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400 13 Nanoaerosol

To take advantage of the unique properties of nanoaerosol, one of the future
trends in nanomedicine is targeted drug delivery to the respiratory system by
nanoaerosol; it is important to improvement of drug therapies, lung imaging, gene
delivery and therapy, tuberculosis diagnosis and treatment. It is important to clearly
understand the toxicological effect of inhaled nanoaerosol to address the increasing
concerns over potentially harmful public and occupational exposure. And this type
of research should be conducted systematically on a global scale.

13.3.2 Noncontinuum Behavior

Nanoaerosol particles are small enough to approach the mean free path of air, which
is about 67 nm under standard conditions. For nanoaerosol the continuum
assumption is no longer valid and can attain free molecular flow; there is a non-
continuum interaction between the particles and the carrier gas. The dimensionless
parameter that defines the nature of the aerosol is the Knudsen number, which is the
ratio of gas mean free path to particle radius.

Kn ¼ 2k=d p ð13:1Þ

where d p is the particle diameter and k is the gas mean free path that was introduced
in Sect. 2.1.7 above. Under normal conditions the mean free path of the air mol-
ecules is 66 nm. Thereby Kn is in the range of 1.32–132 when the diameter of
nanoparticle drops from 100 to 1 nm. Now the air-nanoparticle is in noncontinuum
regime, Cunningham correction factor, C c , becomes much more important than
micro sized particles.

0:999
C c ¼ 1 þ Kn 1:142 þ 0:558 exp ð13:2Þ
Kn
Both Kn and C c are dimensionless parameters. Since the theoretical value of C c is
always greater than 1, the drag force experienced with slipping effect considered is
always smaller than the value calculated with non-slipping assumption. The drag
force exerted by the air on the nanoaerosol is calculated by

3pl V p V g d p
F D ¼ ð13:3Þ
C c
Under normal conditions, air flow immediately surrounding a nanoaerosol particle
is laminar or in the Stokes regime, although the bulk air flow may be turbulent. In
addition, a nanoaerosol particle tends to follow the moving carrier gas and it is very
difficult to separate them simply by inertia.