1,720,969 research outputs found
Temperature and rate effects in damage and decohesion of biological materials
No full textLe incredibili proprietà termomeccaniche dei materiali biologici derivano dalla scala microscopica a causa di un complesso meccanismo gerarchico, che è regolato da microinstabilità a livello molecolare. La descrizione di strutture così complesse è consentita sia dal know-how introdotto dall'avvento degli esperimenti di spettroscopia di forza a singola molecola, che dà la possibilità di studiare tali sistemi in diverse condizioni termiche e meccaniche, sia dalla possibilità di imitare correttamente il loro comportamento al scala più bassa introducendo modelli matematici basati su energie non convesse. In questa tesi, vengono introdotte diverse classi di modelli per descrivere le caratteristiche importanti della transizione di fase, della decoesione e del danno in diverse condizioni di forze applicate e spostamento, campi termici e velocità di carico. Aumentando il livello di complessità di tali modelli, sono stati analizzati diversi fenomeni. Ad esempio, introducendo una catena di unità bistabili per imitare il comportamento di una molecola di titina in fase di dispiegamento, è stato descritto l'effetto del dispositivo di manipolazione in esperimenti su singole molecole, che influenza fortemente la risposta meccanica del sistema, portando a grandi errori nel misura della forza o spostamento risultante. Gli effetti della temperatura sono considerati all'interno di un quadro di Meccanica Statistica, anche nel caso in cui siano introdotte interazioni non locali. Infatti, fenomeni come la presenza di un picco di stress nel diagramma forza-estensione e corrispondente alla generazione e nucleazione di una fase sono osservati sperimentalmente in prove di trazione su nanofili a memoria di forma o materiali polimerici e possono essere descritti come una competizione tra energia interfacciale termini ed effetti entropici. La cooperatività delle interazioni deboli, come i legami idrogeno, è stata anche studiata per evidenziare fenomeni come la decoesione e la frattura nei sistemi biologici. Infatti, gli amminoacidi semplici sono disposti in modo multiscala dando luogo a materiali e strutture gerarchiche ad alte prestazioni, con elevate proprietà meccaniche. Nello specifico, considerando molle elastiche accoppiate con unità fragili, in questa tesi è stato dedotto un modello micromeccanico di sistemi come l'elica del DNA a doppio filamento oi fasci di microtubuli (MT) e le proteine tau disposte all'interno degli assoni con effetti termici e di velocità. Il processo di decoesione risulta essere fortemente regolato dalla rigidità relativa delle due unità pseudoelastiche e il tipo di frattura può variare da un brusco collasso (comportamento fragile) ad un distacco sequenziale dei legami (comportamento duttile). Questo effetto è potenziato anche quando si considera la velocità di carico, dove diventa cruciale la capacità di superare le barriere energetiche che separano gli stati metastabili. I risultati ottenuti nella tesi vengono confrontati con evidenze provenienti da un'ampia rassegna della letteratura e dai comportamenti sperimentali dei sistemi descritti, e vengono dedotte leggi analitiche costitutive microscopiche che illustrano il comportamento complessivo di tali sistemi complessi regolati da microinstabilità multiscala.The incredible thermo-mechanical properties of biological materials arise from the microscopic scale due to a complex hierarchical mechanism, which is regulated by microinstabilities at the molecular level. The description of such complex structures is allowed by both the know-how introduced by the advent of single molecule force spectroscopy experiments, which gives the possibility of studying such systems in different thermal and mechanical conditions, and the possibility of correctly mimicking their behaviour at the lowest scale by introducing mathematical models based on non-convex energies. In this thesis, different classes of models are introduced to describe the important features of phase transition, decohesion and damage under different conditions of applied forces and displacement, thermal fields and rates of loading. By increasing the level of complexity of such models, different phenomena have been analyzed. For instance, by introducing a chain of bistable units to mimic the behaviour of a titin molecule undergoing unfolding, it has been described the effect of the handling device in single molecule experiments, which strongly affects the system's mechanical response, leading to large errors in the measure of the resulting force or displacement. Temperature effects are considered within a Statistical Mechanics framework, also in the case when non local interactions are introduced. Indeed, phenomena such as the presence of a stress peak in the force-extension diagram and corresponding to the generation and nucleation of a phase is experimentally observed in tensile tests on memory shape nanowires or polymer materials and can be described as a competition between interfacial energy terms and entropic effects. The cooperativity of weak interactions, such as hydrogen bonds, has been also studied to highlight phenomena such as decohesion and fracture in biological systems. Indeed, simple amino acids are arranged in a multiscale fashion resulting in high performing hierarchical materials and structures, with elevated mechanical properties. Specifically, considering elastic springs coupled with breakable units, in this thesis a micromechanical model of systems such as the double-stranded DNA helix or the bundles of microtubules (MT) and tau proteins arranged within the axons with thermal and rate effects has been deduced. The decohesion process is found to be highly regulated by the relative stiffness of the two pseudo-elastic units, and the type of fracture may range from an abrupt collapse (fragile behaviour) to a sequential detachment of the bonds (ductile behaviour). This effect is also enhanced when the loading rate is considered, where the ability to overcome energy barriers separating the metastable states becomes crucial. The results obtained in the thesis are compared to pieces of evidence from an extensive literature review and to the experimental behaviours of the systems described, and microscopic constitutive analytic laws are deduced illustrating the overall behaviour of such complex systems regulated by multiscale microinstabilities
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
A surface-induced transition in polymeric nematics
No full textNew polymeric liquid crystals can be treated as standard nematic liquid crystals when only their bulk properties are at issue, but they exhibit peculiar surface properties. The most striking one is that biaxial distributions may be induced on a confining surface. On continuously varying the surface anchoring conditions, we find a first-order phase transition from planar to homeotropic alignment in the bulk. Moreover, the decay towards these uniaxial states is radically different in the two cases: it is asymptotically exponential in the former, whereas it happens abruptly at a finite depth in the latter. There is precisely one surface biaxial distribution that induces bistability between these decay modes: it depends on the elastic constants in the Landau-de Gennes free energy functional. The analysis of the model we propose can prove useful in detecting the sign of the difference between splay and bend constants
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
