Distinct Electrification Regimes at Solid-Liquid Interfaces Revealed by Water Isotope Substitution

Solid-liquid contact electrification is widely studied using diverse experimental geometries that are often implicitly assumed to probe equivalent interfacial processes. Here, we directly test this assumption by comparing immersion-emersion, droplet-based contact, and pressure-driven intrusion-extrusion electrification using the same hydrophobic nanoporous silicon monolith and isotopically substituted liquids (H₂O and D₂O). Despite identical surface chemistry, isotopic substitution produces qualitatively different electrical responses depending on the wetting regime. Immersion-emersion experiments show polarity inversion between H₂O and D₂O, while droplet measurements reveal distinct charge evolution and periodic opposite-polarity events. In contrast, forced nanopore intrusion preserves polarity but strongly enhances electrical output for D₂O. These results demonstrate that wetting of surface nanoroughness during droplet, vs immersion-emersion, vs complete nanopores wetting upon intrusion-extrusion correspond to fundamentally different electrification states. By using isotope substitution as a controlled probe, this work establishes a general framework for disentangling protocol-dependent effects in solid-liquid contact electrification and triboelectric energy conversion.