Cylindre roulant à proximité d'un plan à bas Reynolds

L'écoulement bas Reynolds autour d'un cylindre roulant à proximité d'un plan a été étudié analytiquement et numériquement.En régime de Stokes,une solution analytique exacte a été trouvée lorsque le cylindre est en contact avec la paroi.Ce problème bidimensionnel révèle la nécessité de l'existence d'un interstice entre le cylindre et la paroi.La théorie de lubrification permet alors d'obtenir les efforts sur le cylindre.Des simulations numériques étendent ces résultats jusqu'à un nombre de Reynolds de 80 et présentent l'émergence du sillage à partir de structures tourbillonnaires pré-existante

Surface Engineering for Phase Change Heat Transfer: A Review

Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiencies at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930's to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries

Biomolecular interactions control the shape of stains from drying droplets of complex fluids

When a sessile droplet of a complex fluid dries, a stain forms on the solid surface. The structure and pattern of the stain can be used to detect the presence of a specific chemical compound in the sessile droplet. In the present work, we investigate what parameters of the stain or its formation can be used to characterize the specific interaction between an aqueous dispersion of beads and its receptor immobilized on the surface. We use the biotin-streptavidin system as an experimental model. Clear dissimilarities were observed in the drying sequences on streptavidin-coated substrates of droplets of aqueous solutions containing biotin-coated or streptavidin-coated beads. Fluorescent beads are used in order to visualize the fluid flow field. We show differences in the distribution of the particles on the surface depending on biomolecular interactions between beads and the solid surface. A mechanistic model is proposed to explain the different patterns obtained during drying. The model describes that the beads are left behind the receding wetting line rather than pulled towards the drop center if the biological binding force is comparable to the surface tension of the receding wetting line. Other forces such as the viscous drag, van der Waals forces, and solid–solid friction forces are found negligible. Simple microfluidics experiments are performed to further illustrate the difference in behavior where is adhesion or friction are present between the bead and substrate due to the biological force. The results of the model are in agreement with the experimental observations which provide insight and design capabilities. A better understanding of the effects of the droplet–surface interaction on the drying mechanism is a crucial first step before the identification of drying patterns can be promisingly applied to areas such as immunology and biomarker detection

Surface engineering for phase change heat transfer: A review

Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoverie

Analytical, numerical and an experimental study of flows generated by moving boundaries in microfluidics - Application to micropumps

A l’heure actuelle, la microfluidique est une science en plein développement ayant un besoin croissant de dispositifs permettant de générer des écoulements aux échelles micrométriques. Les phénomènes physiques mis en jeu lors du mouvement d’un fluide sont en effet majoritairement gouvernés par la viscosité (bas nombre de Reynolds) contrairement aux écoulements macroscopiques dominés par les effets inertiels.Dans cette thèse, les écoulements engendrés par le mouvement de parois mobiles ont été étudiés en vue d’une application aux micropompes, dispositifs essentiels en microfluidique.Dans une première partie, une étude analytique et numérique évalue la possibilité de générer un écoulement par un cylindre en rotation à proximité de parois mobiles.Les résultats obtenus du régime de Stokes (Re=0) jusqu’à un nombre de Reynolds Re=60 en régime stationnaire témoignent du potentiel notable d’intégration de cette géométrie dans les microsystèmes en tant que micropompes.Dans une seconde partie, une micropompe, basée sur un principe de fonctionnement novateur, est conçue par l’intermédiaire des techniques de microfabrication. Dans cette optique, le procédé de gravure RIE d’un élastomère est entièrement développé. Les performances de la micropompe en terme de pression et débit générés dépassent l’état de l’art des microsystèmes similaires et ceci en utilisant une technologie simple et bas-coûtCurrently, microfluidic is a science field in constant development with an increasing need of devices able to generate flows at the micrometer order. At these length scales, physical phenomenons occurring in a moving fluid are mainly governed by its viscosity (low Reynolds number) contrary to macroscale flows dominated by inertial effects.In this thesis, a study on flows engendered by moving walls has been carried to fulfill to micropumps devices.In a first part, an analytical and a numerical study evaluates the possibility to generate a flow for a rotating cylinder close to moving boundaries.The results ranging from Stokes flows (Re=0) up to the low Reynolds number Re=60 in the stationary regime reveals the noticeable potential of integrating this device in microsystems as a micropump. In a second part, a new micropump, based on an innovative principle, is designed thanks to microfabrication technologies. In this perspective, the etching process of an elastomer called Silastic S is developed. Micropump performances in terms of pressure and flow rate are beyond the state of the art for similar microsystems and are achieved by using a simple and low-cost technolog

Etude analytique, numérique et expérimentale d’écoulements générés par parois mobiles en microfluidique - Application aux micropompes

Currently, microfluidic is a science field in constant development with an increasing need of devices able to generate flows at the micrometer order. At these length scales, physical phenomenons occurring in a moving fluid are mainly governed by its viscosity (low Reynolds number) contrary to macroscale flows dominated by inertial effects.In this thesis, a study on flows engendered by moving walls has been carried to fulfill to micropumps devices.In a first part, an analytical and a numerical study evaluates the possibility to generate a flow for a rotating cylinder close to moving boundaries.The results ranging from Stokes flows (Re=0) up to the low Reynolds number Re=60 in the stationary regime reveals the noticeable potential of integrating this device in microsystems as a micropump. In a second part, a new micropump, based on an innovative principle, is designed thanks to microfabrication technologies. In this perspective, the etching process of an elastomer called Silastic S is developed. Micropump performances in terms of pressure and flow rate are beyond the state of the art for similar microsystems and are achieved by using a simple and low-cost technologyA l’heure actuelle, la microfluidique est une science en plein développement ayant un besoin croissant de dispositifs permettant de générer des écoulements aux échelles micrométriques. Les phénomènes physiques mis en jeu lors du mouvement d’un fluide sont en effet majoritairement gouvernés par la viscosité (bas nombre de Reynolds) contrairement aux écoulements macroscopiques dominés par les effets inertiels.Dans cette thèse, les écoulements engendrés par le mouvement de parois mobiles ont été étudiés en vue d’une application aux micropompes, dispositifs essentiels en microfluidique.Dans une première partie, une étude analytique et numérique évalue la possibilité de générer un écoulement par un cylindre en rotation à proximité de parois mobiles.Les résultats obtenus du régime de Stokes (Re=0) jusqu’à un nombre de Reynolds Re=60 en régime stationnaire témoignent du potentiel notable d’intégration de cette géométrie dans les microsystèmes en tant que micropompes.Dans une seconde partie, une micropompe, basée sur un principe de fonctionnement novateur, est conçue par l’intermédiaire des techniques de microfabrication. Dans cette optique, le procédé de gravure RIE d’un élastomère est entièrement développé. Les performances de la micropompe en terme de pression et débit générés dépassent l’état de l’art des microsystèmes similaires et ceci en utilisant une technologie simple et bas-coû

Texture and wettability of metallic lotus leaves

Superhydrophobic surfaces with the self-cleaning behavior of lotus leaves are sought for drag reduction and phase change heat transfer applications. These superrepellent surfaces have traditionally been fabricated by random or deterministic texturing of a hydrophobic material. Recently, superrepellent surfaces have also been made from hydrophilic materials, by deterministic texturing using photolithography, without low-surface energy coating. Here, we show that hydrophilic materials can also be made superrepellent to water by chemical texturing, a stochastic rather than deterministic process. These metallic surfaces are the first analog of lotus leaves, in terms of wettability, texture and repellency. A mechanistic model is also proposed to describe the influence of multiple tiers of roughness on wettability and repellency. This demonstrated ability to make hydrophilic materials superrepellent without deterministic structuring or additional coatings opens the way to large scale and robust manufacturing of superrepellent surfaces.This is a manuscript of an article from Nanoscale 8 (2016): 3982, doi:10.1039/C5NR04098A. Posted with permission.</p