RHEOLOGICAL PROPERTIES AND STRUCTURE OF THE LIQUID IN NANOTUBES - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 194

In fact, shear stress is primarily due to van der Waals interaction between
the solid wall and the water molecules. It is noted that the characteristic distance
between the near-wall layer of fluid and pipe wall depends on the equilibrium
distance between atoms O and C and the distribution of the atoms of the solid wall
and bend of the pipe.
From Fig. 3.5, the effective viscosity hincreases by two orders of magnitude

when R changing from 0.67 to 5.4 nm.
According to Eqs. (3.5)-(3.7), the effective viscosity can be calculated as

= π

R
8

4 D

p

h

. The results of calculations are shown in Fig. 3.6.

QL

FIGURE 3.6 Effective viscosity as a function of the nanopore radius and the loading rate.

The dependence of the shear stress on the flow rate is illustrated in Fig. 3.7.
For the tube (20,20) τ increases with v . The growth rate slowed down at higher
values v .
At high speeds v , while water molecules are moving along the surface of the
pipe, the liquid molecules do not have enough time to fully adjust their positions
to minimize the free energy of the system. Therefore, the distance between adja-
cent carbon atoms and water molecules may be less than the equilibrium van der
Waals distances. This leads to an increase in van der Waals forces of repulsion and
leads to higher shear stress.