Friction Reduction at a Superhydrophilic Surface: Role of Ordered Water

Low-friction but superhydrophilic materials are urgently needed in biomedical and engineering fields because of their nonfouling property and biocompatibility, particularly when the surfaces are definitely superhydrophilic, such as metal or TiO₂ as the surface coatings of the intravascular stents. However, generally, there is a higher friction coefficient on the superhydrophilic surfaces than on the hydrophobic surfaces. On the basis of molecular dynamics simulations, we show that the friction on the superhydrophilic surface with appropriate charge patterns is evidently reduced, where the lower friction is similar to that of a rather hydrophobic surface with a contact angle of water droplet of ∼44°. This reduction is attributed to the existence of an ordered water monolayer on the superhydrophilic surface with appropriate charge patterns, and the friction between this ordered water monolayer and the water molecules above is small