Relaxation of a dewetting contact line Part 1: A full-scale hydrodynamic calculation

The relaxation of a dewetting contact line is investigated theoretically in the so-called "Landau-Levich" geometry in which a vertical solid plate is withdrawn from a bath of partially wetting liquid. The study is performed in the framework of lubrication theory, in which the hydrodynamics is resolved at all length scales (from molecular to macroscopic). We investigate the bifurcation diagram for unperturbed contact lines, which turns out to be more complex than expected from simplified 'quasi-static' theories based upon an apparent contact angle. Linear stability analysis reveals that below the critical capillary number of entrainment, Ca_c, the contact line is linearly stable at all wavenumbers. Away from the critical point the dispersion relation has an asymptotic behaviour sigma~|q| and compares well to a quasi-static approach. Approaching Ca_c, however, a different mechanism takes over and the dispersion evolves from |q| to the more common q^2. These findings imply that contact lines can not be treated as universal objects governed by some effective law for the macroscopic contact angle, but viscous effects have to be treated explicitly.Comment: 21 pages, 9 figure

Relaxation of a dewetting contact line Part 2: Experiments

The dynamics of receding contact lines is investigated experimentally through controlled perturbations of a meniscus in a dip coating experiment. We first characterize stationary menisci and their breakdown at the coating transition. It is then shown that the dynamics of both liquid deposition and long-wavelength perturbations adiabatically follow these stationary states. This provides a first experimental access to the entire bifurcation diagram of dynamical wetting, confirming the hydrodynamic theory developed in Part 1. In contrast to quasi-static theories based on a dynamic contact angle, we demonstrate that the transition strongly depends on the large scale flow geometry. We then establish the dispersion relation for large wavenumbers, for which we find that sigma is linear in q. The speed dependence of sigma is well described by hydrodynamic theory, in particular the absence of diverging time-scales at the critical point. Finally, we highlight some open problems related to contact angle hysteresis that lead beyond the current description.Comment: 20 pages, 11 figures Part 1 is stored as Arxiv 0705.357

Collective beating of artificial microcilia

We combine technical, experimental and theoretical efforts to investigate the collective dynamics of artificial microcilia in a viscous fluid. We take advantage of soft-lithography and colloidal self-assembly to devise microcapets made of hundreds of slender magnetic rods. This novel experimental setup is used to investigate the dynamics of extended cilia arrays driven by a precessing magnetic field. Whereas the dynamics of an isolated cilium is a rigid body rotation, collective beating results in a symmetry breaking of the precession patterns. The trajectories of the cilia are anisotropic and experience a significant structural evolution as the actuation frequency increases. We present a minimal model to account for our experimental findings and demonstrate how the global geometry of the array imposes the shape of the trajectories via long range hydrodynamic interactions.Comment: 5 pages, 3 figure

Etude expérimentale d’un écoulement gaz-liquide à contre courant sur un plan incliné

Le ruissellement d’un film d’eau de pluie sur un pare-brise ou une carrosserie est un phénomène important dans un contexte de sécurité des véhicules. Nous décrivons ici une expérience modèle de drainage d’un film liquide sur un plan incliné soumis à un contre-écoulement d’air. Nous caractérisons en particulier le seuil en vitesse de vent qui permet au film liquide de remonter la pente

Force-Velocity Measurements of a Few Growing Actin Filaments

The authors propose a new mechanism for actin-based force generation based on results using chains of actin-grafted magnetic colloids

Tethered fleximags as artificial cilia

Flexible superparamagnetic filaments ('fleximags') are very slender elastic filaments, which can be driven by distributed magnetic torques to mimic closely the behaviour of biological flagella. Previously, fleximags have been used as a basis for artificial micro-swimmers capable of transporting small cargos Dreyfus et al. (Nature, vol. 437, 2005, p. 862). Here, we demonstrate how these filaments can be anchored to a wall to make carpets of artificial micro-magnetic cilia with tunable densities. We analyse the dynamics of an artificial cilium under both planar and three-dimensional beating patterns. We show that the dynamics are controlled by a single characteristic length scale varying with the inverse square root of the driving frequency, providing a mechanism to break the fore and aft symmetry and to generate net fluxes and forces. However, we show that an effective geometrical reciprocity in the filament dynamics creates intrinsic limitations upon the ability of the artificial flagellum to pump fluid when driven in two dimensions

Gouttes et films liquides en aérodynamique automobile

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Cils artificiels (modèle physique pour la propulsion ciliée)

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Mouillage et démouillage de surfaces hétérogènes

L'objet de cette thèse est la caractérisation expérimentale de l'influence de défauts de mouillabilité sur l'étalement et le retrait d'un liquide sur une surface hétérogène. La plus grande partie de cette thèse est consacrée à l'étude morphologique et dynamique de grosses gouttes posées alimentées à débit constant. La démarche adoptée procède du simple au complexe: nous avons étudié l'influence de défauts isolés, le couplage de deux défauts, l'influence d'un réseau périodique et l'influence de la densité sur une distribution désordonnée de défauts. Une partie moins importante mais non moins riche est consacrée à l'étude du démouillage spontané d'une couche de liquide métastable sur une surface hétérogène. Nous avons mesuré l'influence de défauts isolés et nous avons décrit le processus de nucléation de gouttelettes satellites derrière le défaut lors du retrait du liquide sur la surface solide. Les résultats obtenus montrent la possibilité de déplacer et de confiner des volumes contrôlables de liquide sur des surfaces planes.In this thesis, we report an experimental study of the influence of wettability defects on an advancing and on a receding contact line on a heterogeneous surface. The most part of the thesis is dedicated to a morphological and a dynamical study of large drops. The liquid is injected into the drop at constant flow rate. The approach of the problem goes from simplicity to complexity: we have investigated the influence of isolated defects, the coupling between two defects, the influence of a periodic array of defects and the influence of the defect density on a disordered distribution of defects. The last part of the thesis is dedicated to an experimental study of spontaneous dewetting of a metastable liquid layer deposited on a heterogeneous surface. We have measured the influence of isolated defects and we have described the process of nucleation of satellite drops behind the defect during the receding of the liquid layer on the solid surface. The obtained results show the possibility to displace and to confine selected volumes of liquid on plane solid surfaces.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF