Orientador: Alex Antonelli / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-29T05:01:21Z (GMT). No. of bitstreams: 1 CajahuaringaMacollunco_OscarSamuel_D.pdf: 15985103 bytes, checksum: f54d17d6335170c62196c9e329f257d9 (MD5) Previous issue date: 2015 / Resumo: Apesar de serem substâncias muito diferentes, silício e gálio, na fase líquida, compartilham muitas anomalias nas suas propriedades termodinâmicas. Evidências teóricas e experimentais sugerem que esses líquidos podem sofrer a chamada transição de fase líquido-líquido (LLPT). Especula-se que este tipo de transição é acompanhada por uma transição dinâmica entre um líquido frágil e um líquido forte, com base nas hipóteses de que o surgimento de um "dip" na função auto-intermediária de espalhamento logo após o regime balístico e o aparecimento do excesso de modos vibracionais em baixas frequências após a LLPT estariam relacionados com o comportamento de líquidos fortes. Foi realizado um estudo da dinâmica desses sistemas através das funções de correlação nas vizinhanças da LLPT usando simulações clássicas. Observamos apenas no caso do silício o aparecimento do "dip" na função auto-intermediária de espalhamento. A densidade de estados vibracionais reduzida de ambos os líquidos apresenta picos em baixas frequências, sugerindo que ambos os líquidos seriam fortes. Desta forma, nos dois casos estudados, as duas hipóteses não são conclusivas. Visando um melhor entendimento, determinamos a viscosidade de cisalhamento de ambos os líquidos em um amplo intervalo de temperatura. A apresentação desses resultados no chamado gráfico de Angell, indica que, em ambos os casos, a LLPT é acompanhada de uma transição de um líquido frágil para um líquido menos frágil, o que não dá suporte às especulações de uma transição frágil-forte. Além de investigar a LLPT em gálio "bulk", foi também estudado o gálio líquido confinado em nanofendas, ou seja, um sistema quase-bidimensional. Foi observada a transição de fase de primeira ordem entre um líquido isotrópico e um líquido ordenado. Por meio de uma série de análises estruturais, tanto de ordem translacional quanto orientacional, conseguimos identificar o líquido ordenado como a fase hexática, que é prevista ocorrer em sistemas bidimensionais segundo a teoria de Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY). Como a LLPT em gálio bulk foi identificada em simulações de dinâmica clássica, realizamos um estudo visando obter evidência da LLPT em simulações de primeiros princípios, que são em princípio mais realísticas. Utilizando o chamado método-Z, que permite estimar a temperatura de fusão, determinamos que o regime super-resfriado do gálio líquido descrito por cálculos de primeiros princípios ocorre em temperaturas abaixo de 400 K. Partindo de um líquido em equilíbrio em 500 K, realizamos simulações a pressão constante igual a zero em que o líquido foi resfriado para 260 K em um intervalo de 145 ps. Não foi observada uma mudança abrupta no volume durante o resfriamento. A fim de melhorar a amostragem em nossas simulações de primeiros princípios, realizamos um estudo de dinâmica molecular a volume constante com troca de réplicas. Foram usadas um total de 10 réplicas entre 400 K e 260 K, em simulações com a duração de 100 ps. Neste caso, a pressão decresceu monotonicamente com a temperatura, o que é característico de líquidos simples que não sofrem LLPT. Cabe ressaltar que nas simulações clássicas que identificaram a LLPT em gálio, a taxa de resfriamento foi muito mais lenta, da ordem de nano-segundos, que seria inexequível para simulações de primeiros princípios / Abstract: Despite of being very different substances, silicon and gallium, in their liquid phase, share several anomalous thermodynamic properties. Theoretical and experimental evidence suggest that these liquids can undergo the so-called liquid-liquid phase transition (LLPT). There has been speculated that this transition is accompanied by a dynamic transition form a fragile liquid to a strong liquid, based on the hypothesis that the development of a dip in the self-intermediate scattering function, just after the ballistic regime, and the appearance of excess low frequency vibrational modes just after the LLPT would be related to the behavior of strong liquids. It was performed a study of the dynamics of these systems through the correlation functions in the vicinity of the LLPT using classical simulations. We observed the development of a dip in the self-intermediate scattering function only in the case o silicon. The reduced density of vibrational states of both liquids exhibits peaks in low frequencies, suggesting both liquids to be strong. Therefore, for the two cases we studied, the two hypotheses are not conclusive. In order to achieve a better understanding, we determined the shear viscosity of both liquids for a wide range of temperatures. By plotting the data in the so-called Angell plot one can see that, in both cases, the LLPT is accompanied by a transition of a fragile liquid to a less fragile liquid, which does not give support to the speculations of a fragile-to-strong transition. Aside from investigating the LLPT in bulk gallium, it was also studied liquid gallium confined in nanoslits. i.e., a quasi two-dimensional system. It was observed a first-order transition between an isotropic liquid and an ordered liquid. Through a series of structural analyses, both translational and orientational, we were able to identify the ordered liquid as the hexatic phase, which is predict to occur in two-dimensional systems according to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. Since the LLPT in gallium has been found in classical molecular dynamics simulations, we performed a study aimed to obtain evidence of the LLPT in first-principles simulations, which are, in principle, more realistic. Using the so-called Z-method, which allows one to estimate the melting temperature, we determined the supercooled regime of liquid gallium, described by first-principles, to occur below 400 K. Starting from an equilibrated liquid at 500 K, we performed simulations at zero pressure in which the liquid was cooled down to 260 K in an interval of 145 ps. No abrupt change in the volume was observed as the liquid was cooled. In order to improve sampling in our first-principles simulations, we performed a study using constant volume molecular dynamics with replica exchange. Ten replicas were used between 400 K and 260 K, in simulations 100 ps long. In this case, pressure decreased monotonically with temperature, which is the behavior of simple liquids, which do not exhibit the LLPT. It should be emphasized that in the classical simulations that identified the LLPT in gallium, the cooling rates were much slower, of the order of nanoseconds, which would be unfeasible for first-principles simulations / Doutorado / Física / Doutor em Ciências / 1000949 / CAPES

Ce travail étudie comment la mesure des forces hydrodynamiques exercées par un liquide confiné entre une sphère et une surface d'intérêt permet de sonder à distance et sans contact les propriétés mécaniques de cette surface. Nous présentons tout d'abord le principe de cette technique originale de sonde fluide et la machine à forces de surface dynamique que nous utilisons pour mettre en œuvre ces expériences de nano-rhéologie. Puis nous nous intéressons à deux applications que nous avons plus particulièrement étudiées. D'une part, l'étude des propriétés de friction des bicouches lipidiques. Nous mettons en évidence une très faible friction mesurée sur certaines bicouches fluides et son potentiel rôle pour la bio-lubrification. D'autre part l'étude des propriétés élastiques de couche mince d'élastomère. Nous développons pour cela une théorie de l'élastohydrodynamique à géométrie sphère-plan en mode dynamique et présentons des résultats expérimentaux en très bon accord. Nous sommes capables de mesurer le module d'Young de films mince de PDMS d'épaisseur allant jusqu'à 600 nm. Enfin, nous présentons les développements instrumentaux réalisés pour optimiser les performances de la machine à forces de surfaces comme sonde fluide / This work addresses how the measurement of hydrodynamic forces of a liquid confined between a sphere and a surface of interest can lead to probe at a distance and without contact her mechanical properties. Firstly, we introduce this original technique of fluid probe and the dynamic Surface Force Apparatus used to perform these nanorheology experiments. Then, we present two applications for which the fluid probe can be of great importance. The first application is the study of friction properties of lipid bilayers. More particularly we measure very weak fluid friction coefficients on some fluid bilayers, which can be an important issue for biolubrication. The second application is the study of elastic properties of thin elastomer layers. For that we first develop an elasto-hydrodynamic theory for sphere-plan configuration in dynamic mode. Then, we present experimental results in very good agreement with this theory. We are able to measure the Young modulus of thin PDMS films of thickness as small as 600 nm. Finally, we present instrumental developments performed on the Surface Force Apparatus to optimize its potential as fluid probe

Lorsque deux solides séparés par un liquide se rapprochent, le drainage s’accompagne de forces visqueuses normales aux parois. Si elles sont très rapprochées, de l’ordre de quelques nanomètres, les parois sont indentées par ces forces : c’est le "confinement élastique". Indenter un solide par un liquide permet de supprimer l’adhésion, qui limite la résolution en termes de module d’Young des tests classiques d’indentation par un solide, surtout pour les films supportés minces et mous, par exemple des élastomères d’épaisseur micrométrique. Or leurs propriétés, qui peuvent sensiblement différer des propriétés en volume, sont essentielles dans des domaines aussi variés que la microfluidique, l’électronique ou l’usure mécanique. Nous présentons les calculs qui relient les forces normales aux propriétés mécaniques du liquide et des parois lors d’un confinement élastique. Les résultats sont comparés à des expériences de nanorhéologie réalisées sur une machine à forces de surface très sensible. Cette sensibilité nous permet de montrer que l’effet du confinement élastique se manifeste même sans film mou déposé : cela implique que la rhéologie apparente d’un liquide confiné est toujours affectée par les déformations des parois, même très rigides.Nous montrons enfin que nous pouvons effectivement mesurer avec précision des modules d’Young autour du MPa dans des films d’élastomères de quelques centaines de nanomètres à quelques micromètres d’épaisseur. Si le module de stockage ne varie presque pas avec l’épaisseur, un module de pertes apparaît, augmentant sensiblement lorsque l’épaisseur diminue, témoignant d’une visco-élasticité que nous attribuons à la présence d’une couche interfaciale. / When two solids separated by a liquid layer are moving towards each other, the resulting drainage is associated with viscous forces normal to the walls. If these are very close to each other (several nanometers), they are indented by these forces : this is the notion of “elastic confinement”. Indenting a solid by a liquid solves the problem of adhesion, which limits the ability of classical indentationtests to provide accurate measurements on Young’s modulii. Adhesion is especially problematic for soft thin films, for example micrometric layers of elastomers, which mechanical properties can strongly differ from the bulk, but are of the highest importance in various fields such as microfluidics, electronics or mechanical wear. We present here the calculations which link the solid and liquid mechanical properties to the resulting forces in a liquid indentation test. The corresponding results are compared to nanorheology experiments using a very sensitive Surface Force Apparatus. Its sensitivity enables us to show that the elastic confinement is also measurable without any soft films, which implies that a confined liquid apparent rheology is always affected bythe deformations of even very rigid confining walls. Finally, we demonstrate that we are indeed able to measure precisely Young’s modulii in the MPa range for films as thin as several hundreds of nanometers. While the storage modulii are found to be almost independent ofthe film thicknesses, we identify the presence of loss modulii increasing with decreasing thicknesses. We attribute this unexpected viscoelastic behaviour to the presence of an interfacial layer.