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

Authors: 
Voronov, R. S. - Presenter, University of Oklahoma


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 andsr: 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.