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

Abstract

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.