Time Periodic Electroosmotic Flow with Heterogeneous Surface Charge along the Channel

Mixing is an essential process in a lab-on-a-chip microfluidic device for sensing, concentration and amplification of target molecules or compounds. However, effective mixing is a challenging task due to low Reynolds number creeping flow in microfluidic devices.  Mixing can be enhanced by modulating the driving mechanism. In this work, we have presented an analytical model to study the effect of heterogeneous surface charge on time periodic electroosmotic flow in a microchannel. A generalized solution for fluid flow is obtained by solving the Navier-Stokes equations with periodic boundary conditions at the upstream and downstream of the channel and slip boundary conditions at the channel walls. Our results show that various types of vortices and flow patterns can be obtained from the axial variation of surface charges. The location and strength of vortices depend on the channel size, charge distribution and the ratio of diffusion time scale to the period of electric field.  For instance, sinusoidal charge pattern leads to the stronger vortices than step-change in charge pattern. The generalized analytical model presented in this study can be used for optimizing micromixer design.