arXiv subjects
Publications and source records attributed to Christophe Clanet.
A propulsion model has been proposed for front crawl swimming (Carmigniani et al., 2020). The aim of this study is to validate this model through a progressive speed test using instrumented paddles to estimate the propulsion forces in swimming.
A liquid meniscus, a bending rod (also called elastica) and a simple pendulum are all described by the same non-dimensional equation. The oscillatory regime of the pendulum corresponds to buckling rods and pendant drops, and the high-velocity regime corresponds to spherical drops, puddles and multiple rod loopings. We study this analogy in a didactic way and discuss how, despite this common governing equation, the three systems are not completely equivalent. We also consider the cylindrical deformations of an inextensible, flexible membrane containing a liquid, which in some sense interpolates between the meniscus and rod conformations.
The skill to swim fast results from the interplay between generating high thrust while minimizing drag. In front crawl, swimmers achieve this goal by adapting their inter-arm coordination according to the race pace. A transition has been observed from a catch-up pattern of coordination (i.e. lag time between the propulsion of the two arms) to a superposition pattern of coordination as the velocity increases. Expert swimmers choose a catch-up coordination pattern at low velocities with a constant relative lag time of glide during the cycle and switch to a maximum propulsion force strategy at higher velocities. This transition is explained using a burst-and-coast model. At low velocities, the choice of coordination can be understood through two parameters: the time of propulsion and the gliding effectiveness. These parameters can characterize a swimmer and help to optimize their technique.
In each rowing sport, the oars have their very own characteristics most of the time selected through a long time experience. Here we address experimentally and theoretically the problem of rowing efficiency as function of row lengths and blades sizes. In contrast with previous studies which consider imposed kinematics, we set an imposed force framework which is closer to human constraints. We find that optimal row lengths and blades sizes depend on sports and athletes strength, and provide an optimisation scheme.
Contrasting with its sluggish behavior on standard solids, water is extremely mobile on superhydrophobic materials, as shown for instance by the continuous acceleration of drops on tilted water-repellent leaves. For much longer substrates, however, drops reach a terminal velocity that results from a balance between weight and friction, allowing us to question the nature of this friction. We report that the relationship between force and terminal velocity is non-linear. This is interpreted by showing that classical sources of friction are minimized, so that the aerodynamical resistance to motion becomes dominant, which eventually explains the matchless mobility of water. Our results are finally extended to viscous liquids, also known to be unusually quick on these materials.
Although a hydrophobic microtexture at a solid surface most often reflects rain owing to the presence of entrapped air within the texture, it is much more challenging to repel hot water. As it contacts a colder material, hot water generates condensation within the cavities at the solid surface, which eventually builds bridges between the substrate and the water, and thus destroys repellency. Here we show that both "small" (~100 nm) and "large" (~10 μm) model features do reflect hot drops at any drop temperature and in the whole range of explored impact velocities. Hence, we can define two structural recipes for repelling hot water: drops on nanometric features hardly stick owing to the miniaturization of water bridges, whereas kinetics of condensation in large features is too slow to connect the liquid to the solid at impact.
Empirical data reveals a broad variety of hull shapes among the different ship categories. We present a minimal theoretical approach to address the problem of ship hull optimisation. We show that optimal hull aspect ratios result -- at given load and propulsive power -- from a subtle balance between wave drag, pressure drag and skin friction. Slender hulls are more favourable in terms of wave drag and pressure drag, while bulky hulls have a smaller wetted surface for a given immersed volume, by that reducing skin friction. We confront our theoretical results to real data and discuss discrepancies in the light of hull designer constraints, such as stability or manoeuvrability.
Rigid superhydrophobic materials have the ability to repel millimetric water drops, in typically 10 ms. Yet, most natural water-repellent materials can be deformed by impacting drops. To test the effect of deformability, we perform impacts of non-wetting drops onto thin ($\sim$ 10 \micro m), circular PDMS membranes. The bouncing mechanism is markedly modified compared to that on a rigid material: the liquid leaves the substrate as it is kicked upwards by the membrane. We show that the rebound is controlled by an interplay between the dynamics of the drop and that of the soft substrate, so that we can continuously vary the contact time by playing on the membrane's characteristics and reduce it up to 70\%.
We investigate the wicking in granular media by considering layers of grains at the surface of a liquid, and discuss the critical contact angle below which spontaneous impregnation takes place. This angle is found to be on the order of 55° for monodisperse layers, significantly smaller than 90°, the threshold value for penetrating assemblies of tubes: owing to geometry, impregnating grains is more demanding than impregnating tubes. We also consider the additional effects of polydispersity and pressure on this wetting transition and discuss the corresponding shift observed on the critical contact angle.
Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at room temperature. In this article, we first study the deformation of oxygen drops in a magnetic field and show that it can be described via an effective capillary length, which includes the magnetic force. In a second part, we describe how these ultra-mobile drops passing above a magnet significantly slow down and can even be trapped. The critical velocity below which a drop is captured is determined from the deformation induced by the field.
We present a fluid dynamics video on cavities created by explosions of firecrackers at the water free surface. We use three types of firecrackers containing 1, 1.3 and 5 g of flash powder. The firecrackers are held with their center at the surface of water in a cubic meter pool. The movies are recorded from the side with a high-speed video camera. Without confinement the explosion produces an hemispherical cavity. Right after the explosion this cavity grows isotropically, the bottom then stops while the sides continue to expand. In the next phase the bottom of the cavity accelerates backwards to the surface. During this phase the convergence of the flow creates a central jet that rises above the free surface. In the last part of the video the explosion is confined in a vertical open tube made of glass and of centimetric diameter. The explosion creates a cylindrical cavity that develops towards the free end of the tube. Depending on the charge, the cavity can either stop inside the tube or at its exit, but never escapes.
We present a fluid dynamics video showing the behavior of drops of liquid oxygen, at room temperature. Due to their low boiling point, these drops levitate on a cushion of their own vapour. This property gives them a high mobility, as known more generally in such Leidenfrost situations. But liquid oxygen is also paramagnetic, and thus likely to be manipulated using a magnet. It is first shown that the shape of the drop can be modified by changing the drop/magnet distance; approaching the magnet acts as reinforcing gravity, so that the drops get flattened by this action. The transformation is of course reversible: as the magnet is withdrawn, the liquid recovers its quasi-spherical shape. Magnets can also be used to trap the oxygen drops. As they pass above a magnet, they slow down significantly, a consequence of their deformation: despite a very low friction, the vibrations induced by the drop deformations represent an important source of dissipation: below a well-defined velocity, drops can even be stopped in the magnetic trap. Quicker ones can be decelerated and then captured by series of magnets. Oberving the same events from the top reveals the complex shapes adopted by the liquid as it crosses the traps.
A 'splash' is usually heard when a solid body enters water at large velocity. This phenomena originates from the formation of an air cavity resulting from the complex transient dynamics of the free interface during the impact. The classical picture of impacts on free surfaces relies solely on fluid inertia, arguing that surface properties and viscous effects are negligible at sufficiently large velocities. In strong contrast to this large-scale hydrodynamic viewpoint, we demonstrate in this study that the wettability of the impacting body is a key factor in determining the degree of splashing. This unexpected result is illustrated in Fig.1: a large cavity is evident for an impacting hydrophobic sphere (1.b), contrasting with the hydrophilic sphere's impact under the very same conditions (1.a). This unforeseen fact is furthermore embodied in the dependence of the threshold velocity for air entrainment on the contact angle of the impacting body, as well as on the ratio between the surface tension and fluid viscosity, thereby defining a critical capillary velocity. As a paradigm, we show that superhydrophobic impacters make a big 'splash' for any impact velocity. This novel understanding provides a new perspective for impacts on free surfaces, and reveals that modifications of the detailed nature of the surface -- involving physico-chemical aspects at the nanometric scales -- provide an efficient and versatile strategy for controlling the water entry of solid bodies at high velocity.
We report gravity oscillations of a liquid column partially immersed in a bath of liquid. We stress in particular some peculiarities of this system, namely (i) the fact that the mass of this oscillator constantly changes with time; (ii) the singular character of the beginning of the rise, for which the mass of the oscillator is zero; (iii) the sources of dissipation in this system, which is found to be dominated at low viscosity by the entrance (or exit) effects, leading to a long-range damping of the oscillations. We conclude with some qualitative description of a second-order phenomenon, namely the eruption of a jet at the beginning of the rise.