Case Studies in the Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 274

Fig. 8.34 Comparison of
different forces on
nanoparticle deposition at the
flow rate of 10 L/min

effects begin to contribute. In the nanoparticle range ( < 10 nm) the effects of the
flow rate show that deposition efficiency increases with a decrease in the flow rate,
which is quite different in comparison with micron particles. The lower flow rate
for nanoparticles allows more time for the diffusion to take place, leading to greater
particle dispersion and enhancing deposition. In contrast, a lower flow rate for micron
particles reduces the particle inertia (particle relaxation time) and this allows the
particle more time to follow any changes in the air flow field.
The effect of each force applied to a submicron particle on the deposition efficiency
is shown in Fig. 8.34 . The influence of the Brownian force becomes more significant
as the particle size approaches 1 nm. For example, the total deposition efficiency
of particles with the diameter of 1 nm increases by 66 % when the Brownian force
is applied. Without the Brownian force the total deposition efficiency varies around
13 %. The drag force by itself produces a deposition of 11 %. The addition of the
Saffman's lift force only increases the deposition slightly. Interestingly, the addition
of the thermophoretic force decreases the deposition.
In general the deposition pattern for nanoparticles is spread out through the nasal
cavity well. This has interesting applications for drug delivery where traditional nasal
sprays are producing micron-sized droplets that are prone to inertial deposition.
This deposition mechanism leads to high inertial impaction (up to 100 % for a
mean atomized particle droplet of 50

m) in the anterior region of the nasal cavity
(Inthavong et al. 2006a, 2008a). However for high drug efficacy, the delivery of the
droplets needs to be deposited in the middle regions of the nasal cavity, where the
highly vascularised walls exist. Smaller particles such as 1

μ

m were found to be less
affected by inertial properties, which allowed them to bypass the anterior region of
the nasal cavity. However, because of the particles' ability to follow the streamlines
more readily, the particles were less likely to deposit in any region of the nasal
cavity and instead bypassed it completely, leading to the undesired effects of lung
deposition. Delivery of nanoparticles, especially 1-5 nm particles, can therefore

μ