  Water slip at solid surfaces is important for a wide range of
micro/nano-fluidic applications. While it is known that water slip behavior
depends on surface functionalization, how it impacts the molecular level
dynamics and mass transport at the interface is still not thoroughly
understood. In this paper, we use nonequilibrium molecular dynamics simulations
to investigate the slip behavior of water confined between gold surfaces
functionalized by self-assembled monolayer (SAM) molecules with different polar
functional groups. We observe a positive-to-negative slip transition from
hydrophobic to hydrophilic SAM functionalizations, which is found related to
the stronger interfacial interaction between water molecules and more
hydrophilic SAM molecules. The stronger interaction increases the surface
friction and local viscosity, making water slip more difficult. More
hydrophilic functionalization also slows down the interfacial water relaxation
and leads to more pronounced water trapping inside the SAM layer, which both
impede water slip. The results from this work will provide useful insights to
the understanding of the water slip at functionalized surfaces and design
guidelines for various applications.
