This paper proposes the improvement of microfluidic mixture using the unsteady electroosmotic vortices induced by a staggered array of electrodes. The unsteady electroosmotic forces near two walls, having opposite directions near two walls, were shown to drive organized vortices to contribute for the enhanced mixing. Examination was given to the flow field involving unsteady vortices and their role for the particle mixing. Numerical simulation was made to evaluate the particle mixing process and to reveal how the particle mixing occurred under influence of disturbance by unsteady vortices. The Reynolds number was set to 0.005 and 1.0 changing the oscillation frequency and the electric field intensity. Efficiency of particle mixing was evaluated by the variance intensity of particle number fraction. The agreement between the simulation for the steady electroosmotic flow and the existing paper, and the grid convergence vindicated the simulation code presently used. The numerical results for unsteady electroosmotic flow demonstrated that the particle mixing was controlled by the oscillation frequency and the balanced time scales of the flow perturbation and convection was essential to the particle mixing. The particle flow was displayed to discuss the phenomenology related to the particle mixing. This revealed that the intermingling of particles within main and secondary vortices crucial for the mixing process whereas the outside streams exerted by the vortices were not effective on it. (C) 2015 Elsevier B.V. All rights reserved.