Phase transition of contact-electrified negative charges on a thin silicon oxide in air

We investigated the dense contact-electrified negative charges on a thin silicon oxide surface by the reproducible and controllable contact electrification technique using an atomic force microscope (AFM). Time evolution of the contact-electrified negative charges, which was observed as electrostatic force, showed three dissipation processes. First, the contact-electrified negative charges dissipate slowly, then rapidly and finally, slowly again. It was found by comparison between attractive and repulsive force measurements that the first dissipation process was stable for the applied electric field, whereas the second one was unstable. Analysis of contact voltage dependence and time evolution of the spatial integral of the contact-electrified negative charges revealed the charge sites of silicon oxide for the negative charge. Furthermore, it was found that the time evolution from the first stable dissipation process to the second unstable one was a phase transition from a solid phase to a liquid or gas phase of the contact-electrified negative charges, which was investigated in terms of the nondimensional parameter Gamma. By comparison between the spatial distributions of the electrostatic forces measured repulsively and attractively, it was found that the contact-electrified negative charges were very dense and stable in the central region (i.e., solid phase), whereas they were sparse and unstable in the surrounding region (i.e., liquid or gas phase).