1. Electric field driven reconfigurable multistable topological defect patternsSaša Harkai, Bryce S. Murray, Charles Rosenblatt, Samo Kralj, 2020, original scientific article Abstract: Topological defects appear in symmetry breaking phase transitions and are ubiquitous throughout Nature. As an ideal testbed for their study, defect configurations in nematic liquid crystals (NLCs) could be exploited in a rich variety of technological applications. Here we report on robust theoretical and experimental investigations in which an external electric field is used to switch between predetermined stable chargeless disclination patterns in a nematic cell, where the cell is sufficiently thick that the disclinations start and terminate at the same surface. The different defect configurations are stabilized by a master substrate that enforces a lattice of surface defects exhibiting zero total topological charge value. Theoretically, we model disclination configurations using a Landau-de Gennes phenomenological model. Experimentally, we enable diverse defect patterns by implementing an in-house-developed atomic force measurement scribing method, where NLC configurations are monitored via polarized optical microscopy. We show numerically and experimentally that an “alphabet” of up to 18 unique line defect configurations can be stabilized in a 4 × 4 lattice of alternating �=±1 surface defects, which can be “rewired” multistably using appropriate field manipulation. Our proof-of-concept mechanism may lead to a variety of applications, such as multistable optical displays and rewirable nanowires. Our studies also are of interest from a fundamental perspective. We demonstrate that a chargeless line could simultaneously exhibit defect-antidefect properties. Consequently, a pair of such antiparallel disclinations exhibits an attractive interaction. For a sufficiently closely spaced pair of substrate-pinned defects, this interaction could trigger rewiring, or annihilation if defects are depinned.
Keywords: line defects, topological defects, nematic liquid crystals, electric field, atomic force microscopy, numerical techniques, polarized optical microscopy
Published in DKUM: 18.11.2024; Views: 0; Downloads: 2
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2. Fingered core structure of nematic boojumsSamo Kralj, Riccardo Rosso, Epifanio Giovanni Virga, 2008, original scientific article Abstract: Using the Landau-de Gennes phenomenological approach, we study the fine biaxial core structure of a boojum residing on the surface of a nematic liquid crystal phase. The core is formed by a negatively uniaxial finger, surrounded by a shell with maximal biaxiality. The characteristic finger's length and the shell's width are comparable to the biaxial correlation length. The finger tip is melted for topological reasons. Upon decreasing the surface anchoring strength below a critical value, the finger gradually leaves the bulk and it is expelled through the surface.
Keywords: physics, liquid crystals, nematic crystals, line defects, surface phenomena
Published in DKUM: 07.06.2012; Views: 2110; Downloads: 112
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3. The re-characterisation of complex defectsBoštjan Bezenšek, John. W. Hancock, 2004, original scientific article Abstract: The re-characterisation of complex defects with re-entrant sectors has been addressed for cracks extending by fatigue, ductile tearing and cleavage. In Part I crack extension by fatigue and ductile tearing was discussed. In Part II cleavage data are presented for a family of complex defects with re-entrantsectors. Experimental tests on complex and re-characterised profilesare analysed using deterministic and probabilistic approaches. The work addresses the conservatism of re-characterisation procedures when appliedto cleavage failure on the lower shelf and in the ductile-brittle transition.
Keywords: fracture mechanics, structural elements, complex defects, recharacterisation, line-spring analysis, cleavage failure, constraint, statistical fracture mechanics, numerical analysis
Published in DKUM: 01.06.2012; Views: 1461; Downloads: 101
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4. The re-characterisation of complex defectsBoštjan Bezenšek, John. W. Hancock, 2004, original scientific article Abstract: Defect assessment codes idealise complex defects as simple shapes which are amenable to analysis in a process known as re-characterisation. The present work examines the re-characterisation of complex defects which extend by fatigue, ductile tearing or cleavage. A family of representative defects were analysed numerically, while a related experimental programme investigated defect interaction and failure. Part I of the paper focuses on fatigue and ductile tearing. Part II examines cleavage. The numerical and experimental results are discussed within the context of the re-characterisation proceduresdescribed in BS 7910 (Guidance on methods for assessing the acceptability of flaws in metallic structures. London, UK: British Standard Institution; 1999 [Chapter 7] and R6/4 (Assessment of the integrity of structures containing defects. Gloucester: British Energy Generation Ltd.; 2001 [Revision 4, Chapters I and II.3]). The level of conservatism of the re-characterisation prpcedures for fatigue and ductile tearing are discussed. A possible non-conservatism of the re-characterisation for cleavage is discussed in Part II, within the framework of constraint based statistical fracture mechanics.
Keywords: fracture mechanics, structural elements, complex defects, recharacterisation, line-spring analysis, fatigue, ductile tearing, cracks, numerical analysis
Published in DKUM: 01.06.2012; Views: 1647; Downloads: 104
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