Slide electrification charge can exist without residual liquid film: KPFM measurements near receding three-phase contact lines

Spontaneous charge transport near the solid-gas-liquid three-phase contact line (TPCL) is a key phenomenon for interface-driven power generation and related applications, yet its details-especially near a receding TPCL-remain poorly understood. In this study, we experimentally investigated the charge distribution induced by the recession of the TPCL. Specifically, we mapped the charged regions left by glycerol-water droplets on SiO₂ surfaces using Kelvin probe force microscopy (KPFM). It was found that the charge distribution at solid-liquid interfaces near the TPCL is determined by the Debye length of the solution used, which is consistent with a theoretical model proposed in prior studies. However, nanodroplets or residual films, which have been proposed as necessary for the stabilization of charges at the solid-gas interface, were not observed, implying the existence of a different charge stabilization mechanism. Furthermore, we found that the charge distribution unpredictably exists on the solid-gas interface side, possibly induced by charge relaxation. This deviation between the KPFM results and theoretical prediction was resolved by newly introducing a charge-relaxation term to the model. These results deepen our understanding of charge-transfer mechanisms at TPCLs and provide valuable insights for interface-chemistry applications such as energy harvesting.