Mechanism of charge formation in double layer appearing at an hydrocarbon liquid-metal interface (comparison of theoretical models with experiments)

Abstract

To understand some phenomena appearing in conduction in dielectric liquids, it is important to know the mechanism of charge formation at the interface between the metal and the liquid ((1), (2), (3)). In this paper, we study this mechanism without an applied electric field, but the liquid flowing past the metallic surface. At the contact between a metallic surface and an hydrocarbon liquid an electrical double layer appears. The thickness of the compact and the diffuse layers are such that only the diffuse layer can be affected by the flow. For a liquid flowing through a circular tube of finite length the charge convected at the exit of the tube depends on the process of charge formation in the diffuse layer appearing at the inner wall of the tube. A theory is presented for the process of charge convection in a laminar tube flow. The liquid is assumed to be electrically neutral at the entrance of the tube. The development of the diffuse layer is computed in terms of parameters such as the distance from the entrance, the radius of the tube, the Reynolds number of the flow, the conductivity of the liquid and the charge at the wall. In order to understand the mechanism of charge formation inside the pipe we have then computed the value of the space charge convected with different models and compared the results with experimental data. Experimental data are presented for liquid heptane flowing through stainless-steel tubes of radius of 0.24 mn and length between 0.08 m and 4 m. The comparison between experiment and theories permits us to infer the development of the charge on the wall for this experiment, and then to comment on the mechanism of charge formation.