Holes and cracks in rigid foam films

The classical problem of foam film rupture dynamics has been investigated when surfaces exhibit very high rigidity due to the presence of specific surfactants. Two new features are reported. First a strong deviation to the well-known Taylor-Culick law is observed. Then, crack-like patterns can be visualized in the film; these patterns are shown to appear at a well defined deformation. The key role of surface active material on these features is quantitatively investigated, pointing the importance of surface elasticity to describe these fast dynamical processes, and thus providing an alternative tool to characterize surface elasticity in conditions extremely far from equilibrium. The origin of the cracks and their consequences on film rupturing dynamics are also discussed

Yield stress and elasticity influence on surface tension measurements

We have performed surface tension measurements on carbopol gels of different concentrations and yield stresses. Our setup, based on the force exerted by a capillary bridge on two parallel plates, allows to measure an effective surface tension of the complex fluid and to investigate the influence of flow history. More precisely the effective surface tension measured after stretching the bridge is always higher than after compressing it. The difference between the two values is due to the existence of a yield stress in the fluid. The experimental observations are successfully reproduced with a simple elasto-plastic model. The shape of successive stretching-compression cycles can be described by taking into account the yield stress and the elasticity of the gel. We show that the surface tension $\gamma_{LV}$ of yield stress fluids is the mean of the effective surface tension values only if the elastic modulus is high compared to the yield stress. This work highlights that thermodynamical quantities measurements are challenged by the fluid out-of-equilibrium state implied by jamming, even at small scales where the shape of the bridge is driven by surface energy. Therefore setups allowing deformation in opposite directions are relevant for measurements on yield stress fluids.Comment: 12 pages, 16 figures in Soft Matter 201

Dissipation aux interfaces : caléfaction, sillages, filaments visqueux

We investigate in several experiments the link between dissipation and deformable liquid interfaces. First, we study Leidenfrost drops moving on various substrates. Their movement can be either accelerated — on a ratchet-like substrate — or slowed down. The deceleration allows us to study different types of friction. On a smooth solid, the drag is very low and mainly due to the surrounding air. On a crenellated surface, it is a consequence of the liquid impacting the side of the squares. Last, a drop sliding on another liquid decelerates because of wave resistance arising from the wake it generates. In a second part, we characterize viscous threads deformations. Compression and torsion of the filaments result into viscous buckling, which we compare to elastic buckling. However, if the thread is not forced to move, the evolution of its shape comes out of a balance between viscous, gravity and capillary effects.Au travers de plusieurs expériences, nous explorons le lien entre dissipation et mouvement d'interfaces liquides. Dans une première partie, nous étudions le mouvement de gouttes en caléfaction sur différents substrats. Ce mouvement peut être accéléré, si le substrat est asymétrique, ou ralenti, s'il est symétrique. La décélération des gouttes nous permet alors d'étudier différents types de friction spéciale. Sur un solide lisse, celle-ci est très faible et essentiellement due à l'air environnant. Sur un solide crénelé, elle est due à l'impact du liquide sur les marches. Enfin, si la goutte glisse sur un autre liquide, elle ralentit à cause de la résistance de vague qui résulte du sillage qu'elle engendre. La seconde partie de ce travail porte sur les déformations de filaments visqueux. Si ceux-ci sont comprimés ou tordus, nous mettons en évidence plusieurs phénomènes de flambage visqueux, que nous comparons à leurs analogues élastiques. Enfin, si les filaments sont libres de se déformer, leur forme et leur dynamique découlent d'une compétition entre viscosité, gravité et effets capillaires

Buckling of Viscous Filaments of a Fluid under Compression Stresses

International audienceWe study the compression of viscous filaments at constant velocity. If slender enough, the filament bends, a viscous analogue of Euler elastic buckling. We measure the characteristic time of this viscous buckling and discuss the link with the elastic critical compression. We show that the analogy only holds in the limit of large capillary numbers. Otherwise capillarity has a stabilizing effect, which suppresses buckling

SONDe: Contrôle de densité auto-organisante de fonctions réseaux pair à pair

http://algotel2006.lip6.fr/Longtemps dominés par les systèmes de partage de fichiers, les systèmes pair à pair s'ouvrent désormais à un large éventail d'applications telles que l'email, le DNS, la téléphonie, ou les caches répartis. Le bon fonctionnement de ces applications passe par l'utilisation de fonctions de base dont l'accès peut devenir un goulet d'étranglement si elle ne sont pas suffisamment répliquées dans le système. Dans cet article nous présentons SONDe, un algorithme qui permet une réplication automatique et adaptative de ces fonctions dans un réseau à très large échelle. Cet algorithme permet également de borner le nombre de sauts réseaux à effectuer entre un pair et une fonction, rendant ainsi prévisibles et paramétrables les latences attendues. Ceci est rendu possible grâce à une simple prise de décision basée sur l'étude du voisinage de chaque pair du réseau

Trapping Leidenfrost Drops with Crenelations

International audienceDrops placed on very hot solids levitate on a cushion of their own vapor, as discovered by Leidenfrost. This confers to these drops a remarkable mobility, which makes problematic their control and manipulation. Here we show how crenelated surfaces can be used to increase the friction of Leidenfrost drops by a factor on the order of 100, making them decelerate and be trapped on centimetric distances instead of the usual metric ones. We measure and characterize the friction force as a function of the design of the crenelations

Wall slip regimes in jammed suspensions of soft microgels

International audienceWe characterize microfluidic flows of jammed suspensions of soft microgels (Carbopol) behaving as yield-stress fluids. We quantify the wall slip friction, i.e. the slip velocity V versus the tangential stress at the wall σw. We demonstrate a transition in slip regimes, from a non-linear behavior (V ∝ σ 2 w) to a linear one, as the stress at the wall is increased, as expected from scaling arguments. Using fluorescent imaging to characterize the microgel size, we rationalize the two friction regimes for various samples by estimating viscous and elastic forces at the scale of the microgel particle. Only local arguments are thus necessary to predict wall slip friction, in contrast to other complex flow features such as fluidity or shear banding where bulk and surface properties appear to be strongly related

Evaluating topology quality through random walks

A distributed system or network can be modeled as a graph representing the "who knows who" relationship. The conductance of a graph expresses the quality of the connectivity. In a network composed of large dense clusters, connected through only a few links, the risk of partitioning is high; this is typically reflected by a low conductance of the graph. Computing the conductance of a graph is a complex and cumbersome task. Basically, it requires the full knowledge of the graph and is prohibitively expensive computation-wise. Beyond the information carried by the conductance of a graph, what really matters is to identify critical nodes from the topology point of view. In this paper we propose a fully decentralized algorithm to provide each node with a value reflecting its connectivity quality. Comparing these values between nodes, enables to have a local approximation of a global characteristic of the graph. Our algorithm relies on an anonymous probe visiting the network in a unbiased random fashion. Each node records the time elapsed between visits of the probe (called return time in the sequel). Computing the standard deviation of such return times enables to give an information to all system nodes, information that may be used by those nodes to assess their relative position, and therefore the fact that they are critical, in a graph exhibiting low conductance. Based on this information, graph improvement algorithms may be triggered. Moments of order 1 and 2 of the return times are evaluated analytically using a Markov chain model, showing that standard deviation of return time is related to the position of nodes in the graph. We evaluated our algorithm through simulations. Results show that our algorithm is able give informations that are correlated to the conductance of the graph. For example we were able to precisely detect bridges in a network composed of two dense clusters connected through a single link