Electrohydrodynamic instabilities and electroconvection in the transient and A.C. regime of unipolar injection in insulating liquids: A review

Abstract

This paper reviews only the transient regime of unipolar injection into insulating isotropic liquids. Under the action of the Coulomb force, ions injected in such insulating liquids can produce, either under step voltage or a.c., electrohydrodynamic (EHD) instability. The first part of the paper recalls the laws of transient space charge limited current or electrode-limited current in a medium at rest. The possible influence of charges initially present in the bulk or at the interfaces is considered, especially in the case of liquids for which they can produce instability during the transient state. The second part is devoted to the EHD instability following a voltage step: the delay time necessary for the instability to appear is calculated, the various phenomena of transient electroconvection are presented and explained with reference to the hydraulic model of Félici. Particularly, it is underlined that an apparent mobility, greater than the true (ionic) mobility by orders of magnitude, can be measured, especially in liquids of high M numbers (generally viscous ones). Consequently, high values of transient currents are obtained, the efficiency of convective charge transfer being much higher than in the stationary case (motion is better organized in the fluid). Many comparisons are made with both the theoretical and experimental work done in the stationary state (Atten and Moreau; Lacroix and Atten, etc.). The last part shows how the preceding results can be applied to a.c.: existence of a critical voltage above a cut-off frequency, and of a critical field varying as the square root of the frequency. Finally, the influence of EHD motion upon losses is briefly discussed.