Experimental investigations on electrohydrodynamic phenomena at single droplets on insulating surfaces

Polymeric insulating material can lose its hydrophobic proper-ties when is stressed by various environmental factors under service conditions. Humidity is one very important factor for ageing phenomena. This paper reports investigations on single droplets which are electrically stressed on hydrophobic polymer surfaces. With the help of two digital optical diagnostic systems deformations and movements of the droplets can be recorded and analyzed in an electrical ac and dc field. The experiments show several electrohydrodynamic phenomena which have an effect on the process of the very early stage of ageing of polymer insulating surfaces. When a critical field stress is exceeded different phenomena occur, such as transient structures on the surface, creation of filaments, formation of water paths between droplets, collapsing of several droplets, separation of droplets and ejection of very small droplets from bigger ones. These effects lead to a deformation and rearrangement of the original droplets. As a result the droplets cover a wider area of the insulating surface and minimize the dry zones of the solid surface. Thus, bridging by a water path between the electrodes becomes more probable.