Inner-Surface Charge Mapping in Confined Systems via Electrical Double-Layer Dynamics

Abstract Surface charges at solid–liquid interfaces govern diverse emerging interfacial phenomena, yet their quantification in confined geometries remains challenging. Here, we report an electrical double-layer dynamics-enabled method for inner-surface charge characterization (EISC), using water as a mobile and conformal probe. Unlike conventional techniques that require access to exposed surfaces, the EISC method converts charge transfer at water–solid interfaces into measurable, spatially resolved electrical signals, enabling reconstruction of local surface charge density along otherwise inaccessible inner surfaces. Quantitative validation using a roll-up platform, where predefined surface charges are introduced via electrowetting, shows strong agreement with electrostatic voltmeter measurements and enables quantitative reconstruction of diverse charge distributions. Leveraging this capability, we investigate water–solid contact electrification in tubular channels and reveal a velocity-dependent charge separation process at the receding contact line, with grounding effects pronounced at low velocities. Furthermore, direct mapping of charge evolution in droplet electricity generators demonstrates that device performance is governed not only by droplet-carried charges but also by interfacial charge accumulation and redistribution on the solid surface. This work establishes a versatile strategy for probing interfacial charges in confined systems and provides valuable mechanistic insights into charge transfer processes at water–solid interfaces.