Electrification of polymer surface caused by sliding ultrapure water

It has been thought that the conductivity of pure water is high enough to neglect the streaming electrification. However, with the recent advancement of semiconductor industries, ultrapure water appeared to cause electrification problems at the washing and the rinsing processes of semiconductor wafers. The investigation on the electrification phenomenon of polymer (PTFE) surface with ultrapure water is described in this article. Water droplets are always charged positively after sliding on the PTFE surface, and the surface potential on PTFE is mostly negative and varies along the droplet path becoming positive downstream. As the conductivity of water becomes closer to that of ideal water, the amount of droplet charge increases drastically. This electrification phenomenon is also enhanced by decreasing the thickness of the PTFE plate. It suggests that the Coulombic force between the charge on the PTFE surface and the mirror image charge is an important factor. We have proposed a model which is based on the electric double layer similar to conventional streaming electrification for insulating liquid, assuming that the charged droplet leaves the excess positive charge on the polymer surface.