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Effective topological charge cancelation mechanism
Luka Mesarec, Wojciech Góźdź, Aleš Iglič, Samo Kralj, 2016, original scientific article

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: 23.06.2017; Views: 478; Downloads: 259
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Numerical study of membrane configurations
Luka Mesarec, Miha Fošnarič, Samo Penič, Veronika Kralj-Iglič, Samo Kralj, Wojciech Góźdź, Aleš Iglič, 2014, original scientific article

Abstract: We studied biological membranes of spherical topology within the framework of the spontaneous curvature model. Both Monte Carlo simulations and the numerical minimization of the curvature energy were used to obtain the shapes of the vesicles. The shapes of the vesicles and their energy were calculated for different values of the reduced volume. The vesicles which exhibit inplane ordering were also studied. Minimal models have been developed in order to study the orientational ordering in colloids coated with a thin sheet of nematic liquid crystal (nematic shells).The topological defects are always present on the surfaces with the topology of a sphere.The location of the topological defects depends strongly on the curvature of the surface. We studied the nematic ordering and the formation of topological defects on vesicles obtained by the minimization of the spontaneous curvature energy.
Keywords: biological membranes, vesicles, spontaneous curvature model, Monte Carlo simulations, nematic shells, orientational ordering, topological defects
Published: 14.06.2017; Views: 447; Downloads: 286
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Impact of nanoparticles on nematic ordering in square wells
Mitja Slavinec, Eva Klemenčič, Milan Ambrožič, Marjan Krašna, 2015, original scientific article

Abstract: 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.
Keywords: liquid crystals, nanoparticles, nematic ordering, square well, software simulation, visualization
Published: 14.06.2017; Views: 573; Downloads: 272
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[ast]-orderable semigroups
Igor Klep, Primož Moravec, 2010, original scientific article

Abstract: Fiksirajmo ▫$ast$▫-uredljivi obseg ▫$k$▫. V članku vpeljemo razred ▫$ast▫-uredljivih polgrup kot tistih polgrup ▫$S$▫ z involucijo, za katere se da polgrupna algebra ▫$kS$▫, opremljena s kanonično involucijo, ▫$ast$▫-urediti Pokažemo, da je ta razred kvazi varieteta, ki je lokalno in residualno zaprta. Poleg tega dokažemo, da se da nilpotentna polgrupa s krajšanjem in involucijo ▫$ast$▫-urediti natanko tedaj, ko ima lastnost enoličnega izražanja korenov. V splošnem ta ekvivalenca ne drži, po drugi strani pa ima vsaka ▫$ast$▫-uredljiva polgrupa lastnost enoličnega izražanja korenov.
Keywords: algebra, ▫$^ast$▫-ureditev, polgrupa z involucijo, nilpotentna polgrupa, teorija valuacij, algebra, ▫$^ast$▫-ordering, semigroup with involution, nilpotent semigroup, valuation theory
Published: 10.07.2015; Views: 477; Downloads: 9
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Tine Curk, 2012, master's thesis

Abstract: A grafted polymer layer can be used to prevent the deposition of colloidal particles on a solid surface. This thesis presents Monte Carlo simulations of hard-sphere colloids pushed to a polymer brush under the influence of external fields (e.g. gravity). For weak fields colloids can not penetrate the brush and the effective potential acting on a single colloid is approximately quadratic. The phase diagram of three-dimensional hard-sphere colloids, that in one dimension are constrained to a plane by a harmonic potential, is presented. Under the influence of sufficiently strong external fields colloids penetrate the brush and form internally ordered, columnar structures that span the polymer layer. The morphology of the patterns that form depends sensitively on the strength of the applied field. We propose a simple phenomenological theory that accounts for the main characteristics of the observed behaviour. The present results suggest a simple experimental method to determine the surface polymer coverage.
Keywords: Soft Matter, Monte Carlo Method, Wang-Landau Algorithm, Free Energy, Polymer Brush, Colloids, Ordering
Published: 04.03.2013; Views: 1645; Downloads: 73
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Investigation of the bullwhip effect using spreadsheet simulation
Borut Buchmeister, 2008, original scientific article

Abstract: Superior supply chains are one of the best ways to compete in today's marketplaces. In Supply Chain Management, overall supply chain evaluation needs to include an important logistical effect known as the Bullwhip Effect. It shows how small changes at the demand end of a supply chain are progressively amplified up the supply chain. Production plans are based on demand forecasting and suppliers not only react an changed demand, they adapt the level of safety stock (variation of stocks and orders increases). In this paper two special situations in a four-stage supply chain are studied: i) stable demand with a single 5 % change in demand (with application of four different stock keeping policies), and ii) changing demand with alternating 5 % changes in demand (up and down, with another three stock keeping policies). The results of spreadsheet simulations are shown in tables and charts. Increasing variability of production orders and stock levels up the supply chain is evident. The Bullwhip Effect is measured by the standard deviation of orders. The comparison of the results shows that the Bullwhip Effect can be partially reduced by appropriate stock keeping policy.
Keywords: supply chain, changing demand, bullwhip effect, ordering policy
Published: 31.05.2012; Views: 1561; Downloads: 104
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Mario Malačič, 2009, undergraduate thesis

Abstract: V raziskovalnem delu smo predstavili edinstveno tehnologijo RFID (ang. Rario- Frequency Identification) in s pomočjo osnovnih strategij SCM (ang. Supply Chain Management) prikazali možne optimizacijske nastavke na nivoju celotne dobavne verige. RFID je splošen izraz, ki se uporablja za opis sistema, ki brezkontaktno, preko radijskih valov, transmitira identiteto objekta zapisano v obliki unikatne serijske številke. Koncept sistema RFID je zelo preprost. Sestavljata ga RFDI bralnik z anteno in RFID priponka (ang. TAG) z anteno, prenosni medij med čitalnikom in priponko pa so radijski valovi nizke moči. Omrežje EPC pomaga implementirati tehnologijo RFID v odprte sisteme in na ta način doseči integracijo celotne dobavne verige v enoten sistem. S pomočjo online povezave želimo na nivoju posameznega izdelka, označenega z RFID odzivniki, doseči tako imenovani »internet stvari«. Virtualni prostor, v katerem bodo imeli predmeti določeno umetno inteligenco, kar bo omogočalo komunikacijo med predmeti, predmeti in stroji, stroji in ljudmi ter predmeti in ljudmi. Področja uporabe RFID so praktično neomejena. Trenutno se RFID uporablja za označevanje in sledenje živali, prepoznavanje izdelkov v trgovinah, identificiranje ladijskih kontejnerjev, plačilo cestnine, spremljanje pošiljk v logistiki, pri izposoji knjig v knjižnicah, na parkiriščih in v garažah, za kodiranje ključev itn. Tehnologija RFID v primerjavi s črtno kodo omogoča hitrejše in natančnejše logistične procese, kar bo imelo za posledico, da RFID čez čas izpodrine uporabo črtne kode. Zelo pomembna tema pri tehnologiji RFID je standardizacija, kajti ta zagotavlja globalno povezljivost komponent sistemov RFID, ki bodo v skladu s svetovnimi regulacijami, kar je izrednega pomena pri uporabi v globalnih dobavnih verigah. V nadaljevanju smo predstavili šest osnovnih strategij SCM, ki lahko v sinergiji s tehnologijo RFID na nivoju celotne dobavne verige dosega optimizacijske potenciale. Tako smo celotno dobavno verigo razdelili na štiri sklope, kjer smo za vsak posamezni odsek preučili kake prednosti prinaša implementacija nove tehnologije. Z uvedbo nove tehnologije so povezani določeni riziki. Varstvo podatkov predstavlja poglavitnega, ki skuša obvarovati posameznega potrošnika pred zlorabo njegovih podatkov. Veliki trgovski giganti, kot so Metro, Wall Mart in Tesco uporabljajo RFID že nekaj let. Oni predstavljajo gonilno silo pri širjenju uporabe tehnologije in posredno pri zniževanju cen odzivnikov, strojne ter programske opreme. Tehnologija je trenutno predraga za majhna in srednje velika podjetja, ki pa bodo morala v prihodnosti začeti razmišljati o njeni implementaciji, kajti konkurenca z dneva v dan postaja močnejša.
Keywords: RFID, črtna koda, Efficient Consumer Response, Supply Chain Management, Cross Docking, Efficient Administration, Efficient Unit Loads, Vendor Managed Inventory, Computer Assisted Ordering, Efficient Replenishment.
Published: 24.09.2009; Views: 2027; Downloads: 248
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