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We experimentally investigate magnetic surface swimmers on water. These objects self-assemble from ferromagnetic microparticles and a nonmagnetic disk. They are floating on the liquid surface due to interface tension and move under the influence of a harmonically oscillating homogeneous magnetic field oriented vertically, which is distinguished by its amplitude and frequency. The speed of the surface swimmers strongly depends on these parameters. The functional dependencies between speed and amplitude and between speed and frequency are investigated by independently varying both control parameters. In the first case, the data obtained are in good agreement with the predicted scaling while there are some deviations in the latter case. Moreover, due to the interplay between the surface bound swimmers and the ascending liquid meniscus at the edge of the experimental vessel, different dynamics can be realized. We observe periodic and quasiperiodic trajectories in a circular vessel and aperiodic trajectories in a vessel shaped like a Bunimovich stadium.

We dynamically self-assemble an active paramagnetic colloidal network that ballistically transports nonmagnetic fluorescent colloidal particles on top of the network. The transport mechanism is put into effect by a directed hydrodynamic flow, created by the rotation of individual colloids forming the network. The ballistic transport is confined to the network, and tracer beads in meshes show a diffusive instead of a ballistic transport. We develop a simple model to explain the transport.