Self-assembled surface swimmers and micromanipulators

Soft magnetic particles floating on a liquid can spontaneously assemble into ordered structures.\footnote{N.Vandewalle \emph{et al.}, Phys. Rev. E {\bf 85}, 041402 (2012)} This process is controlled through the amplitude and orientation of an external magnetic induction field. Complex behaviors can arise under a time-dependent magnetic field.\footnote{G. Lagubeau \emph{et al.}, Phys. Rev. E {\bf 93}, 053117 (2016)} In particular, assemblies of three particles or more can undergo deformations in non-time-reversible sequences, a necessary condition for low Reynolds number locomotion.\footnote{G. Grosjean \emph{et al.}, Phys. Rev. E {\bf 94}, 021101(R) (2016)} Such microswimmers can follow precisely controlled trajectories.\footnote{G. Grosjean \emph{et al.}, Sci. Rep. {\bf 5}, 16035 (2015)} As a consequence, these self-assembled structures can be used as microrobots to perform different tasks, such as the capture, transport and release of a microcargo, the mixing of fluids at low Reynolds number, and more.