Detecting the Liquid–Solid Contact Electrification Charges in a Liquid Environment

Contact electrification (CE) has been known for more than 2600 years, but its mechanism remains ambiguous, especially for liquid–solid cases. In previous studies on liquid–solid CE, the charges on a dielectric surface in a liquid environment have not been discussed due to the lack of proper measurement techniques, which may be responsible for the poor understanding on the liquid–solid CE. Here, the CE between dielectrics and different liquids, including deionized (DI) water, benzene, and cyclohexane (CYH), is performed by using dual harmonic Kelvin probe force microscopy (DH-KPFM). We focus on the transferred charges on the dielectric surface when it keeps in contact with a liquid. It is observed that the CE surface charges are screened in DI water, but not in organic solutions, suggesting that the electric double layer (EDL) is responsible for the screening of the surface charges. Moreover, it is revealed that the charge transfer in liquid–solid CE occurs not only in the contact but also during the separation process. Based on the observations, a model is proposed to describe the whole process of liquid–solid CE, in which the electron transfer plays a dominant role, and the adsorption of counterions in the EDL on the dielectric surface during separation is considered.