Supershear Rayleigh waves at a soft interface

We report on the experimental observation of waves at a liquid foam surface propagating faster than the bulk shear waves. The existence of such waves has long been debated, but the recent observation of supershear events in a geophysical context has inspired us to search for their existence in a model viscoelastic system. An optimized fast profilometry technique allowed us to observe on a liquid foam surface the waves triggered by the impact of a projectile. At high impact velocity, we show that the expected subshear Rayleigh waves are accompanied by faster surface waves that can be identified as supershear Rayleigh waves.Comment: 4 pages, 4 figures, 2 supplementary video

Active Learning reduces academic risk of students with non-formal reasoning skills. Evidence from an introductory physics massive course in a Chilean public university

We present the findings of a pilot plan of active learning implemented in introductory physics in a Chilean public university. The model is research based as it considered a literature review for adequate selection and design of activities, consistent with the levels of students' reasoning skills. The level of scientific reasoning is positively correlated to student success. By contrast to a control group of students following traditional lectures, we observed a significant reduction in failure rate for students that do not yet posses formal scientific reasoning. This profile of student being the majority, we conclude that implementing active learning is particularly suited to first year of higher education in the context of a developing country. It fits the particularities of student profile and typical classroom size, leading to learning improvement and reduction of academic risk as well as being financially sound

Remote control of self-assembled microswimmers

Physics governing the locomotion of microorganisms and other microsystems is dominated by viscous damping. An effective swimming strategy involves the non-reciprocal and periodic deformations of the considered body. Here, we show that a magnetocapillary-driven self-assembly, composed of three soft ferromagnetic beads, is able to swim along a liquid-air interface when powered by an external magnetic field. More importantly, we demonstrate that trajectories can be fully controlled, opening ways to explore low Reynolds number swimming. This magnetocapillary system spontaneously forms by self-assembly, allowing miniaturization and other possible applications such as cargo transport or solvent flows.Comment: 5 pages, 5 figures articl

Statics and dynamics of magnetocapillary bonds

When ferromagnetic particles are suspended at an interface under magnetic fields, dipole-dipole interactions compete with capillary attraction. This combination of forces has recently given promising results towards controllable self-assemblies, as well as low Reynolds swimming systems. The elementary unit of these assemblies is a pair of particles. Although equilibrium properties of this interaction are well described, dynamics remain unclear. In this letter, the properties of magnetocapillary bonds are determined by probing them with magnetic perturbations. Two deformation modes are evidenced and discussed. These modes exhibit resonances whose frequencies can be detuned to generate non-reciprocal motion. A model is proposed which can become the basis for elaborate collective behaviours

Magnetocapillary self-assemblies: Swimming and micromanipulation

Floating magnetic particles can self-assemble into structures, by a combination of a magnetic dipole-dipole interaction and an attraction due to the interfacial deformation. These structures are periodically deformed in a non reciprocal way using magnetic fields, which leads to controllable low Reynolds number locomotion. Such microswimmers provide a basis for micromanipulation applications such as transport of micro-objects, local mixing of fluids or surface cleaning

Complex Magnetocapillary Microswimmers

When particles are suspended at air-water interfaces in the presence of a vertical magnetic field, dipole-dipole repulsion competes with capillary attraction. Magnetocapillary self-assemblies, composed of N soft-ferromagnetic beads, have been studied as a way to produce controllable micro swimmers at low Reynolds numbers. We hereby investigate the different dynamical behaviors obtained for various self-assemblies. The dynamic interaction of a pair of particles (N = 2) generates a rich behavior at the origin of a non-reciprocal motion, being the major physical ingredient for low Reynolds locomotion. By adding particles to this elementary system up to N = 8, we create new symmetries relevant to generate translational and rotational motions. We propose a model for describing the motion driven by an external field, being the basis for developing elaborated collective behaviors

Self-assembled controllable microswimmers

Because they cause a deformation of the interface, floating particles interact. In particular, identical particles attract each other. To counter this attraction, particles possessing a large magnetic moment m are used. When m is perpendicular to the surface, dipole-dipole interaction is repulsive. This competition of forces can lead to the spontaneous formation of organized structures. By using submillimetric steel spheres for which m is proportional to B, we can precisely tune the interdistances in the system. Here, we deform these self-assemblies by adding an oscillating horizontal field Bx which induces a horizontal contribution mx to the magnetic moment. Time reversal symmetry is broken in the system, leading to locomotion at low Reynolds number. Indeed, a body undergoing non-reciprocal deformations, i.e. breaking time reversal symmetry, is known to be able to swim in viscous regime. Moreover, swimming direction depends on the orientation of field Bx, meaning that swimming trajectories can be finely controlled [8] as shown in figure 2. A simple model allows to understand the breaking of symmetry in a three-particle system. A study of the vibration modes in a pair of particle gives further informations on the dynamics, being the basis for larger assemblies. Because this magnetocapillary system spontaneously forms by self-assembly, it allows miniaturization and various possible applications such as cargo transport or solvent ows. It is highly versatile, being composed of simple passive particles and controlled by magnetic fields of the order of 1 mT

Remote control of self-assembled magnetocapillary microswimmers

Physics governing the locomotion of microorganisms and other microsystems is dominated by viscous damping. An effective swimming strategy involves the non-reciprocal and periodic deformations of the considered body. Herein, we show that a magnetocapillary-driven self-assembly, composed of three soft-ferromagnetic beads, is able to swim along a liquid-air interface when driven by an external magnetic field. Moreover, the system can be fully controled, opening ways to explore low Reynolds number swimming and to create micromanipulators in various applications

Interfaces à grains, et autres situations de mouillage nul

Intercaler une phase intermédiaire (gazeuse ou granulaire) entre une goutte et une surface crée une situation de mouillage nul. Nous étudions les conséquences de la présence de cette phase sur le mouvement des gouttes d eau. Dans une première partie, nous nous intéressons à des situations de non-mouillage induites par la présence d un film gazeux entre le liquide et la surface. Une conséquence de la présence de ce film est la mise en mouvement d une goutte en caléfaction sur une surface chaude de profil périodique et asymétrique (en forme de toit d usine). Nous mettons en évidence que l écoulement asymétrique du gaz dans le film de vapeur entre la goutte et la surface provoque la propulsion de la goutte par effet fusée. À l inverse, la faible épaisseur du film isolant peut induire une dissipation visqueuse supplémentaire que l on observe par exemple lors du rebond d une bulle de savon sur un liquide. La deuxième partie de ce travail est consacrée à une autre situation de non-mouillage réalisée par l ajout de grains à la surface d une goutte (appelée goutte enrobée). On forme alors une interface composite, à la fois capillaire et granulaire qui peut présenter un caractère fortement dissipatif : lorsque la densité de grains est faible, son comportement est celui du liquide qui la compose mais lorsque la densité de grains à la surface approche des densités du jamming, caractère granulaire de l interface l emporte. Le frottement entre grains détermine alors la forme ainsi que la dynamique des gouttes enrobées. Nous montrons que la rhéologie de ces interfaces est alors similaire à celle des écoulements granulaires densesPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Armoring a droplet: Soft jamming of a dense granular interface

Droplets and bubbles protected by armors of particles have found vast applications in encapsulation, stabilization of emulsions and foams, or flotation processes. The liquid phase stores capillary energy, while concurrently the solid contacts of the granular network induce friction and energy dissipation, leading to hybrid interfaces of combined properties. By means of nonintrusive tensiometric methods and structural measurements, we distinguish three surface phases of increasing rigidity during the evaporation of armored droplets. The emergence of surface rigidity is reminiscent of jamming of granular matter, but it occurs differently since it is marked by a step by step hardening under surface compression. These results show that the concept of the effective surface tension remains useful only below the first jamming transition. Beyond this point, the surface stresses become anisotropic