Motion of a massive particle attached to a spherical interface: statistical properties of the particle path

We have studied the motion of a Brownian particle on a spherical interface under gravity, with the aim of setting up a protocol to measure the friction (f) felt by such a particle in experimental conditions. Our analysis is based on the Schmoluchowski equation for particle motion. Essentially we derive a practical criterion to find f from the average particle path. Our statements are illustrated by a few experimental and numerical examples. Numerical paths are obtained by computer simulation and experimental paths are those of micrometre-sized latex or glass particles attached to spherical giant lipid (SOPC) vesicles. From experimental values of f, we estimate the surface shear viscosity of SOPC bilayers to be in the range 3-8×10^(-6) Poise

Motion of a massive microsphere bound to a spherical vesicle

We study the motion of a small solid particle (a few micrometers in size) attached to the membrane of a spherical giant lipid vesicle. By means of optical manipulation, the particle is first brought near the top of the vesicle, and released. We determine the friction experienced by the particle moving along the vesicle surface under the action of gravity. From experiments with latex and glass beads, we check that SOPC membranes are fluid at room temperature (static shear modulus u = 0) and estimate the shear viscosity of SOPC bilayers: nm = 3 * 10^(-6) surface poise

Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water-air interface

We investigate the radial thermocapillary flow driven by a laser-heated microbead in partial wetting at the water-air interface. Particular attention is paid to the evolution of the convective flow patterns surrounding the hot sphere as the latter is increasingly heated. The flow morphology is nearly axisymmetric at low laser power P. Increasing P leads to symmetry breaking with the onset of counter-rotating vortex pairs. The boundary condition at the interface, close to no-slip in the low-P regime, turns about stress-free between the vortex pairs in the high-P regime. These observations strongly support the view that surface-active impurities are inevitably adsorbed on the water surface where they form an elastic layer. The onset of vortex pairs is the signature of a hydrodynamic instability in the layer response to the centrifugal forced flow. Interestingly, our study paves the way for the design of active colloids able to achieve high-speed self-propulsion via vortex pair generation at a liquid interface

An optical fiber based interferometer to measure velocity profiles in sheared complex fluids

We describe an optical fiber based interferometer to measure velocity profiles in sheared complex fluids using Dynamic Light Scattering (DLS). After a review of the theoretical problem of DLS under shear, a detailed description of the setup is given. We outline the various experimental difficulties induced by refraction when using a Couette cell. We also show that homodyne DLS is not well suited to measure quantitative velocity profiles in narrow-gap Couette geometries. On the other hand, the heterodyne technique allows us to determine the velocity field inside the gap of a Couette cell. All the technical features of the setup, namely its spatial resolution ($\approx 50$--$100 \mu$m) and its temporal resolution ($\approx 1$ s per point, $\approx 1$ min per profile) are discussed, as well as the calibration procedure with a Newtonian fluid. As briefly shown on oil-in-water emulsions, such a setup permits one to record both velocity profiles and rheological data simultaneouslyComment: 13 pages, 16 figures, Submitted to Eur. Phys. J. A

The flow of a very concentrated slurry in a parallel-plate device: influence of gravity

We investigate, both experimentally and theoretically, the fow and structure of a slurry when sheared between 2 horizontal plates. The slurry, otherwise called a "wet granular material", is made of non-Brownian particles immersed in a viscous fluid. The particles are heavier than the fluid, consequently, gravity influences the structure and flow profiles of the sheared material. Experiments are carried out in a plane Couette device, with a model slurry composed of approximately monodisperse spherical PMMA particles in oil, at high average solid concentration (about 58%). Optical observation reveals a typical 2-phase configuration, with a fluidized layer in contact with the upper plate and on top of an amorphous solid phase. We provide data on velocity profiles, wall-slip and shear stress versus the average shear rate. To interpret the data, we propose a model for the ideal case of infinite horizontal flat plates. The model, of mean field type, is based on local constitutive equations for the tangential and normal components of the stress tensor and on expressions relating the material viscometric coefficients (the shear viscosity eta and the normal viscosity psi) with the local concentration (phi) and the local shear rate. 1-,2- and 3-phase configurations are predicted, with non linear flow and concentration profiles. We conclude that the model equations correctly describe the experimental data, provided that appropriate forms are chosen for the divergence of eta and psi near the packing concentration (phi_max), namely a (phi_max-phi)^-1 singularity.Comment: 26 pages, 12 figures ; submitted to Physics of Fluid

Legitimacy in conflict: concepts, practices, challenges

The study of legitimacy in situations of conflict and peacebuilding has increased in recent years. However, current work on the topic adopts many assumptions, definitions, and understandings from classical legitimacy theory, which centers on the relationship between the nation-state and its citizens. In this introduction, we provide a detailed critical overview of current theories of legitimacy and legitimation and demonstrate why they have only limited applicability in conflict and post-conflict contexts, focusing on the three main areas that the articles included in this special issue examine: audiences for legitimacy, sources of legitimacy, and legitimation. In particular, we show how conflict and post-conflict contexts are marked by the fragmentation and personalization of power; the proliferation and fragmentation of legitimacy audiences; and ambiguity surrounding legitimation strategies

Application de la méthode semi-Hertzienne pour la simulation du passage d'un TGV sur un appareil de voie

Dans la plupart des simulations en dynamique ferroviaire, le profil de rail est supposé constant le long de la voie. Cette hypothèse ne peut plus être retenue si l'on considère des appareils de voie. Les cœurs à pointes mobiles sont des composants d'appareils de voie conçus pour éviter le franchissement d'une lacune (présente dans les appareils classiques) permettant ainsi un roulement le plus continu possible, recherché dans le cas des grandes vitesses. L'objet des simulations est le calcul des contraintes à l'interface roue-rail lors du franchissement d'une pointe mobile par un TGV. Le problème du contact roue-rail est résolu par la méthode dite "semi-Hertzienne" qui autorise des formes d'empreintes plus réalistes que les ellipses de Hertz. Le principe de la méthode est brièvement présenté. Le cas traité permet de montrer l'avantage de la méthode lorsque l'on s'intéresse aux contraintes de contact

Decoupling Local Ownership? The Lost Opportunities for Grassroots Women’s Involvement in Liberian Peacebuilding

Civil society organizations and grassroots groups are often unable to play an active role in post-conflict reconstruction and peacebuilding. A possible explanation for the observed challenges in peacebuilding is the gap or decoupling between international expectations and norms from practical action, local norms and capacities. External actors are often overly instrumental and operate according to a general template that fails to start from what the local capacities might actually be. This often leads to the decoupling of general values from practical action, which helps account for the observed barriers of engaging local civil and community organizations in reconstruction. We examine the different types of decoupling and the challenges these present. We evaluate our general theoretical argument using evidence based on the experiences of Liberian women’s civil society organizations. Given the compliance of the Liberian government with international norms, we should expect external actors to have an easier task in incorporating civil society and women’s organizations in the post-conflict reconstruction process; yet, the record appears to be the opposite. While we present the ‘tragic’ aspect of this relationship between international norms and local practice, we also suggest opportunities for ‘hybrid’ alternatives