Electrification phenomena of pure water droplets dripping and sliding on a polymer surface

The streaming electrification of pure water is generally thought to be a less problem because of its high conductivity which is six orders of magnitude higher than that of a hydrocarbon liquid. However, due to the advancement of semiconductor industries, ultrapure water has been used for washing and rinsing processes and it turned out that high degree of electrification might be produced by using such ultrapure water. In order to clarify this phenomenon, the electrification of deionized water droplets dripping to and sliding on a polymer (PTFE) surface has been investigated. The surface potential distribution of the polymer plate after electrification is measured by moving a probe for two-dimensional scanning. The charge amount of the droplets after the sliding is measured by a Faraday cage. From these experiments, several interesting results were obtained. The potential distribution on the surface is not uniform, but varies along the streaming path. At the dripping point the surface potential of the polymer plate always becomes negative, then turns to positive downstream. However, the droplet, after sliding on the polymer surface, becomes electrified positively. This electrification phenomenon depends on the velocity and the conductivity of droplets. In order to explain this electrification process, a model based on the electric double layer is proposed.