(86g) Slip Flow in Nanofluidics: Slip Length Vs. Contact Angle on Hydrophobic Surfaces from Nonequilibrium Molecular Dynamics

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, e_r = e_sf /e_ff ,   and size parameter, s_r = s_sf /s_ff, (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 25^o to 147^o

on highly hydrophobic surfaces (as e_r decreases from 0.5 to 0.1) as
expected. 

The slip length attains ~3 micron and
is functionally dependent on the affinity

strength parameters e_r ands_r: increasing logarithmically with decreasing surface

energy e_r (i.e. more hydrophobic)_, while decreasing
with power law with

decreasing size s_r.  The mechanism for the latter is different from the
energetic

case.  While weak wall forces (small e_r) produce hydrophobicity, larger s_r 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 e_r and high s_r should be chosen to

increase slip, and are prime
candidates for drag reduction in nanoscale devices.