This article demonstrates the circular movement of microparticles in water using radial microelectromechanical system electrode arrays. The particles are moved in two polar dimensions, theta and r, by ac electro-osmosis. It is observed that initially particles circle rapidly above the array and move slowly radially to finally form a thin ring at an equilibrium radius. Three-dimensional computational fluid dynamics simulations are performed, cross sectional information is extracted to obtain tangential (U-theta) velocity as a function of r. Orbital velocity and equilibrium radius measurements are performed at various voltages. Velocities and radii are found to be in general agreement with the simulated results. (c) 2006 American Vacuum Society.