111 research outputs found
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
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
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
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
Study of the impacts of droplets deposited from the gas core onto a gas-sheared liquid film
The results of an experimental study on droplet impactions in the flow of a gas-sheared liquid film are presented. In contrast to most similar studies, the impacting droplets were entrained from film surface by the gas stream. The measurements provide film thickness data, resolved in both longitudinal and transverse coordinates and in time together with the images of droplets above the interface and images of gas bubbles entrapped by liquid film. The parameters of impacting droplets were measured together with the local liquid film thickness. Two main scenarios of droplet-film interaction, based on type of film perturbation, are identified; the parameter identifying which scenario occurs is identified as the angle of impingement. At large angles an asymmetric crater appears on film surface; at shallow angles a long, narrow furrow appears. The most significant difference between the two scenarios is related to possible impact outcome: craters may lead to creation secondary droplets, whereas furrows are accompanied by entrapment of gas bubbles into the liquid film. In addition, occurrence of partial survival of impacting droplet is reported
Liquid marbles: topical context within soft matter and recent progress
The study of particle stabilized interfaces has a long history in terms of emulsions, foams and related dry powders. The same underlying interfacial energy principles also allow hydrophobic particles to encapsulate individual droplets into a stable form as individual macroscopic objects, which have recently been called "Liquid Marbles". Here we discuss conceptual similarities to superhydrophobic surfaces, capillary origami, slippery liquids-infused porous surfaces (SLIPS) and Leidenfrost droplets. We provide a review of recent progress on liquid marbles, since our earlier Emerging Area article (Soft Matter, 2011, 7, 5473–5481), and speculate on possible future directions from new liquid-infused liquid marbles to microarray applications. We highlight a range of properties of liquid marbles and describe applications including detecting changes in physical properties (e.g. pH, UV, NIR, temperature), use for gas sensing, synthesis of compounds/composites, blood typing and cell culture
Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing
The Leidenfrost effect occurs when an object near a hot surface vaporizes
rapidly enough to lift itself up and hover. Although well-understood for
liquids and stiff sublimable solids, nothing is known about the effect with
materials whose stiffness lies between these extremes. Here we introduce a new
phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost
effect. By dropping hydrogel spheres onto hot surfaces we find that, rather
than hovering, they energetically bounce several times their diameter for
minutes at a time. With high-speed video during a single impact, we uncover
high-frequency microscopic gap dynamics at the sphere-substrate interface. We
show how these otherwise-hidden agitations constitute work cycles that harvest
mechanical energy from the vapour and sustain the bouncing. Our findings
unleash a powerful and widely applicable strategy for injecting mechanical
energy into soft materials, with relevance to fields ranging from soft robotics
and metamaterials to microfluidics and active matter
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
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