  Recent studies of dynamic self-assembly in ferromagnetic colloids suspended
in liquid-air or liquid-liquid interfaces revealed a rich variety of dynamic
structures ranging from linear snakes to axisymmetric asters, which exhibit
novel morphology of the magnetic ordering accompanied by large-scale
hydrodynamic flows. Based on controlled experiments and first principle theory,
we argue that the transition from snakes to asters is governed by the viscosity
of the suspending liquid where less viscous liquids favor snakes and more
viscous, asters. By obtaining analytic solutions of the time-averaged
Navier-Stokes equations, we gain insights into the role of mean hydrodynamic
flows and an overall balance of forces governing the self-assembly. Our results
illustrate that the viscosity can be used to control the outcome of the dynamic
self-assembly in magnetic colloidal suspensions.
