Liquid-Polymer Triboelectricity:Chemical Mechanisms in the Contact Electrification Process.
The liquid-polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as function of pH and ionic strength of the drop as well as ambient relative humidity (RH). The PTFE surface was characterized by SEM, water-contact-angle measurements, FTIR, XPS, and Raman spectroscopy. The charge acquired by the drops was calculated by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become positively charged at pH> pHzch (being pHzch the zero charge point of the polymer), while negatively charged for pH< pHzch. The addition to the water of non-hydrolysable salts (NaCl, CaCl2) decreases the electrical charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liquid to the hydrophobic polymer surface. A proposed thermodynamic model for the ion transfer process allows explaining the observed effects of RH, pH and ionic strength.