Ultralow Liquid/Solid
Friction in Carbon Nanotubes:
Comprehensive Theory for Alcohols, Alkanes, OMCTS, and Water
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Abstract
In this work, we perform a theoretical study of liquid
flow in
graphitic nanopores of different sizes and geometries. Molecular dynamics
flow simulations of different liquids (water, decane, ethanol, and
OMCTS) in carbon nanotubes (CNT) are shown to exhibit flow velocities
1–3 orders of magnitude higher than those predicted from the
continuum hydrodynamics framework and the no-slip boundary condition.
These results support previous experimental findings obtained by several
groups that reported exceptionally high liquid flow rates in CNT membranes.
The liquid/graphite friction coefficient is identified as the crucial
parameter for this fast mass transport in CNT. The friction coefficient
is found to be very sensitive to wall curvature: friction is independent
of confinement for liquids between flat graphene walls with zero curvature,
whereas it decreases with increasing positive curvature (liquid inside
CNT), and it increases with increasing negative curvature (liquid
outside CNT). Furthermore, we present a theoretical approximate expression
for the friction coefficient, which predicts qualitatively and semiquantitatively
its curvature dependent behavior. The proposed theoretical description,
which works well for different kinds of liquids (alcohols, alkanes,
and water), sheds light on the physical mechanisms at the origin of
the ultra low liquid/solid friction in CNT. In fact, it is due to
their perfectly ordered molecular structure and their atomically smooth
surface that carbon nanotubes are quasiperfect liquid conductors compared
to other membrane pores like nanochannels in amorphous silica