Atomic scale friction properties of confined water layers

The atomic scale friction properties of water layers are a highly important subject for understanding the lubricating mechanism of ice. Recently, two-dimensional (2D) materials gave rise to an excellent platform for investigating the tribological properties of water, since water layers can be easily intercalated between 2D layers and hydrophilic substrates. In this Review, we highlight recent research on the friction properties of the confined water between hydrophilic (mica and silica) substrates and 2D materials. Recent friction force microscopy work has revealed the nanotribological properties of water intercalated between 2D materials (e.g., graphene and MoS₂), and a hydrophilic substrate increases the friction force. Moreover, the friction on both graphene and MoS₂ increased as the number of stacking water layers increased. On the other hand, the magnitude of friction increase went down as the number of covering 2D layers above the intercalated water layer increased; the friction is eventually indistinguishable from the multilayer stack excluded water adsorption. The isotope effect of frictional enhancement has been addressed, and it was shown that the intercalation of deuterium oxide (D₂O) leads to the decrease of friction at H₂O intercalated graphene on mica due to the lower vibrational frequency of D₂O adsorbate, which is associated with the low rate of frictional energy dissipation at the interface. Water exfoliated by 2D layers prepared with mechanical exfoliation and chemical vapor deposition exhibits similar frictional enhancement, indicating the universal tendency of friction by intercalated water between 2D atomic layers and hydrophilic surfaces.