Impact of High Electric Fields on the Behavior of Water Droplets under Various Ambient Conditions: Experiments and Theory

The interaction of water droplets and electric fields is present in various applications like electrowetting, electrostatic lenses or high-voltage composite insulators. Depending on the application, the electric field is used to actively control the behaviour of the liquid as for example for electrostatic lenses, or is only considered as a boundary condition which has a negative influence on, as for instance, the properties of high-voltage composite insulators. The understanding of the underlying physical mechanisms is essential to predict and control the behaviour of liquids under the impact of electric fields. Even though the general behaviour of water under the impact of constant, alternating and transient electric fields was already investigated experimentally, theoretical and numerically in the past, the influence of electric charges on the behaviour of water droplets or the interaction of nearby droplets under the impact of transient electric fields are still not completely clear. It is well known that water droplets are significantly influenced by external electric fields, resulting in oscillations or deformations. Hence, the presence of electric fields might indirectly impact other physical mechanism like ice nucleation, which is controversially discussed and not unambiguously proven. The present work aims to expand the knowledge of the impact of electric fields on sessile droplets under various ambient conditions. Therefore, the influence of electric charges on the motion of single droplets and the inception field strength for electrical partial discharges for single and multiple droplets are experimentally investigated depending on the electric field strength, droplet volume as well as frequency of the electric field. The oscillation behaviour and partial discharge inception of water droplets are significantly influenced by electric charges. In addition, the interaction of nearby droplets under the impact of transient electric fields is determined and the behaviour is categorized depending on the influencing factors. Furthermore, it is confirmed that heterogeneous ice nucleation can be clearly promoted by the presence of electric fields. While constant electric fields have an almost negligible influence, alternating and transient electric fields significantly promote ice nucleation depending on the frequency, type and strength of the electric field. This present experimental work expands the understanding of the impact of electric fields on water droplets under various boundary conditions and might help to improve and optimize applications like the operation of high-voltage composite insulators at ambient and cold conditions