Homogeneous interfacial water structure favors realizing a low-friction coefficient state

The use of water to reduce friction has always played a significant role in a wide range of areas ranging from biology to engineering. Many efforts have been made to extensively investigate the water behavior between two contacted surfaces, but its role in water-based friction remains incompletely understood.Herein, we utilize the sum-frequency generation (SFG) spectroscopy to identify interfacial water structures upon adjusting the wettability of titanium dioxide (TiO2) and silicon surfaces. And the corresponding wettability-tunable underwater friction is measured by atomic force microscopy (AFM).It demonstrates that enhanced wettability induces higher friction on the TiO2 surface but lower friction on the silicon surface. Although the tribological properties of TiO2 show independence of surface forces in contrast to the case of silicon, both TiO2 and silicon surfaces covered with homogeneous water molecules correspond to a lower friction coefficient. This observation indicates that a homogeneous interfacial water structure, dominating over surface forces, is of the utmost importance for achieving low friction. Our results shed new light on the origins of friction in the presence of water and reveal the ubiquitous role of interfacial water structures on friction.