This paper is a theoretical study of dynamic self assembly in a system of millimeter-sized magnetized disks floating at a liquid-air interface and spinning under the influence of a rotating magnetic field. Equations of motions are derived that account for the hydrodynamic and magnetic forces acting in the system. Numerical integration of these equations predicts formation of ordered structures of spinning disks; the simulated structures reproduce the patterns observed experimentally.T he formation of ordered structures by self assembly is interesting both theoretically and practically, with implications for chemistry (1, 2), physics (3-5), materials science (6-9), and biology (10-12). Although self assembly and self organization in systems operating at or near thermodynamic equilibrium are relatively well understood, the theoretical description of dynamic self-assembling systems (13, 15)-those that operate away from equilibrium and develop order only when dissipating energy-is incomplete. In the absence of a general analytical description of such systems, numerical analysis is a convenient (and often the only) method for studying their dynamics.In previous work (16, 17), we described a dynamic selfassembling system composed of a limited number (Ͻ40) of millimeter-sized magnetized disks floating on a liquid-air interface and subject to an external magnetic field produced by a rotating permanent magnet with a dipole length much larger (ϳ6 cm) than the radii (ϳ0.5 mm) of the disks. In the presence of the rotating external field, the disks spin around their axes with angular frequency equal to that of the external magnet ( ϳ200-1,200 rpm). All disks are attracted toward the axis of rotation of the magnet and are repelled by one another by hydrodynamic interactions associated with the motion of the fluid surrounding the disks. The interplay between attractive magnetic and repulsive hydrodynamic interactions in this system leads to the formation of macroscopic patterns. We quantified both the hydrodynamic repulsive force between the disks and the central magnetic force acting on all disks. On the basis of the experimental data for the simplest case of two interacting disks, we derived the equations that describe the forces acting in the system (17) and proposed that interactions in aggregates composed of a larger number of disks can be treated pairwise.This work describes the equations of motion of the rotating disks and uses them to model self organization of different numbers of such disks in ordered aggregates. The simulated patterns are in excellent agreement with those observed experimentally. Our model not only correctly predicts the symmetry and dimensions of the aggregates at various rotational speeds but also allows estimation of the frequencies of occurrence of polymorphic patterns (i.e., different patterns that arise in the same system). We identify dimensionless parameters that control the formation of the aggregates and compare and contrast the results of our simulations with those for systems of twodimen...