1/2}, {-1/2,-1/2} and {-1/2,1/2}. The collisions are imposed by the relative rotation of the azimuthal angle θ of the substrates that strongly pin the defect end points. Pairs {1/2,1/2} and {-1/2,-1/2} "rewire" at the critical angle θ 4 in all cases studied. On the other hand, two qualitatively different scenarios are observed for {-1/2,1/2}. In the thinner film regime h < h c , the disclinations rewire at θ 4 . The rewiring process is mediated by an additional chargeless loop nucleated in the middle of the cell. In the regime h > h c , the colliding disclinations at θ (2) c reconfigure into boojum-like twist disclinations.
We demonstrate the strong impact of electrostatic properties on radial-bipolar structural transit... more We demonstrate the strong impact of electrostatic properties on radial-bipolar structural transitions in nematic liquid crystal (LC) droplets dispersed in different aqueous environments. In the experimental part of the study, we systematically changed the electrostatic properties of both LC droplets and aqueous solutions. Mixtures of nematics were studied by combining LC materials with negative (azoxybenzene compounds) and strongly positive (cyanobiphenyl) dielectric anisotropy. The aqueous solutions were manipulated by introducing either polyvinyl alcohol, glycerol, electrolyte or amphiphilic anionic surfactant SDS into water. In the supporting theoretical study, we identified the key parameters influencing the dielectric constant and the electric field strength of aqueous solutions. We also estimated the impact of different electrolytes on the Debye length at the LC-aqueous interface. The obtained results are further analysed for chemical and biological sensing applications.
Influence of <i>K</i> <sub>24</sub> on the structure of nematic liquid crystal droplets
Liquid Crystals, Oct 1, 1992
A phenomenological free energy is used to describe the stable ordering of nematic liquid crystals... more A phenomenological free energy is used to describe the stable ordering of nematic liquid crystals confined to supramicron spherical cavities. In particular the effects of the saddle splay elastic constant, K24, on the equilibrium structures and phase diagram of droplets with homeotropic surface anchoring are discussed. Some structures are illustrated by the corresponding simulated polarization microscope textures. Possibilities for an
Behavior of nanoparticles within liquid crystal phases
Elsevier eBooks, 2021
Abstract This chapter provides a brief presentation of dilute mixtures of simple liquid crystal (... more Abstract This chapter provides a brief presentation of dilute mixtures of simple liquid crystal (LC) phases and diverse nanoparticles (NPs). A unique combination of softness, liquid behavior, and optical anisotropy introduces LCs by themselves into a wide variety of applications, in particular, in electro-optics (e.g., liquid crystal display technology). Recently, LCs have moved into the attractive provinces of nanoscience and nanotechnology, which increases complexity of resulting structures, often introducing new material properties and functionalities. Consequently, such configurations enormously widen window of possible LC-based applications. In this contribution, representative examples of different types of NPs within several types of LC configurations are presented. Dispersions of nanomaterials of various dimensions (zero, one, and two) in LCs yields either effective systems with anomalously enhanced or new material properties. Phase behavior of LCs, NPs self-assembly and alignment in liquid crystalline media, synthesis of NPs by using LCs as templates or precursors are discussed. The importance of NPs–LC synthesis and interactions to obtain desired synergetic properties is highlighted.
Topological defects (TDs) constitute topologically protected frustrated regions in a host field o... more Topological defects (TDs) constitute topologically protected frustrated regions in a host field of an ordered manifold. They are ubiquitous in nature and appear at all scales, including the realms of particle physics, condensed matter, cosmology… We demonstrate that a simple plane parallel cell that confines a nematic liquid crystal (LC) could host diverse complex and multistable configurations of TDs, which we stabilized using the AFM scribing method. These competitive states could be reversibly and robustly reconfigured by appropriate external electric fields. Furthermore, we show that complex lattices of line defects, which are otherwise unstable or stable in a narrow interval of temperatures, could be stabilized efficiently by doping LCs with appropriate nanoparticles. We demonstrate that such TD configurations have potential for diverse applications, particularly in nanoand biotechnology: e.g., for nanotechnology-based devices based on reconfigurable conducting nanowires, tunable photonic devices, sensitive sensors… Furthermore, our study of TDs might provide some insight into still unresolved problems of fundamental physics. Namely, LCs could exhibit so-called "chargeless" twist disclinations, which commonly decay into a defectless state. Twist TDs could simultaneously act as defects and antidefects [3], and such neighboring pairs could be mutually annihilated. These configurations bear some resemblance to intriguing Majorana particles.
Thin Nematic Films: Laboratory of Physics for Topological Defects
Molecular Crystals and Liquid Crystals, Feb 2, 2012
In this pedagogical presentation we demonstrate basic physics of topological defects in orientati... more In this pedagogical presentation we demonstrate basic physics of topological defects in orientational ordering. We limit to two dimensional (2D) films. Different defect structures and pairs defect-antidefect are visualized in the applet presentation, which shows also the corresponding interference textures simulating typical polarizing microscopy experiment. The conservation of topological defect is shown on the case of 2D films on curved
Material properties of mixtures : invited paper at the 2007 SEM Annual Conference, Sheraton Springfield Monarch Place Hotel Springfield, Massachusetts, USA, June 4-6, 2007
Stable Assemblies of Topological Defects in Nematic Orientational Order
We consider history-dependent behavior in domain-type configurations in orientational order that ... more We consider history-dependent behavior in domain-type configurations in orientational order that are formed in configurations reached via continuous symmetry-breaking phase transitions. In equilibrium, these systems exhibit in absence of impurities a spatially homogeneous order. We focus on cases where domains are formed via (i) Kibble-Zurek mechanism in fast enough quenches or by (ii) Kibble mechanism in strongly supercooled phases. In both cases, domains could be arrested due to pinned topological defects that are formed at domain walls. In systems exhibiting polar or quadrupolar order, point and line defects (disclinations) dominate, respectively. In particular, the disclinations could form complex entangled structures and are more efficient in stabilizing domains. Domain patterns formed by fast quenches could be arrested by impurities imposing a strong enough random-field type disorder, as suggested by the Imry-Ma theorem. On the other hand, domains formed in supercooled systems...
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