Self-assembly of nanorod motors into geometrically regular multimers and their propulsion by ultrasound.

Segmented gold-ruthenium nanorods (300 ± 30 nm in diameter and 2.0 ± 0.2 μm in length) with thin Ni segments at one end assemble into few-particle, geometrically regular dimers, trimers, and higher multimers while levitated in water by ∼4 MHz ultrasound at the midpoint of a cylindrical acoustic cell. The assembly of the nanorods into multimers is controlled by interactions between the ferromagnetic Ni segments. These assemblies are propelled autonomously in fluids by excitation with ∼4 MHz ultrasound and exhibit several distinct modes of motion. Multimer assembly and disassembly are dynamic in the ultrasonic field. The relative numbers of monomers, dimers, trimers, and higher multimers are dependent upon the number density of particles in the fluid and their speed, which is in turn determined by the ultrasonic power applied. The magnetic binding energy of the multimers estimated from their speed-dependent equilibria is in agreement with the calculated strength of the magnetic dipole interactions. These autonomously propelled multimers can also be steered with an external magnetic field and remain intact after removal from the acoustic chamber for SEM imaging.