Numerical study of ionic current in dielectric liquid layer subjected to ac voltage

Ionic current flowing in a thin layer of dielectric liquid under the action of ac voltage was studied numerically. Strongly blocking electrodes and equal mobilities of anions and cations were assumed. Field dependent dissociation and recombination were taken into account. The spatial distributions of ion concentrations and of electrical potential along coordinate normal to the layer and the ionic current density during a single voltage cycle were calculated as functions of time. At sufficiently low frequency and sufficiently high voltage, the ionic current was non-sinusoidal function of time. Significant contribution of higher harmonics were present. In certain frequency region this contribution gave rise to a bump of the current. The moment at which the bump occurred was not related with the value of ions mobility. These effects resulted from changes of spatial distribution of ions. In most cases the changes occurred simultaneously in the whole layer. In sufficiently thin layer, two gradually diffusing groups of ions wandering across the layer, one formed by cations and the other formed by anions, could be distinguished. Their appearance coincided with the maximum of the bump which was particularly high in such case. In thicker layers, similar groups of ions did not arise, although the current bumps were present.