Beating the teapot effect

We investigate the dripping of liquids around solid surfaces in the regime of inertial flows, a situation commonly encountered with the so-called "teapot effect". We demonstrate that surface wettability is an unexpected key factor in controlling flow separation and dripping, the latter being completely suppressed in the limit of superhydrophobic substrates. This unforeseen coupling is rationalized in terms of a novel hydro-capillary adhesion framework, which couples inertial flows to surface wettability effects. This description of flow separation successfully captures the observed dependence on the various experimental parameters - wettability, flow velocity, solid surface edge curvature-. As a further illustration of this coupling, a real-time control of dripping is demonstrated using electro-wetting for contact angle actuation.Comment: 4 pages; movies at http://lpmcn.univ-lyon1.fr/~lbocque

Making a splash with water repellency

A 'splash' is usually heard when a solid body enters water at large velocity. This phenomena originates from the formation of an air cavity resulting from the complex transient dynamics of the free interface during the impact. The classical picture of impacts on free surfaces relies solely on fluid inertia, arguing that surface properties and viscous effects are negligible at sufficiently large velocities. In strong contrast to this large-scale hydrodynamic viewpoint, we demonstrate in this study that the wettability of the impacting body is a key factor in determining the degree of splashing. This unexpected result is illustrated in Fig.1: a large cavity is evident for an impacting hydrophobic sphere (1.b), contrasting with the hydrophilic sphere's impact under the very same conditions (1.a). This unforeseen fact is furthermore embodied in the dependence of the threshold velocity for air entrainment on the contact angle of the impacting body, as well as on the ratio between the surface tension and fluid viscosity, thereby defining a critical capillary velocity. As a paradigm, we show that superhydrophobic impacters make a big 'splash' for any impact velocity. This novel understanding provides a new perspective for impacts on free surfaces, and reveals that modifications of the detailed nature of the surface -- involving physico-chemical aspects at the nanometric scales -- provide an efficient and versatile strategy for controlling the water entry of solid bodies at high velocity.Comment: accepted for publication in Nature Physic

Effets du mouillage en hydrodynamique macroscopique : traînée, impacts et ruissellement

The influence of the surface properties on a micro-scale liquid flow near a solid wall has been pointed out in the last years; surface properties means here the liquid-solid affinity, in terms of the wetting angle formed between the liquid and the solid phase when a drop is deposited on the solid surface. In particular superhydrophobic surfaces allowed to reach a huge slip in microfluidic flows. One can wonder if the wetting properties do have any influence in macroscopic hydrodynamic situations. This thesis work aspires to provide some answers to this issue.In a first part, drag force measurements on immersed solids show that a superhydrophobic coating on the surfaces does not lead to the great expected drag reduction. The second aspect of this work concerns solid-liquid impacts situations, where triple contact lines are created : first, solid spheres falling on a liquid-gas interface, then liquid jet hitting solid impactors, what leads to liquid bells formation. In both cases, a huge coupling between surface properties and macroscopic hydrodynamics has been evidenced and rationalized.L'influence des propriétés de surfaces sur un écoulement liquide microscopique au voisinage d'une paroi a été mise en évidence au cours des dernières années. On entend ici par propriétés de surface l'affinité des solides avec les liquides mis en œuvre, caractérisée par l'angle de mouillage formé par une goutte liquide déposée sur la paroi solide. L'usage de surfaces superhydrophobes a permis notamment d'observer un glissement important du liquide sur la paroi solide pour des écoulements de microfluidique. On peut se demander si les propriétés de mouillage ont également une influence dans des situations hydrodynamiques de plus grande échelle, et c'est à cette problématique que l'on tente d'apporter des réponses dans cette thèse.Dans un premier temps, des mesures de forces réalisées sur des solides immergés dans un écoulement montrent que l'application d'un revêtement superhydrophobe ne permet pas d'obtenir la réduction radicale espérée de la traînée hydrodynamique. Par la suite, nous nous intéressons à des situations d'impact solide-liquide, mettant en œuvre des lignes de contact triples : impacts de sphères solides sur une interface liquide-gaz, puis impacts d'un jet liquide sur des impacteurs solides, conduisant à la formation de cloches liquides. Nous mettons en évidence et rationalisons dans ces deux cas le couplage fort qui relie les propriétés des surfaces à l'hydrodynamique macroscopique

Effets du mouillage en hydrodynamique macroscopique (traînée, impacts et ruissellement)

The influence of the surface properties on a micro-scale liquid flow near a solid wall has been pointed out in the last years; surface properties means here the liquid-solid affinity, in terms of the wetting angle formed between the liquid and the solid phase when a drop is deposited on the solid surface. In particular superhydrophobic surfaces allowed to reach a huge slip in microfluidic flows. One can wonder if the wetting properties do have any influence in macroscopic hydrodynamic situations. This thesis work aspires to provide some answers to this issue. In a first part, drag force measurements on immersed solids show that a superhydrophobic coating on the surfaces does not lead to the great expected drag reduction. The second aspect of this work concerns solid-liquid impacts situations, where triple contact lines are created : first, solid spheres falling on a liquid-gas interface, then liquid jet hitting solid impactors, what leads to liquid bells formation. In both cases, a huge coupling between surface properties and macroscopic hydrodynamics has been evidenced and rationalizedLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Discrimination of positional isomers by ion mobility mass spectrometry Application to organic semiconductors

International audienceOrganic semiconductors are increasingly being used in organic-based opto-electronic devices. Since regioselective synthetic approaches are not always controlled, promising compounds are sometimes prepared following non-regioselective routes. Ion mobility mass spectrometry is here introduced as a direct method to distinguish isomers in mixtures, instead of undertaking time-consuming analytical chromatography procedures. © 2018 The Royal Society of Chemistry