1. Annihilation of highly-charged topological defectsEva 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: 3
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2. Multiple twisted chiral nematic structures in cylindrical confinementMilan 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: 2
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3. Liquid crystal based active electrocaloric regeneratorAndrej Dobovišek, Milan Ambrožič, Zdravko Kutnjak, Samo Kralj, 2023, izvirni znanstveni članek Opis: The active electrocaloric (EC) regenerator exploiting electric conversion into thermal energy has recently become important for developing a new generation of heat-management devices. We analyze an active EC regenerator numerically. We establish a temperature span across the regenerator ΔT by commuting a liquid crystalline (LC) unit between regions with and without an external electric field E. In modelling, we use Landau-de Gennes mesoscopic approach, focusing on the temperature regime where isotropic (paranematic) and nematic phase order compete. We determined conditions enabling a large enough value of ΔT suitable for potential applications. In particular, (i) the vicinity of the paranematic-nematic (P–N) phase transition, (ii) large enough latent heat of the transition, (iii) strong enough applied external field (exceeding the critical field Ec at which the P–N transition becomes gradual), and (iv) relatively short contact times between LC unit and heat sink and heat source reservoirs are advantageous. Our analysis reveals that Δ T >> 1K could be achieved using appropriate LC material. Ključne besede: liquid crystals, electrocaloric effect, phase behavior Objavljeno v DKUM: 18.09.2024; Ogledov: 0; Prenosov: 2
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4. Analitična mehanikaMilan Ambrožič, Arbresha Hölbl, 2023, univerzitetni, visokošolski ali višješolski učbenik z recenzijo Opis: Pri predmetu Analitična mehanika obravnavamo številne probleme iz mehanike, predvsem dinamike, na bolj matematično sistematičen način, kot je navada pri običajnih fizikalnih nalogah, npr. v zvezi z drugim Newtonovim zakonom. S tem lahko vključimo tudi kompleksnejše geometrije pri gibanju teles. Ključni del analitične mehanike je vpeljava generaliziranih koordinat kot neodvisnih spremenljivk gibanja, s katerimi izrazimo Lagrangeevo funkcijo ali pa Hamiltonian. Nazadnje moramo rešiti ustrezne diferencialne enačbe, da najdemo časovno odvisnost generaliziranih koordinat. Gravitacija in nihanje sta značilni področji, kjer koristno uporabimo matematični formalizem analitične mehanike. Ključne besede: Lagrangeeva funkcija, Hamiltonian, Euler-Lagrangeeve enačbe, mehanika, variacijski račun, enostavna telesa in sistemi teles Objavljeno v DKUM: 07.12.2023; Ogledov: 410; Prenosov: 51
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5. Monte Carlo simulation of air resistance on an ellipsoid in motionVeronika Bukina, 2021, magistrsko delo Opis: The main goal of the master's thesis was the analysis of air resistance on the body in motion in a model that does not require solving the Navier-Stokes equations, but works on the basis of mechanics and statistical physics. The model was a Monte Carlo (MC) simulation of the motion of ideal gas molecules in a closed container in which a body was placed, moving along one of the axes. For the most part of calculations, the approach was used when the body was fixed in the middle of the simulation cell, and one of the components of the molecular velocity had an additional term that simulated the flow, as if the body was moving at this speed in the opposite direction. First of all, a linear dependence of the drag force on speed was found for low flow speed for a flat plate, which was predicted by linear drag law. For high molecular flow rates, the quadratic dependence predicted by the Bernoulli equation was clearly observed. The results of calculating the corresponding resistivity coefficients for the flat plate were in agreement with the analytical values for both regimes of speeds. By analogy, a simulation was made for a spherical body, which also demonstrated a strong quadratic dependence at high speeds and the drag coefficient value is approximately equal to the analytical one. In the following, we studied systematically ellipsoids with circular cross-section, where we varied the ratio between semiaxes in the direction of motion and perpendicular direction, respectively. The results for the ellipsoid showed that the drag coefficient value is maximum for a flat plate (a limiting case of an ellipsoid, when the semiaxis in the direction of motion tends to 0) and decreases with stretching of the body along the flow axis. When the Maxwell distribution of molecular speeds that was mainly used was replaced with uniform Root-Mean-Square (RMS) speed the results for drag coefficient were slightly different. Ključne besede: Air resistance, drag force, quadratic drag law, drag coefficient, Monte Carlo (MC) simulation, Maxwell distribution. Objavljeno v DKUM: 13.10.2021; Ogledov: 1038; Prenosov: 65
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6. Numerična analiza toka izpušnih plinov in termične obremenitve izpušnega kolektorjaRok Peklar, 2020, magistrsko delo Opis: Z rastočo tematiko okoljevarstvenih težav je avtomobilska industrija primorana implementirati številne ukrepe, kot so znižati potrošnjo goriva, zmanjšati neželene izpuste in celo hrup vozila. Računska dinamika tekočin je nedvomno velika gonilna sila pri razvoju tehnoloških inovacij, saj nam da vpogled v fenomene, katerih obravnava z eksperimentalnimi metodami ni mogoča, ali pa je preprosto predraga. V primerjavi z drugimi vejami industrije, je avtomobilska industrija ena izmed prvih, ki so računsko dinamiko tekočin pričele vključevati pri razvoju komponent. Magistrska naloga prestavlja numerično analizo termične obremenitve izpušnega sistema. Proces je deljen na dva dela: analizo toka izpušnih plinov in termično analizo strukture izpušnega sistema. Za vse simulacije uporabimo programski paket za računsko dinamiko tekočin z metodo končnih volumnov AVL FIRE™. Rezultati simulacij so bili primerjani z meritvami obstoječega izpušnega sistema. Ključne besede: Računska dinamika tekočin, metoda končnih volumnov, izpušni sistem, katalizator, prenos toplote, termična analiza, AVL FIRE Objavljeno v DKUM: 16.12.2020; Ogledov: 1403; Prenosov: 96
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7. Poučevanje o gravitacijskih valovih pri fiziki v slovenskih gimnazijahSebastjan Krajnc, 2017, magistrsko delo Opis: Splošna teorija relativnosti je že skoraj stoletje nazaj napovedala obstoj gravitacijskih valov, vendar nam je šele razvoj tehnologije omogočil neposredno opazovanje njegovega fizikalnega vpliva. Ta dosežek ni le ponovno potrdil splošne teorije relativnosti, temveč nam je podal revolucionaren način pridobivanja informacij na področju astrofizike, neodvisno od spektra elektromagnetnega valovanja, s čimer smo nazadnje pridobili tudi nov vidik razumevanja številnih astrofizikalnih objektov in procesov ter razvoja zgodnjega vesolja.
Za odkritje obstoja gravitacijskih valov je bilo izkazano precejšnje zanimanje, tudi s strani širše javnosti, saj je bilo njihovo odkritje izpostavljeno v različnih medijih. Mladoletniki, na katere imajo mediji prav največji vpliv, pogosto iščejo dodatne informacije o aktualni temi, bodisi s samostojnim raziskovanjem ali pri svojem učitelju (v tem primeru učitelju fizike). Učni načrt fizike za gimnazijo ne vsebuje vsebin v zvezi z odkritjem in opisom gravitacijskih valov, zato bi bilo treba za vsaj osnovno razumevanje tega kompleksnega pojava dijakom zagotoviti ustrezno fizikalno predznanje v obliki poglobitve ali dopolnitve nekaterih učnih vsebin.
S kvalitativnim raziskovalnim pristopom je bilo ugotovljeno, da je učni načrt za fiziko fleksibilen v tolikšni meri, da ob uvodni poglobitvi in dopolnitvi nekaterih vsebin omogoča zadostno fizikalno predznanje, potrebno za razumevanje vsebin o gravitacijskih valovih, v okviru vsebinsko nerazporejenih ur pa celo v formalnem smislu omogoča njihovo obravnavo. Veljavnost in učinkovitost tega spoznanja sta bili preverjeni z didaktičnim eksperimentom. Ključne besede: gravitacijski valovi (GV), splošna teorija relativnosti, učni načrt za fiziko, didaktični eksperiment. Objavljeno v DKUM: 02.10.2017; Ogledov: 1623; Prenosov: 117
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8. Symmetry breaking and structure of a mixture of nematic liquid crystals and anisotropic nanoparticlesMarjan Krašna, Matej Cvetko, Milan Ambrožič, 2010, izvirni znanstveni članek Opis: Orientational ordering of a homogeneous mixture of uniaxial liquid crystalline(LC) molecules and magnetic nanoparticles (NPs) is studied using the Lebwohl-Lasher lattice model. We consider cases where NPs tend to be oriented perpendicularly to LC molecules due to elastic forces. We study domain-type configurations of ensembles, which are quenched from the isotropic phase. We show that for large enough concentrations of NPs the long range uniaxial nematic ordering is replaced by short range order exhibiting strong biaxiality. This suggests that the impact of NPs on orientational ordering of LCs for appropriate concentrations of NPs is reminiscent to the influence of quenched random fields which locally enforce a biaxial ordering. Ključne besede: liquid crystals, nanoparticles, disorder, orientational order, quenched disorder, symmetry breaking Objavljeno v DKUM: 03.08.2017; Ogledov: 1514; Prenosov: 382
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9. Impact of nanoparticles on nematic ordering in square wellsMitja Slavinec, Eva Klemenčič, Milan Ambrožič, Marjan Krašna, 2015, izvirni znanstveni članek Opis: Nematic liquid crystalline structures within square wells are studied numerically using both Lebwohl-Lasher lattice semimicroscopic and the Landau-de Gennes mesoscopic approach. At lateral boundary wall strong planar anchoring is enforced. The cell thickness h along the z Cartesian coordinate is assumed to be smaller than the characteristic square well size R. Using semimicroscopic modelling we restrict to effectively two-dimensional systems which we study in terms of the tensor nematic order parameter. We consider impact of appropriate nanoparticles (NPs) on nematic configuration for cases where R becomes comparable to the biaxial order parameter correlation length. In this case a star-like order reconstruction biaxial profile could be formed in absence of NPs. We demonstrate existence of a rich variety of different nematic structures, including topological defects, which are enabled by presence of appropriate NPs. Ključne besede: liquid crystals, nanoparticles, nematic ordering, square well, software simulation, visualization Objavljeno v DKUM: 14.06.2017; Ogledov: 1261; Prenosov: 372
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10. History-dependent patterns in randomly perturbed nematic liquid crystalsAmid Ranjkesh Siahkal, Milan Ambrožič, George Cordoyiannis, Zdravko Kutnjak, Samo Kralj, 2013, izvirni znanstveni članek Opis: We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the impact of the samples history on the universal behavior. The obtained results are of interest for every randomly perturbed system exhibiting a continuous symmetry-breaking phase transition. A semimicroscopic lattice simulation is used where the LC molecules are treated as cylindrically symmetric, rod-like objects interacting via a Lebwohl-Lasher (LL) interaction. Pure LC systems exhibit a first order phase transition into the orientationally ordered nematic phase at T = Tc on lowering the temperature T. The orientational ordering of LC molecules is perturbed by the quenched, randomly distributed rod-like impurities of concentration p. Their orientation is randomly distributed, and they are coupled with the LC molecules via an LL-type interaction. Only concentrations below the percolation threshold are considered. The key macroscopic characteristics of perturbed LC structures in the symmetry-broken nematic phase are analyzed for two qualitatively different histories at T << Tc. We demonstrate that, for a weak enough interaction among the LC molecules and impurities, qualitatively different history-dependent states could be obtained. These states could exhibit either short-range, quasi-long-range, or even long-range order. Ključne besede: liquid crystals Objavljeno v DKUM: 14.06.2017; Ogledov: 2300; Prenosov: 340
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