Dynamic Assembly of Magnetic Colloidal Vortices

Magnetic
colloids in external time-dependent fields are subject
to complex induced many-body interactions governing their self-assembly
into a variety of equilibrium and out-of-equilibrium structures such
as chains, networks, suspended membranes, and colloidal foams. Here,
we report experiments, simulations, and theory probing the dynamic
assembly of superparamagnetic colloids in precessing external magnetic
fields. Within a range of field frequencies, we observe dynamic large-scale
structures such as ordered phases composed of precessing chains, ribbons,
and rotating fluidic vortices. We show that the structure formation
is inherently coupled to the buildup of torque, which originates from
internal relaxation of induced dipoles and from transient correlations
among the particles as a result of short-lived chain formation. We
discuss in detail the physical properties of the vortex phase and
demonstrate its potential in particle-coating applications.