Dipolar organization and magnetic actuation of flagella-like nanoparticle assemblies

Abstract

Modeled after the design of eukaryotic protozoa, we fabricated artificial microscopic swimmers through the dipolar assembly of a bidisperse mixture of 250 nm superparamagnetic magnetite colloids and 24 nm ferromagnetic cobalt nanoparticles. The cobalt nanoparticles self-assemble into long, 1-D chains measuring approximately 24 nm × 5 µm. These chains then co-assemble with the magnetite beads to form “head” + “tail” structures. These types of asymmetric “flagella-like” colloidal assemblies were formed and maintained solely through dipolar interactions and is the first demonstration using randomly mixed dispersions of disparate magnetic colloids. When actuated by a pair of orthogonal static and sinusoidal magnetic fields, they undergo an asymmetric undulation that is the essential condition for locomotion at low Reynolds numbers. Based upon their shape, size, and articulation, these assemblies are potentially among the smallest structures capable of overcoming Brownian motion to perform useful locomotion. In addition to the head and tail structure, a variety of irregular structures formed that were incapable of swimming. A design of experiments (DOE) study was therefore implemented to optimize the production of artificial swimmers within a large parameter space that included concentration, the amount of sonication, and magnetic field strength. The artificial swimmers were most prevalent for intermediate concentrations of Co and magnetite particles. Statistical analysis suggested that the permanent dipole of the Co nanoparticles stimulated the assembly of the bidisperse mixture into complex, heterogeneous structures. Demonstration of in situ imaging of the magnetic actuation of these dipolar NP assemblies was conducted by optical microscopy.