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1.
Biaxial structures of localized deformations and line-like distortions in effectively 2D nematic films
Luka Mesarec, Samo Kralj, Aleš Iglič, 2024, izvirni znanstveni članek

Opis: We numerically studied localized elastic distortions in curved, effectively two-dimensional nematic shells. We used a mesoscopic Landau-de Gennes-type approach, in which the orientational order is theoretically considered by introducing the appropriate tensor nematic order parameter, while the three-dimensional shell shape is described by the curvature tensor. We limited our theoretical consideration to axially symmetric shapes of nematic shells. It was shown that in the surface regions of stomatocyte-class nematic shell shapes with large enough magnitudes of extrinsic (deviatoric) curvature, the direction of the in-plane orientational ordering can be mutually perpendicular above and below the narrow neck region. We demonstrate that such line-like nematic distortion configurations may run along the parallels (i.e., along the circular lines of constant latitude) located in the narrow neck regions of stomatocyte-like nematic shells. It was shown that nematic distortions are enabled by the order reconstruction mechanism. We propose that the regions of nematic shells that are strongly elastically deformed, i.e., topological defects and line-like distortions, may attract appropriately surface-decorated nanoparticles (NPs), which could potentially be useful for the controlled assembly of NPs.
Ključne besede: nematic shells, orientational order, topological defects, stomatocytes, order reconstruction mechanism
Objavljeno v DKUM: 28.03.2025; Ogledov: 0; Prenosov: 6
.pdf Celotno besedilo (4,60 MB)
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2.
Criticality controlling mechanisms in nematic liquid crystals
Maha Zid, George Cordoyiannis, Zdravko Kutnjak, Samo Kralj, 2024, izvirni znanstveni članek

Opis: We theoretically study the generic mechanisms that could establish critical behavior in nematic liquid crystals (NLCs). The corresponding free energy density terms should exhibit linear coupling with the nematic order parameter and, via this coupling, enhance the nematic order. We consider both temperature- and pressure-driven, order–disorder phase transitions. We derive a scaled effective free energy expression that describes how qualitatively different mechanisms enforce critical behavior. Our main focus is on the impact of nanoparticles (NPs) in homogeneous NP-NLC mixtures. We illustrate that in the case of pressure-driven phase changes, lower concentrations are needed to impose critical point conditions in comparison with pure temperature variations.
Ključne besede: nematic liquid crystals, critical point, critical behavior, phase transition
Objavljeno v DKUM: 28.03.2025; Ogledov: 0; Prenosov: 6
.pdf Celotno besedilo (4,10 MB)
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3.
Dynamics and pretransitional effects in C60 fullerene nanoparticles and liquid crystalline dodecylcyanobiphenyl (12CB) hybrid system
Sylwester Rzoska, Szymon Starzonek, Joanna M. Łoś, Aleksandra Drozd-Rzoska, Samo Kralj, 2020, izvirni znanstveni članek

Opis: The report shows the strong impact of fullerene C60 nanoparticles on phase transitions and complex dynamics of rod-like liquid crystal dodecylcyanobiphenyl (12CB), within the limit of small concentrations. Studies were carried out using broadband dielectric spectroscopy (BDS) via the analysis of temperature dependences of the dielectric constant, the maximum of the primary loss curve, and relaxation times. They revealed a strong impact of nanoparticles, leading to a ~20% change of dielectric constant even at x = 0.05% of C60 fullerene. The application of the derivative-based and distortion-sensitive analysis showed that pretransitional effects dominate in the isotropic liquid phase up to 65 K above the clearing temperature and in the whole Smectic A mesophase. The impact of nanoparticles on the pretransitional anomaly appearance is notable for the smectic–solid phase transition. The fragility-based analysis of relaxation times revealed the universal pattern of its temperature changes, associated with scaling via the “mixed” (“activated” and “critical”) relation. Phase behavior and dynamics of tested systems are discussed within the extended Landau–de Gennes–Ginzburg mesoscopic approach.
Ključne besede: nanoparticles, liquid crystals, soft materials, fullerenes, dielectric spectroscopy, phase transitions, dynamics
Objavljeno v DKUM: 10.03.2025; Ogledov: 0; Prenosov: 3
.pdf Celotno besedilo (1,89 MB)
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4.
Enzyme cascade to enzyme complex phase-transition-like transformation studied by the maximum entropy production principle
Andrej Dobovišek, Tina Blaževič, Samo Kralj, Aleš Fajmut, 2025, izvirni znanstveni članek

Opis: In biological cells, soluble enzymes often spontaneously reorganize into higher-order complexes called metabolons, providing regulatory advantages over individual soluble enzymes under specific conditions. Despite their importance, the mechanisms underlying metabolon formation remain unclear. Here we report a theoretical model that elucidates the spontaneous transition between soluble enzyme cascades and complexes, driven by fluctuations in intermediate metabolite concentrations. The model integrates the maximum entropy production principle (MEPP) and the Shannon information entropy (MaxEnt), Landau phase-transition theory, kinetic modeling, stability analysis, and metabolic control analysis. Our results show that soluble enzymes and enzyme complexes represent two distinct catalytic states with unique kinetic and regulatory properties. The transition from an enzyme cascade to an enzyme complex displays features of a first-order phasetransition, highlighting the system's tendency to reorganize into its most thermodynamically favorable state, providing a potential pathway for metabolic regulation.
Ključne besede: theoretical modeling, irreversible thermodynamics, maximum entropy production principle, Shannon information entropy, first-order phase transition, enzyme organization, enzyme cascade, enzyme complex
Objavljeno v DKUM: 06.02.2025; Ogledov: 0; Prenosov: 1
.pdf Celotno besedilo (1,52 MB)
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5.
Annihilation of highly-charged topological defects
Eva Klemenčič, Pavlo Kurioz, Milan Ambrožič, Charles Rosenblatt, Samo Kralj, 2020, izvirni znanstveni članek

Opis: We studied numerically external stimuli enforced annihilation of a pair of daughter nematic topological defect (TD) assemblies bearing a relatively strong topological charge |m|=3/2. A Landau- de Gennes phenomenological approach in terms of tensor nematic order parameter was used in an effectively two-dimensional Cartesian coordinate system, where spatial variations along the z-axis were neglected. A pair of {m=3/2,m=−3/2} was enforced by an appropriate surface anchoring field, mimicking an experimental sample realization using the atomic force microscope (AFM) scribing method. Furthermore, defects were confined within a rectangular boundary that imposes strong tangential anchoring. This setup enabled complex and counter-intuitive annihilation processes on varying relevant parameters. We present two qualitatively different annihilation paths, where we either gradually reduced the relative surface anchoring field importance or increased an external in-plane spatially homogeneous electric field E. The creation and depinning of additional defect pairs {12,−12} mediated the annihilation in such a geometry. Furthermore, we illustrate the absorption of TDs by sharp edges of the confining boundary, accompanied by m=±1/4↔∓1/4 winding reversal of edge singularities, and also E-driven zero-dimensional to one-dimensional defect core transformation.
Ključne besede: liquid crystals, topological defects, annihilation, order reconstruction
Objavljeno v DKUM: 24.01.2025; Ogledov: 0; Prenosov: 10
.pdf Celotno besedilo (3,58 MB)
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6.
Multiple twisted chiral nematic structures in cylindrical confinement
Milan Ambrožič, Apparao Gudimalla, Charles Rosenblatt, Samo Kralj, 2020, izvirni znanstveni članek

Opis: In this article, we theoretically and numerically study the chirality and saddle-splay elastic constant (K24) -enabled stability of multiple twist-like nematic liquid crystal (LC) structures in cylindrical confinement. We focus on the so-called radially z-twisted (RZT) and radially twisted (RT) configurations, which simultaneously exhibit twists in different spatial directions. We express the free energies of the structures in terms of dimensionless wave vectors, which characterise the structures and play the roles of order parameters. The impact of different confinement anchoring conditions is explored. A simple Landau-type analysis provides an insight into how different model parameters influence the stability of structures. We determine conditions for which the structures are stable in chiral and also nonchiral LCs. In particular, we find that the RZT structure could exhibit macroscopic chirality inversion upon varying the relevant parameters. This phenomenon could be exploited for the measurement of K24.
Ključne besede: liquid crystals, chirality, confinement, saddle-splay elasticity, double twist deformations
Objavljeno v DKUM: 14.01.2025; Ogledov: 0; Prenosov: 3
.pdf Celotno besedilo (2,60 MB)
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7.
Nano and micro-structural complexity of nematic liquid crystal configurations
Andreja Jelen, Maha Zid, Kaushik Pal, Remya Rajan Renuka, Dejvid Črešnar, Samo Kralj, 2024, izvirni znanstveni članek

Opis: Of our interest are frustration-driven pattern generating mechanisms in systems which in bulk equilibrium display spatially homogeneous long-range orientational order in absence of perturbations. As testbed material, we select thermotropic nematic liquid crystals. In bulk, they exhibit weakly discontinuous order-disorder phase transformation on varying temperature where the ordered nematic phase features spatially uniform axial order along an arbitrary symmetry breaking direction. However, due to continuous symmetry breaking (CSB) the established order is extremely susceptible to various perturbations which are in real systems in general always present. We theoretically illustrate how diverse complex patterns could be excited. Particularly intriguing configurations could appear if topological defects are present that could be generated via CSB. Our analysis is based on a relatively simple Lebwohl-Lasher-type model in which we could get analytical insight into phenomena of our interest. Using it we illustrate history dependent early stage isotropic-nematic phase evolution and final patterns in presence of "impurities" (e.g., nanoparticles). We show how characteristic effective interaction characteristics predict qualitatively different emerging patterns. Our analysis is based on CSB which is ubiquitous in nature. Consequently, demonstrated mechanisms are expected to manifest also in other condensed matter systems whose ordered phase is formed via CSB. We illustrate how kinetics and impurities could impact key structural properties of the systems of our interest.
Ključne besede: continuous symmetry breaking, patterns, topological defects, nematic liquid crystals
Objavljeno v DKUM: 05.12.2024; Ogledov: 0; Prenosov: 7
.pdf Celotno besedilo (9,33 MB)
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8.
Electric field driven reconfigurable multistable topological defect patterns
Saša Harkai, Bryce S. Murray, Charles Rosenblatt, Samo Kralj, 2020, izvirni znanstveni članek

Opis: 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.
Ključne besede: line defects, topological defects, nematic liquid crystals, electric field, atomic force microscopy, numerical techniques, polarized optical microscopy
Objavljeno v DKUM: 18.11.2024; Ogledov: 0; Prenosov: 4
.pdf Celotno besedilo (3,79 MB)
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9.
Curvature potential unveiled topological defect attractors
Luka Mesarec, Aleš Iglič, Veronika Kralj-Iglič, Wojciech Góźdź, Epifanio Giovanni Virga, Samo Kralj, 2021, izvirni znanstveni članek

Opis: 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.
Ključne besede: topological defects, nematic liquid crystals, nematic shells, geometric potentials, curvature
Objavljeno v DKUM: 30.09.2024; Ogledov: 0; Prenosov: 10
.pdf Celotno besedilo (3,69 MB)
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10.
Reconfiguration of nematic dislocations
Saša Harkai, Mitja Kralj, Samo Kralj, 2023, objavljeni znanstveni prispevek na konferenci (vabljeno predavanje)

Ključne besede: fields, topological defects, topological charge, disclinations, liquid crystals
Objavljeno v DKUM: 19.09.2024; Ogledov: 0; Prenosov: 4
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