RHEOLOGICAL PROPERTIES AND STRUCTURE OF THE LIQUID IN NANOTUBES - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 189

In the experiments for the water moving in microchannels on smooth hydro-
phobic surfaces, there are sliding at about 20 nm. If the wall of the channel is not
smooth but twisty or rough (and hydrophobic) such a structure would lead to an
accumulation of air in the cavities and become super hydrophobic (with contact
angle greater than 160°). It is believed that this leads to creation of contiguous ar-
eas with high and low slippage, which can be described as “effective slip length.”
This effective length of the slip occurring on the rough surface can be several tens
of microns, which was confirmed experimentally.
It should be noted that for practical use of advantages of nanotubes with slip-
page is necessary to solve many more problems. Scientists have already shown
that hydrophobic surfaces tend to form bubbles. On the other hand, the surfaces
used by most researchers were rough, but the use of smooth surfaces could gener-
ally reduce the formation of bubbles.
Another possible problem is filling of the hydrophobic systems with liquid.
Filling of micron size hydrophobic capillaries is not a big problem, because pres-
sure of less than 1 atm is sufficient. Capillary pressure, however, is inversely
proportional to the diameter of the channel, and filling for nanochannels can be
very difficult.

3.2 THE DENSITY OF THE LIQUID LAYER NEAR A WALL OF CARBON
NANOTUBE
Scientists showed radial density profiles of oxygen averaged in time and hydro-
gen atoms in the “zigzag” carbon nanotube with chiral vector (20, 20) and a radius
356 = R nm (Fig. 3.3). The distribution of molecules in the area near the wall

of the carbon nanotube indicated a high density layer near the wall of the carbon
nanotube. Such a pattern indicates the presence of structural heterogeneity of the
liquid in the flow of the nanotube.

1

( ) ( ) ( )

r = , whereas 2712 nm diameter pipe is completely
filled with water molecules at 300°K. The overall density is

In Fig. 3.3, r

*

r r / r 0

The arrows denote the location of distinguishable layers of the water molecules
and the vertical line is the position of the CNT wall.