Hydrodynamic Pair Attractions Between Driven Colloidal Particles
  Colloidal spheres driven through water along a circular path by an optical ring trap display unexpected dynamical correlations. We use Stokesian Dynamics simulations and a simple analytical model to demonstrate that the path's curvature breaks the symmetry of the two-body hydrodynamic interaction, resulting in particle pairing. The influence of this effective nonequilibrium attraction diminishes as either the temperature or the stiffness of the radial confinement increases. We find a well defined set of dynamically paired states whose stability relies on hydrodynamic coupling in curving trajectories. 
Yulia Sokolov, Derek Frydel, David G. Grier, Haim Diamant, and Yael   Roichman
cond-mat.soft cond-mat.stat-mech