  When tiny soft ferromagnetic particles are placed along a liquid interface
and exposed to a vertical magnetic field, the balance between capillary
attraction and magnetic repulsion leads to self-organization into well-defined
patterns. Here, we demonstrate experimentally that precessing magnetic fields
induce metachronal waves on the periphery of these assemblies, similar to the
ones observed in ciliates and some arthropods. The outermost layer of particles
behaves like an array of cilia or legs whose sequential movement causes a net
and controllable locomotion. This bioinspired many-particle swimming strategy
is effective even at low Reynolds number, using only spatially uniform fields
to generate the waves.
