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Curvatura, flessibilità e fluttuazioni orientazionali: come le caratteristiche molecolari controllano le proprietà elastiche e la struttura mesoscopica nei nematici
No full textI cristalli liquidi sono una vasta classe di materiali soffici con un'ampia gamma di campi d'interesse. La caratteristica fondamentale che li accomuna è la combinazione di ordine e fluidità. Da un lato, questa combinazione è uno dei requisiti di base per l'auto-organizzazione e la formazione di strutture nei sistemi viventi, e infatti la fisica dei cristalli liquidi sta emergendo come una delle nuove frontiere nella ricerca in biologia cellulare. Dall'altro, tale caratteristica rende i cristalli liquidi sensibili a una grande varietà di stimoli come luce, campi elettrici e magnetici, forze meccaniche e gradienti termici, con conseguente rilevanza per applicazioni in fotonica, optofluidica, elettronica organica e bio-sensoristica. In tutti questi casi, l'elasticità delle fasi liquido cristalline, cioè la loro risposta meccanica a distorsioni dello stato ordinato, è cruciale dal momento che il loro comportamento alla micro- e mesoscala è il risultato della competizione fra forze elastiche interne, vincoli geometrici e campi esterni.
Il filo conduttore di questa tesi è lo studio dell'elasticità dei nematici, ovvero di cristalli liquidi con puro ordine orientazionale. L'argomento viene trattato da una prospettiva microscopica e teorica, e in particolare vengono affrontate tre questioni aperte principali. Stimolati dal bisogno di chiarificare il ruolo della costante elastica detta di saddle-splay, K24, in recenti esperimenti controversi, nella Parte I sviluppiamo un formalismo in cui le costanti elastiche di superficie (K24 e K13) sono trattate allo stesso modo di quelle di bulk (K11, K22 e K33). L'applicazione del metodo ad un sistema semplice e paradigmatico, gli hard rod non chirali, svela un nuovo meccanismo che porta alla rottura spontanea della simmetria di riflessione, con potenziali implicazioni in un'ampia classe di sistemi rilevanti dal punto di vista sperimentale. Nella Parte II presentiamo un approccio microscopico, detto non-locale, che oltrepassa le classiche assunzioni delle teorie per la nemato-elasticità (trattamento di ogni particella come un rod efficace) e che rende conto dell'accoppiamento fra la specifica morfologia di una particella e la geometria delle deformazioni del direttore. Questa mutua interazione, che si manifesta tangibilmente in nuovi tipi di ordine orientazionale in nematogeni con specifiche forme in risposta a certe distorsioni, è cruciale per la formazione delle nuove fasi liquido cristalline modulate che recentemente stanno attraendo particolare attenzione. Infine, nella Parte III, applichiamo il modello microscopico non-locale al caso più generale di particelle flessibili, con l'obiettivo di modellizzare l'elasticità dei nematici polimerici, la quale rappresenta un problema aperto da diverso tempo. Mettiamo a punto una metodologia teorico-computazionale in particolare senza ricorrere alla approssimazione di disaccoppiamento che invece viene tipicamente utilizzata nelle teorie esistenti. I risultati per sistemi modello nel regime semiflessibile mostrano effetti distinti sulle costanti elastiche, con una speciale sensibilità della K33 che decresce drasticamente al crescere della flessibilità. Questo rappresenta un passo promettente verso la comprensione e il controllo delle proprietà meccaniche dei materiali liquido cristallini bio-polimerici.Liquid crystals are a broad class of soft materials with a wide range of interest across different fields. Their common fundamental feature is the combination of order and fluidity. On the one hand, this is a basic requirement for self-organization and structure formation in living systems and indeed the physics of liquid crystals has emerged as a flourishing new research frontier in cell biology. On the other hand, this characteristic makes them sensitive and responsive to a large variety of stimuli such as light, electric and magnetic fields, mechanic forces and thermal gradients, with consequent technological interest in photonics, optofluidics, organic electronics and biosensing. In all cases, the elasticity of liquid crystalline phases, i.e. their mechanical response to distortions of their ordered state, is paramount since their behavior at the micro- and meso-scale is the result of the competition between internal elastic forces, geometric constraints and external fields.
The common thread of this thesis is the elasticity of nematics - i.e. of liquid crystals with pure orientational order. The topic is approached from a microscopic and theoretical perspective, and three major open questions are addressed. Stimulated by the need to clarify the role of the saddle-splay elastic constant, K24, in recent controversial experiments, in Part I we develop a consistent formulation in which the surface-like elastic constants (K24 and K13) are treated on the very same footing of the bulk ones (K11, K22 and K33). The application of the method to the simple paradigmatic system of hard achiral rods uncovered a novel mechanism leading to spontaneous mirror symmetry breaking, with potential implications on a broad range of experimentally relevant systems. In Part II we present a microscopic approach, denominated non-local, that overcomes the classical assumptions of nemato-elasticity theories (effective-rod treatment of any particle type) and that accounts for the coupling between the specific particle morphology and the geometry of the director distortions. This interplay, manifesting tangibly in the onset of new types of orientational order among specifically shaped nematogens in response to certain deformations, is crucial for the formation of novel modulated liquid crystal phases that are attracting considerable attention. Lastly, in Part III, we extend the non-local microscopic model to the more general case of particles endowed with flexibility in order to describe the elasticity of nematic polymers, which constitutes a long standing issue in the field. We set up a theoretical-computational methodology that fully accounts for the interplay between the director field and both the orientational and the conformational degrees of freedom of polymer chains, without resorting to the decoupling approximation which is generally adopted in existing theories. Calculations for model systems in the semiflexible regime show distinct effects on the bulk elastic constants, with a special sensitivity of K33, which dramatically decreases with increasing flexibility. This is a promising step towards understanding and controlling the mechanical properties of biopolymeric liquid crystalline materials
Microscopic modelling of nematic elastic constants beyond Straley theory
No full textRecent findings on various classes of nematics, whose microscopic structure differs from the prototypical rod-like shape, evidence unusual elastic properties, which challenge existing theories. Here we develop a theoretical and numerical methodology for the calculation of Frank elastic constants, accounting for the coupling between the molecular shape and each specific deformation mode. This is done in the framework of Onsager-Straley's second-virial theory, using a non-local form of the orientational distribution function. The comparison between two benchmark systems, a straight and a bent rod, allows us to illustrate the distinct features of this approach, which include additional order parameters induced by the deformation and, related to this, an ideal contribution to the deformation free energy. Then, using a simple system that can be seen as a minimalist model of liquid crystal trimers, we discuss the impact of different molecular conformations on elastic constants
On the elusive saddle–splay and splay–bend elastic constants of nematic liquid crystals
Full text linkThe elastic behavior of nematics is commonly described in terms of the three so-called bulk deformation modes, i.e., splay, twist, and bend. However, the elastic free energy contains also other terms, often denoted as saddle–splay and splay–bend, which contribute, for instance, in confined systems. The role of such terms is controversial, partly because of the difficulty of their experimental determination. The saddle–splay (K24) and splay–bend (K13) elastic constants remain elusive also for theories; indeed, even the possibility of obtaining unambiguous micro- scopic expressions for these quantities has been questioned. Here, within the framework of Onsager theory with Parsons–Lee correction, we obtain microscopic estimates of the deformation free energy density of hard rod nematics in the presence of different director deformations. In the limit of a slowly changing director, these are directly compared with the macroscopic elastic free energy density. Within the same framework, we derive also closed microscopic expressions for all elastic coefficients of rodlike nematics. We find that the saddle–splay con- stant K24 is larger than both K11 and K22 over a wide range of particle lengths and densities. Moreover, the K13 contribution comes out to be crucial for the consistency of the results obtained from the analysis of the microscopic deformation free energy density calculated for variants of the splay deformation
Interplay of Particle Morphology and Director Distortions in Nematic Fluids
No full textThe existing microscopic theories for elasticity of nematics are challenged by recent findings on systems, whether bent molecules or semiflexible polymers, which do not comply with the model of rigid rodlike particles. Here, we propose an extension of Onsager-Straley second-virial theory, based on a model for the orientational distribution function that, through explicit account of the director profile along a particle, changes in the presence of deformations. The elastic constants reveal specific effects of particle morphology, which are not captured by the existing theories. This paves the way to microscopic modeling of the elastic properties of semiflexible liquid crystal polymers, which is a longstanding issue
Spontaneous Twisting of Achiral Hard Rod Nematics
No full textSince Onsager’s seminal work, hard rods have been taken as a prototype of nematic liquid crystals, characterized by uniaxial order and a uniform director field as a ground state. Here, using Onsager theory to calculate the free energy in the presence of arbitrary deformations, we find that hard rod nematics have an intrinsic tendency to twist around their ordering axis (double twist), driven by a mechanism in which the orientational fluctuations of particles play a key role. The anisotropic hard core potential used here is arguably the simplest form of interaction able to originate spontaneous breaking of mirror symmetry in a 3D fluid. Our results are discussed in relation to the recent discovery of a double twisted ground state in cylindrically confined lyotropic chromonic liquid crystals
Elasticity of lyotropic nematic liquid crystals: a review of experiments, theory and simulation
No full textThe elastic response of liquid crystals (LCs) to external perturbations is of eminent importance for both their theoretical understanding and practical application. While the elasticity of thermotropic nematic LCs has been widely studied, far less is known about the elastic properties of lyotropic nematic LCs. In recent years however, several observations of spontaneous mirror symmetry breaking in lyotropic LCs (LLCs) confined in curved geometries stimulated increasing scientific interest in the elastic behaviour of lyotropic nematics. In this review article, we have now compiled the known experimental data on the elastic constants of different classes of nematic LLCs, ranging from micellar via chromonic and polymeric to particle-based LLCs. The experimental findings are compared with simulation results on various model systems and discussed in the light of current theoretical concepts. As a result of these considerations, it is shown that the elastic properties of nematic LLCs can indeed be very different and strongly depending on the nature of their particular nanoscopic building blocks (such as micelles, molecular stacks, semiflexible polymer chains, solid nanoparticles, biofilaments), namely their size, aspect ratio and flexibility, as well as their specific interactions. Nevertheless, in all cases the twist constant K22 is by far the smallest of the three elastic constants of a certain system and an order of magnitude smaller than typical values found in thermotropic nematics. It also appears that the splay constant K11 is mainly determined by the aspect ratio of the particular LLC building blocks and the bend constant K33 by their flexibility. The one-constant approximation, often used for thermotropic nematics, clearly fails in the case of LLCs. Finally, our observations also make obvious that the current knowledge about the elastic properties of LLCs is still incomplete and thus improvements are necessary, both in terms of experimental investigations and theoretical studies.Abbreviations: 5CB: 4-Cyano-4'-pentylBiphenyl; BLG: beta-LactoGlobulin; CCNC: Carboxylated Cellulose NanoCrystal; CDEAB: N; N-Dimethyl-N-Ethylhexadecyl-Ammonium Bromide; CNC: Cellulose NanoCrystal; COM: Centre Of Mass; CsPFO: Caesium PerFluoroOctanoate; DACl: DecylAmmonium Chloride; DCF: Direct Correlation Function; DFT: Density Functional Theory; DLS: Dynamic Light Scattering; DOH: 1-Decanol; DSCG: DiSodium CromoGlycate; GB: Gay-Berne; HSC: Hard SpheroCylinder; LLC: Lyotropic Liquid Crystal; LCLC: Lyotropic Chromonic Liquid Crystal; MC: Monte Carlo; MD: Molecular Dynamics; NC: Nematic phase of rod-like (Calamitic) building blocks; ND: Nematic phase of Disc-like building blocks; ODF: Orientational Distribution Function; PBG: Poly-gamma-Benzyl-Glutamate; SDS: Sodium Dodecyl Sulfate; SCNC: Sulfated Cellulose NanoCrystal; SSY: SunSet Yellow; TMV: Tobacco Mosaic Viru
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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