Drop Formation in Non-Newtonian Fluids

International audienceWe study the pinch-off dynamics of droplets of yield stress and shear thinning fluids. To separate the two non-Newtonian effects, we use a yield stress material for which the yield stress can be tuned without changing the shear thinning behavior, and a shear thinning system (without a yield stress) for which the shear thinning can be controlled over a large range, without introducing too much elasticity into the system. We find that the pinch-off remains very similar to that of constant viscosity Newtonian liquids, and consequently thinning in shear flow does not imply a thinning in elongational flow

Width distribution of contact lines on a disordered substrate

We have studied the roughness of a contact line of a liquid meniscus on a disordered substrate by measuring its width distribution. The comparison between the measured width distribution and the width distribution calculated in previous works, extended here to the case of open boundary conditions, confirms that the Joanny-de Gennes model is not sufficient to describe the dynamics of contact lines at the depinning threshold. This conclusion is in agreement with recent measurements which determine the roughness exponent by extrapolation to large system sizes.Comment: 4 pages, 3 figure

Rugosité et dynamique d'une ligne de contact sur un substrat désordonné

We have studied the roughness and the dynamics of the contact line of a viscous liquid on a disordered substrate. With a controlled disorder of typical scale 10 µm, the roughness of the contact line depends neither on the viscosity nor on the velocity of the line. The roughness exponent zeta is found equal to 0.52±0.04, this value desagrees the theoritical prediction (zeta = 0.39). Thus, we have checked the assumptions of the model. A precise study of the motion of the contact line confirms the assumption of an overdamped motion. On the other hand, the dissipation is not due to the viscosity. A realistic simulation shows that the non linearities of the dissipation forces do not explain the unexpected value of zeta. Both experiment and simulation show that the dynamics close to the depinning threshold is mainly controlled by the dissipation at microscopic scale, that is not universal.Nous avons étudié la rugosité et la dynamique de la ligne de contact d'un liquide visqueux sur un substrat désordonné. Sur des substrats présentant un désordre contrôlé avec une échelle caractéristique de 10 µm, nous avons trouvé que la rugosité de la ligne de contact ne dépend ni de la vitesse de la ligne, ni de la viscosité du liquide. L'exposant de rugosité zeta mesuré vaut 0,52±0,04, en désaccord avec les prédictions théoriques (zeta = 0,39). Nous avons donc testé les hypothèses du modèle. Une étude précise du mouvement de la ligne confirme l'hypothèse d'un mouvement quasi statique. En revanche, la dissipation n'est pas de type visqueux. Une simulation réaliste du problème montre que la non-linéarité en vitesse de la dissipation n'expliquent pas la valeur mesurée de zeta. Expérience et simulation montrent que la dynamique au voisinage du seuil de dépiégeage est contrôlée essentiellement par la dissipation à l'échelle microscopique, qui n'est pas universelle

Experimental Evidence of Thermal-Like Behavior in Dense Granular Suspensions

International audienceWe experimentally investigate the statistical behavior of a model two-dimensional granular system undergoing stationary sedimentation. Buoyant cylindrical particles are rotated in a liquid-filled drum, thus confined in a harmonic centripetal potential with tunable curvature, which competes with gravity to produce various stationary states: though heterogeneous, the packing fraction of the system can be tuned from fully dispersed to crystallized as the rotation rate is increased. We show that this dynamical system is in mechanical equilibrium in the confining potential and exhibits a thermal-like behavior, where the granular pressure and the packing fraction are related through an equation of state. We obtain an expression of the equation of state allowing us to probe the nature of the hydrodynamic interactions between the particles. This description is valid in the whole range of the physical parameters we investigated and reveals a buoyant energy scale that we interpret as an effective temperature. We finally discuss the behavior of our system at high packing fractions and the relevance of the equation of state to the liquid-solid phase transition

Structure and mechanics of aegagropilae fiber network

International audienceFiber networks encompass a wide range of natural and man-made materials. The threads or filaments from which they are formed span a wide range of length scales: from nanometers, as in biological tissues and bundles of carbon nanotubes, to millimeters , as in paper and insulation materials. The mechanical and thermal behavior of these complex structures depends on both the individual response of the constituent fibers and the density and degree of entanglement of the network. A question of paramount importance is how to control the formation of a given fiber network to optimize a desired function. The study of fiber clustering of natural flocs could be useful for improving fabrication processes, such as in the paper and textile industries. Here, we use the example of aegagropilae that are the remains of a seagrass (Posidonia oceanica) found on Mediterranean beaches. First, we characterize different aspects of their structure and mechanical response, and second, we draw conclusions on their formation process. We show that these natural aggregates are formed in open sea by random aggregation and compaction of fibers held together by friction forces. Although formed in a natural environment, thus under relatively unconstrained conditions, the geometrical and mechanical properties of the resulting fiber aggregates are quite robust. This study opens perspectives for manufacturing complex fiber network materials. fiber network | fiber aggregation | Posidonia oceanica | fibrous material A egagropilae are a natural aggregate of fibers produced by the decomposition of leaves and roots of Posidonia ocean-ica. The fibers are entangled by sea motion until the clusters reach the shore (1) (Fig. 1). P. oceanica is an endemic plant of the Mediterranean Sea with very long and thin leaves (about 1-m long, 1-cm wide, and less than 1-mm thick). The name aega-gropilae originates from the Greek [α´ιγαγρoςα´ιγαγρoς (wild goat) and π˜ιλoςπ˜ιλoς (fur)] and refers to the resemblance between the shape of these balls and those regurgitated by goats. Natural fiber clustering occurs for different species of aquatic plants, such as the so-called seaballs that can be found on the Atlantic Ocean and lake shores (2, 3). P. oceanica meadows play an important ecological role in the preservation of Mediterranean coasts. They constitute plant barriers that promote sediment trapping and oxygen production in seabed, and the accumulated remains protect the beaches from erosion. Moreover, aegagropilae fibers are suitable materials for insulation in construction and automotive industries. After submitting them to various tests, they recently landed in the marketplace under the name of Neptutherm. Beyond the curiosity that these natural objects can provoke, aegagropilae samples found on beaches raise several fundamental questions. The first set of questions is on their formation process. How can fibers be entangled and packed by a flow without any confinement? How long does it take to form a cluster? Can one explain the size distribution of these aggregates? The second set of questions is on the cohesion of these structures. How can we relate the apparent stiffness of these balls to the interaction of the fibers and the topological properties of the network? Natural and manmade fiber networks are abundant structures and arise on a wide range of length scales. Examples may be found in both biological systems, such as the cytoskeleton of a cell (4), blood clots (5), and biological tissues (6), and technology , such as nanotube bundles (7), paper (8), textiles, and felts (9–11). The mechanical response of fiber assemblies depends on both the properties of the elementary thread and the density , connectivity, and ordering of the network. Therefore, the functional properties of these materials can be tuned by controlling their formation processes. The manufacturing of some athermal networks, such as paper, involves transport of fibers in a fluid flow (8). During this process, fibers are advected and deformed elastically, and they interact through interfiber friction (12, 13). From this perspective, aegagropilae are an archetype of these fiber networks, and understanding the clustering mechanism can shed light on the fundamental aspects of fiber aggre-gation dynamics. Here, we perform various measurements on aegagropilae to characterize their structural and mechanical properties. Our observations provide a qualitative understanding of the formation process of this fiber network. Structural Properties of Aegagropilae Aegagropilae were collected at two different locations of the Mediterranean shore: at Six Fours, France (43 • 06 03 N, 5 • 49 20 E) and on Porquerolles Island, France (43 • 00 02 N, 6 • 13 38 E) with the authorization of Port-Cros National Park. On the first beach, nearly 2,000 samples were collected to determine their size and mass distributions. To avoid any bias in the sampling caused by damage of aegagropilae by human activity, all samples were collected on a morning in winter, the day after a storm. Moreover, only balls found at a distance less than 10 m from the sea were collected, assuming that the ones located farther away from the sea might have been released by a precedent storm. At the second spot, few samples were picked up in seabed to investigate the geometrical and mechanical properties of fiber Significance Aegagropilae are centimeter-sized, nearly spherical fiber aggregate found on Mediterranean beaches. They result from aggregation and compaction of the remains of the seagrass Posidonia oceanica. We find that they possess remarkable mechanical properties, which are unexpected for a structure formed by random agitation caused by sea motion and held together solely by friction between the fibers. The study of such material offers perspectives for complex materials, such as felts and insulation materials

Drop Formation in Non-Newtonian Fluids

We study the pinch-off dynamics of droplets of yield stress and shear thinning fluids. To separate the two non-Newtonian effects, we use a yield stress material for which the yield stress can be tuned without changing the shear thinning behavior, and a shear thinning system (without a yield stress) for which the shear thinning can be controlled over a large range, without introducing too much elasticity into the system. We find that the pinch-off remains very similar to that of constant viscosity Newtonian liquids, and consequently thinning in shear flow does not imply a thinning in elongational flow

The effects of repetitive neck-muscle vibration on postural disturbances after a chronic stroke

International audienceObjective. - We aimed to test a repeated program of vibration sessions of the neck muscles (rNMV) on postural disturbances and spatial perception in patients with right (RBD) versus left (LBD) vascular brain damage. Methods. -Thirty-two chronic stroke patients (mean age 60.9 +/- 10 yrs and mean time since stroke 4.9 +/- 4 yrs), 16 RBD and 16 LBD, underwent a program of 10 sessions of NMV over two weeks. Posturography parameters (weight-bearing asymmetry (WBA), Xm, Ym, and surface), balance rating (Berg Balance Scale (BBS), Timed Up and Go (TUG)), space representation (subjective straight ahead (SSA), longitudinal body axis (LBA), subjective visual vertical (SW)), and post-stroke deficiencies (motricity index, sensitivity, and spasticity) were tested and the data analyzed by ANOVA or a linear rank-based model, depending on whether the data were normally distributed, with lesion side and time factor (D-15, D0, D15, D21, D45). Results. -The ANOVA revealed a significant interaction between lesion side and time for WBA (P< 0.0001) with a significant shift towards the paretic lower limb in the RBD patients only (P=0.0001), whereas there was no effect in the LBD patients (P=0.98). Neither group showed a significant modification of spatial representation. Nonetheless, there was a significant improvement in motricity (P=0.02), TUG (P=0.0005), and BBS (P< 0.0001) in both groups at the end of treatment and afterwards. Conclusions. - rNMV appeared to correct WBA in RBD patients only. This suggests that rNMV could be effective in treating sustainable imbalance due to spatial cognition disorders. (C) 2020 Elsevier Masson SAS. All rights reserved