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.