  Suspended microparticles subjected to AC electrical fields collectively
organize into band patterns perpendicular to the field direction. The bands
further develop into zigzag shaped patterns, in which the particles are
observed to circulate. We demonstrate that this phenomenon can be observed
quite generically by generating such patterns with a wide range of particles:
silica spheres, fatty acid, oil, and coacervate droplets, bacteria, and ground
coffee. We show that the phenomenon can be well understood in terms of second
order electrokinetic flow, which correctly predicts the hydrodynamic
interactions required for the pattern formation process.Brownian particle
simulations based on these interactions accurately recapitulate all of the
observed pattern formation and symmetry-breaking events, starting from a
homogeneous particle suspension. The emergence of the formed patterns can be
predicted quantitatively within a parameter-free theory.
