29 research outputs found
Elastocapillary instability under partial wetting conditions: bending versus buckling
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 , 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 . 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
Séparation des générations et manque de liens intergénérationnels : mythe ou réalité ? Qu’en est-il vraiment ? Que propose le Cyberthé de Neuchâtel? Répond-il à des besoins réels exprimés ? Si oui, lesquels ? Quels sont les vécus et les perceptions des participants sur les échanges et les relations qui se créent ? Que viennent-ils réellement chercher dans ces lieux de rencontre ? Ce travail de bachelor entend répondre à ces questions et faire découvrir aux lecteurs et lectrices des concepts actuels grâce à ce projet innovant, qui nous remet en question sur nos propres échanges sociaux et les partages avec les générations qui nous entourent. Le Semestre de Motivation et Pro Senectute se sont unis pour proposer des ateliers innovants autour de l’informatique. Par des entretiens et des observations, ce travail va mettre en lumière ce que les seniors et les jeunes viennent y chercher et leur vécu personnel des liens établis lors des échanges. Le but premier de ces ateliers est d’offrir une prestation autour de l’informatique, mais les initiateurs du projet sont convaincus des bénéfices secondaires que peut apporter le Cyberthé. Vous pourrez constater que les motivations varient selon chaque témoignage et parcours de vie
Elastic deformation due to tangential capillary forces \ud
A sessile liquid drop can deform the substrate on which it rests if the solid is sufficiently “soft.” 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
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 reaches a few molecular sizes
Origin of line tension for a Lennard-Jones nanodroplet
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
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
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 thèse met en évidence différents effets géométriques intervenant dans des phénomènes de mouillage. La première partie est dédiée à l'élaboration d'un modèle d'interactions dans le liquide permettant de déterminer, à partir de la géométrie de l'interface, la distribution des forces capillaires à l'échelle moléculaire. Nous proposons dans ce cadre une interprétation de la construction d'Young en tant qu'équilibre des forces dans un coin de liquide. Ce modèle est ensuite appliqué dans la deuxième partie à diverses situations mettant en jeu la capillarité aux échelles moléculaires. La tension de ligne est étudiée grâce à des simulations de dynamique moléculaire et une interprétation géométrique du phénomène est présentée. L'existence d'un film de prémouillage est prédite lors de la saturation du phénomène d'électromouillage. Ce modèle fait en outre ressortir une distribution des forces tout à fait particulière dans un solide au voisinage de la ligne de contact, dont les effets ne sont visibles que lorsque le substrat est déformable. Ainsi, une confirmation expérimentale de l'existence d'une pression de Laplace supplémentaire lorsqu'un solide est immergé est apportée. Nous étudions ensuite l'influence de la mouillabilité du liquide sur le fléchissement et le flambage d'une plaque élastique sous l'effet de cette distribution de forces capillaires. Pour finir, la transition de démouillage dynamique par entraînement d'air est examinée, et nous mettons en évidence le rôle crucial de la dissipation dans l'écoulement de l'air lorsque celui-ci est entraîné et confiné sous le liquide