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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
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 . We also propose an analogy with waveguide models based on the
observation of the Faraday instability within the channels.Comment: 8 pages, 6 figure
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
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