Dynamics of water droplets immersed in dielectric liquids submitted to electric stress

This paper deals with the parameters influencing the dynamics of water droplets immersed in dielectric liquids when submitted to a uniform electric field stress. We mainly investigate the influence of the sizes and number of water droplets as well as their position with respect to the electrodes and to each other on their deformation (shape - elongation), their break-up and/or breakdown of oil gap. The experimental observations are discussed using electrohydrodynamic approach. It is shown that the deformation can be symmetric or asymmetric depending on the geometrical parameters (electrodes gap, the positions, sizes and number of water droplets within this gap), the applied electric field and its application time as well as the surface charge density. The conditions required to bridge oil gap and then breakdown are also analysed. It is also shown that even if the sizes of water droplets are insufficient to bridge the gap when elongated, local vaporization and discharges can be initiated at their poles (ends) resulting in mixed bridge consisting of water and vapour phase (water vapour and/or liquid vapour) leading then to breakdown.