INFLUENCE OF ELECTRODE PROCESSES ON TRANSIENT CONDUCTION PHENOMENA IN NONPOLAR LIQUIDS

Experimental studies of charge injection and the onset of electrohydrodynamic (EHD) motion in non-polar liquids, Voltesso(R) 35 and Norpar(R) 12 contaminated with an ionic additive, Stadis(R) 450 are reported. Measurements were made using both optical and electrical methods with the application of a step voltage to parallel-plane electrodes (d = 4 mm and E = 1.8 MV/m). It is shown that EHD instability in non-polar liquids is caused by the sudden injection of charge carriers into the liquid due to the pre-existence of charges in the diffuse electric double layer near the injecting electrode rather than by true charge injection. The total charge in the diffuse layer is a function of the impurity concentration in the liquid and the elapsed time (rest time) between the successive voltage applications. The measured transient currents in Norpar 12, unlike those measured in Voltesso 35, did not indicate any sudden change in current with the onset of EHD instability unless the additive Stadis 450 is used. The charge injection into the liquid with the application of a voltage step was sufficient to cause a hydrodynamic disturbance in the liquid which was observed on Schlieren images under all conditions. These results demonstrate the limitation of the transient current measurements in detecting EHD instability in non-polar liquids unless a 'critical' level of ionic concentration is present.