Spelling suggestions: "subject:"nanorheology"" "subject:"damorheology""
1 |
Computer simulation of nanorheology for inhomogeneous fluidsZhang, Junfang, junfang.zhang@csiro.au January 2005 (has links)
In this thesis, we use nonequilibrium molecular dynamics (NEMD) methods to investigate the structural and dynamic properties of highly confined atomic and polymeric fluids undergoing planar Poiseuille flow. We derive 'method of planes' expressions for pressure tensor and heat flux vector for confined inhomogeneous atomic fluids under the influence of three-body forces. Our derivation is validated against NEMD simulations of a confined atomic fluid acted upon
by a two-body Barker-Fisher-Watts force coupled with the Axilrod-Teller three-body force. Our method of planes calculations are in excellent agreement with the equivalent mesoscopic route of integrating the momentum and energy continuity equations directly from the simulation data. Our calculations reveal that three-body forces have an important consequence for the isotropic pressure, but have negligible in�uence on the shear stress and heat flux vector for a confined simple fluid. We use the non-local linear hydrodynamic constitutive model, proposed by Evans and Morriss [1] for computing a viscosity kernel, a function of compact support, for inhomogeneous nonequilibrium fluids. Our results show that the viscosity kernel, �(y),
has a peak at y = 0, and gets smaller and decays to zero as y increases. Physically, it
means that the strain rate at the location where we want to know the stress contributes
most to the stress, and the contribution of the strain rate becomes less significant as the
relative distance y increases. We demonstrate that there is a limitation in the model
when it is applied to our confined fluids due to the effect of domain restriction on inverse
convolution. We study the nanorheology of simple polymeric fluids. Our NEMD simulation results show that sufficiently far from the walls, the radius of gyration for molecules under shear in the middle of the channel follows the power law, Rg / N�, where N is the number of
bonds and the exponent has a value � = 0:60�0:04, which is larger than the melt value
of 0:5 for a homogeneous equilibrium �uid. Under the conditions simulated, we find that
viscous forces dominate the flow, resulting in the onset of plug-like flow velocity pro�les
with some wall slippage. An examination of the streaming angular velocity displays a strong correlation with the radius of gyration, being maximum in those regions where Rg is minimum and vice-versa. The angular velocity is shown to be proportional to half the strain rate su�ciently far from the walls, consistent with the behaviour for homogeneous fluids in the linear regime. Finally, we make some concluding remarks and suggestions for future work in the
final chapter.
|
2 |
Caractérisation et applications des écoulements de polymères en films minces nanoimprimés / Characterization and applications of flowing nanoimprinted thin polymer filmsRognin, Etienne 04 February 2013 (has links)
Cette thèse présente des résultats théoriques et expérimentaux portant sur des écoulements à l'échelle nanoscopique de polymères fondus. L'étude analytique et numérique de l'écoulement d'un film de polymère, préalablement nanostructuré par nanoimpression puis recuit au dessus de sa température de transition vitreuse, a permis de dégager différentes dynamiques de nivellement selon la topographie initiale du film. Certaines ont été appliquées à l'élaboration d'éléments optiques par recuit de nanostructures complexes. Une méthode de mesure de la viscosité Newtonienne et du temps terminal de relaxation d'un polymère déposé en film mince a également pu être développée. Enfin, un travail exploratoire portant sur un procédé de nanoimpression sans épaisseur résiduelle par démouillage est présenté. L'accent a porté sur le calcul précis de la pression de disjonction dans un milieu stratifié en utilisant la théorie moderne de Lifshitz basée sur les propriétés optiques des matériaux en interaction. / This thesis presents a theoretical and experimental work on nanoscale flows of polymer melts. Different leveling dynamics emerge from the analytical and numerical study of the reflow of a polymer film that is first nanoimprinted and then annealed above its glass transition temperature, depending on the initial topography of the film. These concepts were applied to the manufacturing of optical devices from the reflow of complex nanostructures. A method to measure the Newtonian viscosity and the terminal relaxation time of a thin polymer film was also developed. Finally, an exploratory work on a residual-layer-free nanoimprint process based on dewetting is presented. Emphasis was put on the accurate computation of the disjoining pressure in stratified media with the modern Lifshitz theory based on the optical properties of the interacting materials.
|
3 |
Nanorhéologie des liquides confimés : application à la nanomécanique des couches minces / Confined liquids nanorheology : application to thin films nanomechanicsVilley, Richard 05 December 2013 (has links)
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.
|
Page generated in 0.0255 seconds