Modulating interfacial shear of nanoconfined hydration layer via surface charging

Hydration lubrication provides a significant paradigm for reducing friction at rubbing interfaces, with widespread relevance in engineering and human health. Although the frictional energy dissipation behind hydration lubrication is largely attributed to the shear between hydration layers, the influence of ionic distribution within the confined hydration layer remains unclear. Here, we show that ionic distribution critically modulates the frictional behavior through controlling the shear plane. By systematically tuning the surface charging of sliding interfaces, continuous control was achieved over the friction coefficient—from a superlubricity state under negative charging to a high-friction state under positive charging. Surface characterization and numerical analyses confirm that the contact zone is flattened and the hydration layer remains nanometers thick during sliding, with the tuning process governed by physical interactions. Molecular dynamics simulations reveal that surface charging modulates the spatial distribution of hydrated ions in the confined hydration layer, leading to asymmetric ion compositions across the shear plane. This asymmetry reshapes the energy barrier encountered during sliding, thereby altering the friction behavior significantly. Our results further elucidate the energy dissipation mechanism of electrotunable hydration lubrication and establish a theoretical framework for understanding the macroscopic modulation of interfacial friction.