Surface effects are overpowering for flows in nanofluidic devices because
of the large surface-to-volume ratios. To reduce the surface drag and
increase the throughput, one hopes to exploit hydrophobic surfaces
where slip boundary flow might occur. Previous experiments on channels
from micrometer to nanometer scale confirmed the slip behavior. However,
the mechanisms of slip are open to interpretations. Here we employ the
nonequilibrium molecular dynamics (NEMD) to characterize the effects
of surface wetting properties (such as the contact angle) on the slip length
for a Lennard-Jones fluid in Couette flow between graphite-like hexagonal-lattice
walls. The fluid-wall interaction is varied by modulating the interfacial energy
parameter, er = esf /eff , and size parameter, sr = ssf /sff, (s= solid, f= fluid)
to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity).
Effects of surface chemistry, as well as the effects of temperature and shear
rate on the slip length are determined. Contact angle increases from 25o to 147o
on highly hydrophobic surfaces (as er decreases from 0.5 to 0.1) as expected.
The slip length attains ~3 micron and is functionally dependent on the affinity
strength parameters er and sr: increasing logarithmically with decreasing surface
energy er (i.e. more hydrophobic), while decreasing with power law with
decreasing size sr. The mechanism for the latter is different from the energetic
case. While weak wall forces (small er) produce hydrophobicity, larger sr smoothes
out the surface roughness. Both tend to increase slip. Slip length grows rapidly
with high shear rate, as wall velocity increases three decades from 100 m/s to 105 m/s.
We demonstrate that fluid-solid interfaces with low er and high sr should be chosen to
increase slip, and are prime candidates for drag reduction in nanoscale devices.