“An experiment is a question which we ask of Nature, who is always ready to give a correct answer, provided we ask properly, that is, provided we arrange a proper experiment."
Charles Vernon Boys, 1896
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Birds such as cape gannets exhibit a remarkable fishing strategy: they dive head-first in the water. Before hitting the water, they streamline their body shape to minimize impact force. To better understand the force generation during impact, we study the different aspects of the water entry of a two-dimensional wedge. We focus specifically on the time-dependent impact force during entry (e.g., the existence of a maximum before total immersion of a wedge), but also on the the shape(s) of the splash, which proved to be strongly influenced by aerodynamics.
paper coming soon!
In the botanical world like in the animal world, survival depends on the ability to outperform competitors. In particular, seed dispersal rely on the flight performance of seedpods, the shape of which have striking similarities despite the variety of size and flight modes. We explore experimentally the fitness of auto-rotating flyers of various shapes, flexibility, and layout in a carefully controlled drop setup. High-speed camera and in-house post-processing script are used to record all time-dependent parameters, such as velocity, and tumbling rate. In parallel, we conduct wind tunnel experiments to quantify the aerodynamic differences. Ultimately, we seek to find design rules for optimal flight performance, that will likely reach beyond our initial scope.
paper coming soon!
Some organisms such as fish, seals and copepods are known to exhibit rheotaxis, that is a reaction to hydrodynamic changes in their environment. Bacterial rheotaxis is on the other hand not well understood -yet it has the potential to shed light on many natural mechanisms. V. fischeri is a bioluminscent marine bacteria recruited in a very selective process by the juvenie bobtail squid in order to help him hide on the seafloor. To test wether rheotaxis is part of the process, we probe the reaction of V. fischeri to simple, controlled flows in a microfluidics setup.
Fish exhibit remarkable abilities to respond to minute water motions. Among several sensory modalities, the fish lateral line sensory system is thought to play a major role in detecting hydrodynamic signals. Here, we propose an experimental model, consisting of two pressure sensors mounted in parallel or in tandem and placed in the wake of a cylinder towed at various speeds (i.e., Reynolds numbers) in an otherwise still fluid. Our goal is to reconstruct the salient flow features from local pressure measurements.
Faraday waves are parametrically-excited waves spontaneously occurring at the free-surface of a pool of liquid undergoing vertical vibrations. Rather than using a large pool of liquid, we characterize the properties of these wave in a Hele-Shaw cell, where the cell thickness is smaller than any other dimension in the setup. The waves' periodic motion excite a complex mixing flow both above and below the free-surface. We investigate its dependance on the excitation parameters and explore the possibility to use these findings in engineering applications.
The free fall of heavy bodies in a viscous fluid medium is a problem of interest to many engineering and scientific disciplines, including unpowered flight, seed dispersal, and sedimentation. Axisymmetric bodies such as coins may be one of the simplest body one can thinks of --yet they exhibit a large variety of behaviors: disks have previously been found to fall according to 4 different modes: straight, fluttering, tumbling, and chaotic (a combination of tumbling and chaotic). In our lab, we investigate how simple geometric changes, such as a hole in the center, trigger transition from one descent mode to another. We also investigate how the falling dynamics is affected by the medium stratification, thus extending the relevance of the results to naturally stratified environments such as the Earth atmosphere.
Pele's hair (see here), named after the Hawaiian goddess of volcanoes, are long solid filamentary structures found near volcanoes. Their formation is believed to occur as followed : explosions into flowing lava generate locally high stretching rates that deform bridges of lava (between bigger blobs) into long and thin ligaments, able to solidify before breaking into droplets. If we understand how Rayleigh-Plateau instability is affected by viscosity alone, the role of stretching (combined or not with viscosity) is, in contrast, poorly understood. In our experiment, a liquid ligament is created by accelerating axially a solid millimetric cylinder (called rod) initially supporting a drop, or the top end of a liquid bridge. The fluid volume deforms into a long needle-like ligament, that eventually disconnect from the top rod and atomize (shortly after). We described the forces in play during stretching, and gave a prediction of the breakup time (which unexpectedly turned out to be independent of surface tension), as well as the breakup length.
We investigated, numerically and theoretically, the dynamics of a liquid bridge held between two facing coaxial cylinders (rods), while moving one of the rod with various types of displacement. The numerical part is based on a code making use of one-dimensionnal equations (known as "viscous slice model") that prove to be remarkably accurate for such free-surface problems. The theoretical part relied on a weakly non-linear analysis of the same set of equations. We explained how the static shapes of the bridge (Delaunay curves) are corrected by viscosity and the forcing of the right rod. For large stretching or viscosity values, inducing large deformations of the bridge, one can expect the bridge to break while being stable according to the Rayleigh-Plateau criteria.
A viscous fluid layer is deposited upon a fast-rotating cylinder. For it is subject to high radial acceleration, the fluid layer destabilize according to Rayleigh-Taylor instability, giving birth, under a set of particular conditions, to nice elongated ligamentary structures ended by a head drop (coming from the capillar retraction of the tip of the filament). The long lifetime of this unstable ligaments are favoured by both viscosity and elongation process that hinder the growth of perturbation by Rayleigh-Plateau mechanism. While the head drop is close to ballistic motion, the body of the ligament is likely to be slowed down by the surrounding air, giving rise to this slight "S" shape. The head drop eventually detaches from the ligament, leading, some moment later, to the atomization of the liquid filament.