Fig. 1.

(A) The geometry of “surface walkers”. Each chain is composed of superparamagnetic beads that move according to the dynamics of the magnetic field B that induces a magnetic moment m in each bead. In particular, we focus on rotation along the x-z plane at a frequency ν. (B) Top: The aggregate moves along the surface in both experiment (frames taken 16 ms apart in a 5 Hz rotating field, scale bar is 5 μm) and simulation upon confinement at the surface. Bottom: When the field rotation is raised to 7 Hz, the rotors fragment periodically. Notice that the agreement is excellent between both experiments and theory, even in the shape of the fragmented aggregates.