In this study, the transient response of the fluid with combined electrokinetic and capillary effect in a microchannel is investigated theoretically. A second-order differential governing equation is obtained which represent the electroosmotic and capillary flow in a rectangular microchannel. This governing equation takes care of inertial force, dynamic contact angle, electrokinetic and entrance effect. The electrokinetic effect is modeled through additional electrokinetic force, which varies along the penetration depth of fluid. The non-dimensional analysis is presented by normalizing the gravity, viscous, electrokinetic forces with surface tension force. New non-dimensional group for electrokinetic force is reported which represents the ratio of electrokinetic and surface tension forces. The numerical solution of the governing equation is validated with experimental and analytical results for capillary flows, which shows good agreement. It is observed that for smaller Eo the capillary front advancement follows the similar trend as pure capillary flow but for higher Eo, the electroosmotic force dominates over surface tension force. It is demonstrated that this model provides simple approach to predict the penetration depth in electroosmotically driven capillary flow.

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