  Many fascinating phenomena such as large-scale collective flows, enhanced
fluid mixing and pattern formation have been observed in so-called active
fluids, which are composed of particles that can absorb energy and dissipate it
into the fluid medium. For active particles immersed in liquids, fluid-mediated
viscous stresses can play an important role on the emergence of collective
behavior. Here, we experimentally investigate their role in the dynamics of
self-assembling magnetically-driven colloidal particles which can rapidly form
organized hexagonal structures. We find that viscous stresses reduce hexagonal
ordering, generate smaller clusters, and significantly decrease the rate of
cluster formation, all while holding the system at constant number density.
Furthermore, we show that time and length scales of cluster formation depend on
the Mason number (Mn), or ratio of viscous to magnetic forces, scaling as t /
Mn and L / Mn^(1/2). Our results suggest that viscous stresses hinder
collective behavior in a self-assembling colloidal system.
