Collective Behavior of Reconfigurable Magnetic Droplets via Dynamic Self-Assembly.

Dynamic self-assembly represents an effective approach to form energy-dissipative structures by introducing interactions among multiple building blocks with a continuous energy supply. Time-dependent magnetic fields are treated as convenient energy inputs to construct such a dynamic self-assembled system. The induced interactions can be further tuned by modulating the input field, resulting in a diversity of assembled patterns. However, formation of a functional dynamic pattern with controllability remains a challenge. Herein, we report the formation and pattern control of dynamically self-assembled magnetic droplets at an air-liquid interface, energized by a precessing magnetic field. The formation process involves the assembly of magnetic microparticles into particle chains inside droplets, and then highly ordered patterns are generated by balancing the induced interactions among droplets. By modulating the input field, the interactions and collective behaviors are adjusted and the pattern can be reversibly tuned, i.e., expand and shrink, in a controlled manner. Furthermore, the assembled droplets are able to be steered in two-dimensional as an entity by applying a magnetic field gradient. Utilizing dynamic pattern control and steerability of the assembled structure, cargoes are successfully trapped, transported, and released in a noncontact fashion, indicating that the dynamically assembled droplets can act as a reconfigurable untethered robotic end-effector for manipulation.