Loose interfacial water molecule induced low friction on ice

Reducing the ice surface friction coefficient is crucial for improving the performance of winter sports equipment and enhancing the energy efficiency of icebreakers. The existence of a lubricating layer on the ice surface is the fundamental reason for the slippery ice surface. However, understanding the role of lubricating layer in ice friction presents challenges due to the complex interactions between water molecules and various materials. This study investigates the underlying mechanisms that govern ice-surface friction through advanced material design, utilizing a multiscale characterization approach that incorporates Rheometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and sum frequency generation (SFG) spectroscopy. We examined the effects of interfacial lubricating layer on friction across varying sliding velocities, contact pressures, and temperatures. Our findings indicate that three types distinct elements doped diamond-like carbon (DLC) films can achieve ultralow friction on ice, with a-C:H, a-C:H:Si, and a-C:H:WC showing reductions in the friction coefficient of 21.5%, 38.7%, and 58.6%, respectively, when compared to bearing steel (SUJ2). Here, we reveal the mechanism underlying the low friction of DLC on both ice surfaces and in water. In addition to the conventional graphitization induced low friction mechanism, the low friction coefficient is also closely associated with the presence of loosely structured interfacial water molecules, which exhibit a "liquid-like" arrangement and a disordered hydrogen bonding network that collectively contribute to the reduced friction. Additionally, molecular dynamics simulations directly visualize the contribution of loosely structured interfacial water molecules to low interfacial shear. This work presents a novel low friction material of DLC for ice surfaces. The groundbreaking discovery offers an innovative solution for friction reduction engineering in aqueous environments and for advancing competitive winter sports technology. Achieving friction reduction by regulating the molecular structure of interfacial water would be significant for the lubrication industry.