Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 53

d 0 , ρ 0

Increasing airway residence time
and uptake of water vapor

d i , ρ i

FIGURE 3.1 Variation in particle properties due to hygroscopic growth in a lung airway (not to scale).

been performed for NaCl 25,26 and cigarette smoke. 27 Computational models of the deposition of
hygroscopic particles have been developed for sulfate aerosols, 28,29 cigarette smoke, 30,31 saline, 32-35
phosphoric acid aerosols, 36 atmospheric salts, 37 and several inhaled pharmaceutics. 38

3.3 DOMAINS OF PARTICLE DYNAMICS
The theory employed to quantify particle dynamics in a transporting medium will depend on the
properties of both the particles and gases involved. Particle dynamics can be classiied by size into
three regimes of behavior. In the free-molecule regime, the behavior of very small particles is inlu-
enced by the motion of individual gas molecules. In the continuum regime, the surrounding gas acts
on large particles as a continuum or viscous luid. In the slip-low regime, a correction for luid slip
at the surface of the particle is employed. A fourth (transition) regime, situated between the slip-low
and free-molecule regimes, is sometimes considered, 38 although the behavior of particles within this
regime is not independently deined. In some texts, the entire region between the free-molecule and
continuum regimes is called the transition regime. 4
The regimes are deined by Kn values, or (at constant temperature and pressure) by particle
sizes. However, the actual values of the threshold Kn or d p parameters for each regime vary in the
literature. The regimes, their corresponding particle characteristics, and their associated dynamic
theories are discussed in the following.

3.3.1 F ree -M olecule r egiMe
The free-molecule regime has been deined to include particles having Kn > 10, 38 or Kn > 20. 7 In air
at STP, these values correspond to particle diameters of 0.01 and 0.02 μm, respectively.
In the free-molecular regime, the motion of particles can be inluenced by interactions with
individual gas molecules. Particles and gas molecules in Brownian motion collide randomly, and
after many collisions, the force exerted by the gas molecules will affect the direction of motion
of the much more massive particles. In this case, particle motion must be quantiied using gas
dynamic theory.
Particle diffusion becomes a very important consideration in the study of particle dynamics
in the free-molecule regime. In this regime, the diffusional velocity of the particle due to random
Brownian motion is several orders of magnitude greater than the terminal settling velocity 38 and,
thus, in this regime the effects of gravitational forces may be neglected.