This paper presents a numerical study on the enhancement of mixing within a micro cavity by use of transient induced-charge electro-osmotic flows created by a periodic on-off-switching of the electric field applied across a pair of electrodes. For the first time, we demonstrate that the electro-osmotic flows observed during the charging and decharging stages are not symmetric with respect to each other and can lead to a chaotic mixing. Lyapunov exponents and particle scattering patterns are obtained for the symptom of the chaotic advection at various external fields.