29 research outputs found

    Elastocapillary instability under partial wetting conditions: bending versus buckling

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    The elastocapillary instability of a flexible plate plunged in a liquid bath is analysed theoretically. We show that the plate can bend due to two separate destabilizing mechanisms, when the liquid is partially wetting the solid. For contact angles őłe>ŌÄ/2\theta_e > \pi/2, the capillary forces acting tangential to the surface are compressing the plate and can induce a classical buckling instability. However, a second mechanism appears due to capillary forces normal to surface. These induce a destabilizing torque that tends to bend the plate for any value of the contact angle őłe>0\theta_e > 0. We denote these mechanisms as "buckling" and "bending" respectively and identify the two corresponding dimensionless parameters that govern the elastocapillary stability. The onset of instability is determined analytically and the different bifurcation scenarios are worked out for experimentally relevant conditions.Comment: 12 pages, 13 figure

    Cyberthé: un projet intergénérationnel : perceptions et expériences des participants

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    S√©paration des g√©n√©rations et manque de liens interg√©n√©rationnels : mythe ou r√©alit√© ? Qu‚Äôen est-il vraiment ? Que propose le Cyberth√© de Neuch√Ętel? R√©pond-il √† des besoins r√©els exprim√©s ? Si oui, lesquels ? Quels sont les v√©cus et les perceptions des participants sur les √©changes et les relations qui se cr√©ent ? Que viennent-ils r√©ellement chercher dans ces lieux de rencontre ? Ce travail de bachelor entend r√©pondre √† ces questions et faire d√©couvrir aux lecteurs et lectrices des concepts actuels gr√Ęce √† ce projet innovant, qui nous remet en question sur nos propres √©changes sociaux et les partages avec les g√©n√©rations qui nous entourent. Le Semestre de Motivation et Pro Senectute se sont unis pour proposer des ateliers innovants autour de l‚Äôinformatique. Par des entretiens et des observations, ce travail va mettre en lumi√®re ce que les seniors et les jeunes viennent y chercher et leur v√©cu personnel des liens √©tablis lors des √©changes. Le but premier de ces ateliers est d‚Äôoffrir une prestation autour de l‚Äôinformatique, mais les initiateurs du projet sont convaincus des b√©n√©fices secondaires que peut apporter le Cyberth√©. Vous pourrez constater que les motivations varient selon chaque t√©moignage et parcours de vie

    Elastic deformation due to tangential capillary forces \ud

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    A sessile liquid drop can deform the substrate on which it rests if the solid is sufficiently ‚Äúsoft.‚ÄĚ In this paper we compute the detailed spatial structure of the capillary forces exerted by the drop on the solid substrate using a model based on Density Functional Theory. We show that, in addition to the normal forces, the drop exerts a previously unaccounted tangential force. The resultant effect on the solid is a pulling force near the contact line directed towards the interior of the drop, i.e., not along the interface. The resulting elastic deformations of the solid are worked out and illustrate the importance of the tangential force

    Contact angles on a soft solid: from Young's law to Neumann's law

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    The contact angle that a liquid drop makes on a soft substrate does not obey the classical Young's relation, since the solid is deformed elastically by the action of the capillary forces. The finite elasticity of the solid also renders the contact angles different from that predicted by Neumann's law, which applies when the drop is floating on another liquid. Here we derive an elasto-capillary model for contact angles on a soft solid, by coupling a mean-field model for the molecular interactions to elasticity. We demonstrate that the limit of vanishing elastic modulus yields Neumann's law or a slight variation thereof, depending on the force transmission in the solid surface layer. The change in contact angle from the rigid limit (Young) to the soft limit (Neumann) appears when the length scale defined by the ratio of surface tension to elastic modulus ő≥/E\gamma/E reaches a few molecular sizes

    Origin of line tension for a Lennard-Jones nanodroplet

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    The existence and origin of line tension has remained controversial in literature. To address this issue we compute the shape of Lennard-Jones nanodrops using molecular dynamics and compare them to density functional theory in the approximation of the sharp kink interface. We show that the deviation from Young's law is very small and would correspond to a typical line tension length scale (defined as line tension divided by surface tension) similar to the molecular size and decreasing with Young's angle. We propose an alternative interpretation based on the geometry of the interface at the molecular scale

    Capillary Pressure and Contact Line Force on a Soft Solid

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    The surface free energy, or surface tension, of a liquid interface gives rise to a pressure jump when the interface is curved. Here we show that a similar capillary pressure arises at the interface of soft solids. We present experimental evidence that immersion of a thin elastomeric wire into a liquid induces a substantial elastic compression due to the solid capillary pressure at the bottom. We quantitatively determine the effective surface tension from the elastic displacement field, and find a value comparable to the liquid-vapor surface tension. Most importantly, these results also reveal the way the liquid pulls on the solid close to the contact line: the capillary force is not oriented along the liquid-air interface, nor perpendicularly to the solid surface, as previously hypothesized, but towards the interior of the liquid

    Mouillage statique et dynamique : Influences géométriques aux échelles moléculaires

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    This thesis highlights different geometric effects involved in wetting phenomena. The first part is dedicated to the development of a model of interactions in liquids to determine, from the geometry of the interface, the distribution of capillary forces at a molecular level. In this context we propose an new microscopic interpretation of Young's force balance that leads to the equilibrium contact angle in a corner of fluid. In the second part, this model is applied to phenomena as line tension, electrowetting and elastocapillary interactions -- these all involve capillary forces at molecular scales. The effects of line tension are studied by molecular dynamics simulations of nanodrops, and we present a geometrical interpretation of the phenomenon. The existence of a pre-wetting film is predicted during the saturation of the phenomenon of electrowetting. The microscopic capillary model also shows a peculiar distribution of forces in the solid in the vicinity of the contact line, which can be observed when the substrate is deformable. In particular, the model predicts an additional Laplace pressure when a solid is immersed in a liquid -- this is confirmed experimentally. We also study the influence of the wettability of liquids on the bending and buckling of an elastic plate under the influence of capillary forces. Finally, the dynamic transition of air entrainment is examined, and we highlight the crucial role of dissipation in the air flow when it is driven and confined by the liquid.Cette th√®se met en √©vidence diff√©rents effets g√©om√©triques intervenant dans des ph√©nom√®nes de mouillage. La premi√®re partie est d√©di√©e √† l'√©laboration d'un mod√®le d'interactions dans le liquide permettant de d√©terminer, √† partir de la g√©om√©trie de l'interface, la distribution des forces capillaires √† l'√©chelle mol√©culaire. Nous proposons dans ce cadre une interpr√©tation de la construction d'Young en tant qu'√©quilibre des forces dans un coin de liquide. Ce mod√®le est ensuite appliqu√© dans la deuxi√®me partie √† diverses situations mettant en jeu la capillarit√© aux √©chelles mol√©culaires. La tension de ligne est √©tudi√©e gr√Ęce √† des simulations de dynamique mol√©culaire et une interpr√©tation g√©om√©trique du ph√©nom√®ne est pr√©sent√©e. L'existence d'un film de pr√©mouillage est pr√©dite lors de la saturation du ph√©nom√®ne d'√©lectromouillage. Ce mod√®le fait en outre ressortir une distribution des forces tout √† fait particuli√®re dans un solide au voisinage de la ligne de contact, dont les effets ne sont visibles que lorsque le substrat est d√©formable. Ainsi, une confirmation exp√©rimentale de l'existence d'une pression de Laplace suppl√©mentaire lorsqu'un solide est immerg√© est apport√©e. Nous √©tudions ensuite l'influence de la mouillabilit√© du liquide sur le fl√©chissement et le flambage d'une plaque √©lastique sous l'effet de cette distribution de forces capillaires. Pour finir, la transition de d√©mouillage dynamique par entra√ģnement d'air est examin√©e, et nous mettons en √©vidence le r√īle crucial de la dissipation dans l'√©coulement de l'air lorsque celui-ci est entra√ģn√© et confin√© sous le liquide
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