Search the digital library catalog

Abstract: We consider the theoretical and positional assembling of topological defects (TDs) in effectively two-dimensional nematic liquid crystal films. We use a phenomenological Helfrich–Landau–de Gennes-type mesoscopic model in which geometric shapes and nematic orientational order are expressed in terms of a curvature tensor field and a nematic tensor order parameter field. Extrinsic, intrinsic, and total curvature potentials are introduced using the parallel transport concept. These potentials reveal curvature seeded TD attractors. To test ground configurations, we used axially symmetric nematic films exhibiting spherical topology.
Keywords: topological defects, nematic liquid crystals, nematic shells, geometric potentials, curvature
Published in DKUM: 30.09.2024; Views: 0; Downloads: 7
Full text (3,69 MB)
This document has many files! More...

Abstract: Membrane budding often leads to the formation and release of microvesicles. The latter might play an important role in long distance cell-to-cell communication, owing to their ability to move with body fluids. Several mechanisms exist which might trigger the pinching off of globular buds from the parent membrane (vesiculation). In this paper, we consider the theoretical impacts of topological defects (frustrations) on this process in the membranes that exhibit global in-plane orientational order. A Landau–de Gennes theoretical approach is used in terms of tensor orientational order parameters. The impact of membrane shapes on position and the number of defects is analyzed. In studied cases, only defects with winding numbers m = ±1/2 appear, where we refer to the number of defects with m = 1/2 as defects, and with m = –1/2 as anti-defects. It is demonstrated that defects are attracted to regions with maximal positive Gaussian curvature, K. On the contrary, anti-defects are attracted to membrane regions exhibiting minimal negative values of K. We show on membrane structures exhibiting spherical topology that the coexistence of regions with K > 0 and K < 0 might trigger formation of defect–anti-defect pairs for strong enough local membrane curvatures. Critical conditions for triggering pairs are determined in several demonstrative cases. Then the additionally appeared anti-defects are assembled at the membrane neck, where K < 0. Consequent strong local fluctuations of membrane constituent anisotropic molecules might trigger membrane fission neck rupture, enabling a membrane fission process and the release of membrane daughter microvesicles (ie, vesiculation).
Keywords: structural transitions, topological defects, membrane microvesicles, membrane curvature, membrane fission, vesiculation
Published in DKUM: 03.08.2017; Views: 1339; Downloads: 433
Full text (4,92 MB)
This document has many files! More...

Abstract: Topological defects (TDs) appear almost unavoidably in continuous symmetry breaking phase transitions. The topological origin makes their key features independent of systems’ microscopic details; therefore TDs display many universalities. Because of their strong impact on numerous material properties and their significant role in several technological applications it is of strong interest to find simple and robust mechanisms controlling the positioning and local number of TDs. We present a numerical study of TDs within effectively two dimensional closed soft films exhibiting in-plane orientational ordering. Popular examples of such class of systems are liquid crystalline shells and various biological membranes. We introduce the Effective Topological Charge Cancellation mechanism controlling localised positional assembling tendency of TDs and the formation of pairs {defect, antidefect} on curved surfaces and/or presence of relevant “impurities” (e.g. nanoparticles). For this purpose, we define an effective topological charge Δmeff consisting of real, virtual and smeared curvature topological charges within a surface patch Δς identified by the typical spatially averaged local Gaussian curvature K. We demonstrate a strong tendency enforcing Δmeff → 0 on surfaces composed of Δς exhibiting significantly different values of spatially averaged K. For Δmeff ≠ 0 we estimate a critical depinning threshold to form pairs {defect, antidefect} using the electrostatic analogy.
Keywords: topological defects, topological charge, numerical studies, orientational ordering, nematic liquid crystals, liquid crystalline shells, biological membranes, nanoparticles, Gaussian curvature, electrostatic analogy, annihilation, topology
Published in DKUM: 23.06.2017; Views: 1141; Downloads: 350
Full text (2,15 MB)
This document has many files! More...