For a smooth hydrophobic surface, slip occurs during fluid flow close to the boundary, known as boundary slip which can reduce fluid drag. The surface charge present at the interface is believed to affect the boundary slip. In this paper, the effect of surface charge on the fluid flow on a hydrophobic surface is studied in both deionized (DI) water and saline. Positive and negative voltages are applied to the substrate to vary the surface charge density. The electrostatic force and slip lengths are measured with different applied electric field. Results are the subject of the paper. (c) 2012 Elsevier Inc. All rights reserved.