Magnetocapillary self-assemblies: locomotion and micromanipulation along a liquid interface

This paper presents an overview and discussion of magnetocapillary self-assemblies. New results are presented, in particular concerning the possible development of future applications. These self-organizing structures possess the notable ability to move along an interface when powered by an oscillatory, uniform magnetic field. The system is constructed as follows. Soft magnetic particles are placed on a liquid interface, and submitted to a magnetic induction field. An attractive force due to the curvature of the interface around the particles competes with an interaction between magnetic dipoles. Ordered structures can spontaneously emerge from these conditions. Furthermore, time-dependent magnetic fields can produce a wide range of dynamic behaviours, including non-time-reversible deformation sequences that produce translational motion at low Reynolds number. In other words, due to a spontaneous breaking of time-reversal symmetry, the assembly can turn into a surface microswimmer. Trajectories have been shown to be precisely controllable. As a consequence, this system offers a way to produce microrobots able to perform different tasks. This is illustrated in this paper by the capture, transport and release of a floating cargo, and the controlled mixing of fluids at low Reynolds number.Comment: 10 pages, 8 figures review pape

Commodities in a global vision for agriculture towards 2010

It is impossible to see a hundred years into the future. So, how can we imagine what will become of commodity crops in Malaysia by 2100? Yet, worldwide, these commodity crops provide a living for hundreds of millions of people: how they evolve will have considerable economic, social and environmental consequences for many countries. It is therefore worth trying to steer that future rather than submitting to it. We adopted a three-stage approach to launch our deliberations on possible futures: an historical approach, an analysis of the current context, and hypotheses on possible developments. The historical approach draws lessons from a study of commodity crops up to the 20th century, taking the example of the cotton and palm oil sectors. The analysis of the current context brings out variables that are important for understanding the current situation and whose evolution could play a decisive role in future changes. Lastly, we propose imagining the future from three angles: the first, a trend hypothesis, prolongs the current major trends, with progressive and constructed changes; the second, a disaster hypothesis, describes certain key variables that are subject to sudden and imposed disruptions, and which could have serious repercussions for commodity crops; the third hypothesis is a positive fiction scenario applied to the palm oil sector, with severe but controlled and constructed breaks with trends. The conclusion draws lessons and identifies the limitations of the exercise, and proposes avenues for further deliberation. In order to construct the desired changes and keep abreast of foreseeable developments, it is essential beforehand to carry out a fine analysis of the variables of the context, their interrelations, and mechanisms for monitoring what becomes of them. Such an analysis requires the active participation of stakeholders in the sectors and players in the global context, some of whom have already undertaken foresight exercises: initially, it would be worth bringing together their results with a view to a true international foresight study. (Résumé d'auteur

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

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

Waveguides for walking droplets

When gently placing a droplet onto a vertically vibrated bath, a drop can bounce without coalescing. Upon increasing the forcing acceleration, the droplet is propelled by the wave it generates and becomes a walker with a well defined speed. We investigate the confinement of a walker in different rectangular cavities, used as waveguides for the Faraday waves emitted by successive droplet bounces. By studying the walker velocities, we discover that 1d confinement is optimal for narrow channels of width of $D \simeq 1.5 \lambda_F$. We also propose an analogy with waveguide models based on the observation of the Faraday instability within the channels.Comment: 8 pages, 6 figure