Wetting of crossed fibers: multiple steady states and symmetry breaking

We investigate the wetting properties of the simplest element of an array of random fibers: two rigid fibers crossing with an inclination angle and in contact with a droplet of a perfectly wetting liquid. We show experimentally that the liquid adopts different morphologies when the inclination angle is increased: a column shape, a mixed morphology state where a drop lies at the end of a column, or a drop centered at the node. An analytical model is provided that predicts the wetting length as well as the presence of a non-symmetric state in the mixed morphology regime. The model also highlights a symmetry breaking at the transition between the column state and the mixed morphology. The possibility to tune the morphology of the liquid could have important implications for drying processes

Dynamics of elastocapillary rise

We present the results of a combined experimental and theoretical investigation of the surface-tension-driven coalescence of flexible structures. Specifically, we consider the dynamics of the rise of a wetting liquid between flexible sheets that are clamped at their upper ends. As the elasticity of the sheets is progressively increased, we observe a systematic deviation from the classical diffusive-like behaviour: the time to reach equilibrium increases dramatically and the departure from classical rise occurs sooner, trends that we elucidate via scaling analyses. Three distinct temporal regimes are identified and subsequently explored by developing a theoretical model based on lubrication theory and the linear theory of plates. The resulting free-boundary problem is solved numerically and good agreement is obtained with experiments

Obstacle-induced lateral dispersion and nontrivial trapping of flexible fibers settling in a viscous fluid

The motion of flexible fibers through structured fluidic environments is ubiquitous in nature and industrial applications. Most often, their dynamics results from the complex interplay between internal elastic stresses, contact forces and hydrodynamic interactions with the walls and obstacles. By means of numerical simulations, experiments and analytical predictions, we investigate the dynamics of flexible fibers settling in a viscous fluid embedded with obstacles of arbitrary shapes. We identify and characterize two types of events: trapping and gliding, for which we detail the mechanisms at play. We observe nontrivial trapping conformations on sharp obstacles that result from a subtle balance between elasticity, gravity and friction. In the gliding case, a flexible fiber reorients and drifts sideways after sliding along the obstacle. The subsequent lateral displacement is large compared to the fiber length and strongly depends on its mechanical and geometrical properties. We show how these effects can be leveraged to propose a new strategy to sort particles based on their size and/or elasticity. This approach has the major advantage of being simple to implement and fully passive, since no energy is needed.Comment: 18 pages, 9 figure

Oscillations of confined fibers transported in microchannels

We investigate the trajectories of rigid fibers as they are transported in a pressure-driven flow, at low Reynolds number, in shallow Hele Shaw cells. The transverse confinement and the resulting viscous friction on these elongated objects, as well as the lateral confinement (i.e. the presence of lateral walls), lead to complex fibers trajectories that we characterize with a combination of microfluidic experiments and simulations using modified Brinkman equations. We show that the transported fiber behaves as an oscillator for which we obtain and analyze a complete state diagram

Liquid film coating a fiber as a model system for the formation of bound states in active dispersive-dissipative nonlinear media

We analyze the coherent-structure interaction and the formation of bound states in active dispersivedissipative nonlinear media using a viscous film coating a vertical fiber as a prototype. The coherent structures in this case are droplike pulses that dominate the evolution of the film.We study experimentally the interaction dynamics and show evidence for formation of bound states. A theoretical explanation is provided through a coherent-structures theory of a simple model for the flow

Écoulement et capture d’aérosols dans les masques respiratoires

Une stratégie essentielle dans la lutte contre la pandémie de Covid-19 est le port de masques composés de fibres enchevêtrées non tissées, qui capturent les gouttes chargées de virus émises par le porteur du masque ou présentes dans l’air ambiant. Deux mécanismes principaux de capture des gouttes sont observés : l’interception directe de grosses gouttes qui ne sont capturées qu’en dessous d’une vitesse critique, et la capture par impact inertiel de petites gouttes qui augmente avec la vitesse des gouttes. Il est nécessaire de mieux quantifier ces phénomènes en fonction de la microstructure du matériau filtrant pour optimiser l’efficacité des masques selon les usages

Instabilités d'un film liquide en écoulement sur une fibre verticale

We consider the instability of a viscous liquid film flowing down a vertical fibre. We explore experimentally the various dynamical regimes arising from the interplay of viscosity, surface tension and inertia. We demonstrate the existence of two different dynamics, the instability being either absolute or convective due to the competition between the capillary Rayleigh-Plateau instability and the advection of the waves by the gravity-induced flow. We obtain a theoretical criterion describing this transition. When the instability is convective, the system behaves as a noise amplifier. We then study the selective spatial response of the film to periodic perturbations at inlet. The destabilisation of the flow leads to the creation of non-linear travelling waves. Those waves are characterised both experimentally and numerically from a non-linear model based on lubrication theory. When viscous and surface tension effects are of the same order, the waves are localised; their particular shape gives rise to attractive or repulsive interactions leading to the formation of bound states. Finally, we focus on the surface tension dominated regime and study the change of dynamics related to the development of recirculation zones in the drops.Cette thèse porte sur l'étude de l'instabilité d'un film liquide visqueux en écoulement sur une fibre verticale. Nous explorons expérimentalement les différents régimes dynamiques issus des effets combinés de la viscosité, de la tension de surface et de l'inertie du fluide. Nous démontrons l'existence de deux dynamiques différentes, correspondant à une instabilité absolue ou convective, résultant de la compétition entre l'écoulement induit par la gravité et la croissance de l'instabilité capillaire de Rayleigh-Plateau. Nous obtenons théoriquement un critère pour décrire cette transition. Dans le cas convectif, le système se comporte comme un amplificateur de bruit et nous étudions alors la réponse spatiale sélective du film à un forçage périodique. La déstabilisation de l'écoulement s'accompagne de la création d'ondes non-linéaires propagatives caractérisées expérimentalement et numériquement à partir d'un modèle faiblement non-linéaire formulé dans le cadre de la théorie de la lubrification. Lorsque les effets visqueux et capillaires sont du même ordre, les ondes sont localisées et adoptent une forme particulière qui induit des interactions complexes entre structures; ces interactions à la fois attractives et répulsives conduisent à la formation d'états liés. Finalement, nous étudions les régimes dominés par la tension de surface et notamment le changement de dynamique lié à l'apparition d'une zone de recirculation dans les gouttes

Controlling wet adhesion with elasticity

International audienceWe consider the wet adhesion between two deformable fibers. We show that a strong adhesive force can be maintained by coupling the fibers deformation with capillarity. We further identify a regime where, contrary to capillary adhesion, the pull-off force remains constant throughout debonding. In this peeling regime, elasticity, which tends to minimize the deformation of the object, is balanced by capillarity which maximizes the liquid spreading. We show that the adhesive force and the existence region of this self-adjusted peeling regime depend on a single dimensionless parameter, and can thus be controlled by tuning the material properties

Instabilités d'un film liquide en écoulement sur une fibre verticale

Cette thèse porte sur l instabilité d'un film liquide visqueux en écoulement sur une fibre verticale. Nous explorons expérimentalement les différents régimes issus des effets combinés de la viscosité, de la tension de surface et de l'inertie du fluide. Nous démontrons l'existence de deux dynamiques différentes correspondant à une instabilité absolue ou convective et obtenons théoriquement un critère pour décrire cette transition. Dans le cas convectif, nous étudions la réponse spatiale sélective du film à un forçage périodique. La déstabilisation de l'écoulement s'accompagne de la création d'ondes non-linéaires propagatives caractérisées expérimentalement et numériquement. Lorsque les effets visqueux et capillaires sont du même ordre, les ondes adoptent une forme particulière qui induit des interactions complexes entre structures. Finalement, nous étudions le changement de dynamique lié à l apparition d une zone de recirculation dans les gouttes lorsque la tension de surface domine.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF