Particle deposition mechanisms

Tassopoulos, M. Relationships between particle deposition mechanism, deposit microstructure and effective transport properties . PhD Thesis, Yale University, USA (1991). 

Description of particle deposition mechanisms at an airway branching site... 

The behavior shown in Figure 11.9 seems to be universal, because it has been observed in other polyelectrolyte/substrate systems, for example, poly(ethylene imine) (PEI)/mica [30] orpDMAEMA/silica [28]. Because of similarities in chemical composition, it can be expected that an analogous particle deposition mechanism will appear in the case of surfaces covered by protein layers. 

Two other deposition mechanisms, in addition to the six listed, may be in operation under particular circumstances. Some dust particles may be collected on filters by sieving when the pore diameter is less than the particle diameter. Except in small membrane filters, the sieving mechanism is probably limited to surface-type filters, in which a layer of collected dust is itself the principal filter medium. 

In addition to the deposition mechanisms themselves, methods for preliminary conditioning of aerosols may be used to increase the effectiveness of the deposition mechanisms subsequently apphed. One such conditioning method consists of imposing on the gas nigh-intensity acoustic vibrations to cause collisions and flocculation of the aerosol particles, producing large particles that can be separated by simple inertial devices such as cyclones. This process, termed sonic (or acoustic) agglomeration, has attained only hmited commercial acceptance. 

Mean airflow velocities approach zero as the inspired airstream enters the lung parenchyma, so particle momentum also approaches zero. Most of the particles reaching the parenchyma, however, are extremely fine (< 0.5 pm MMAD), and particle buoyancy counteracts gravitational forces. Temperature gradients do not exist between the airstream and airway wall because the inspired airstream has been warmed to body temperature and fully saturated before reaching the parenchyma. Consequently, diffusion driven by Brownian motion is the only deposition mechanism remaining for airborne particles. Diffusivity, can be described under these conditions by... 

The particle size is the most important factor that contributes to the clearance of particles. For particles deposited in the anterior parts of the nose, wiping and blowing are important mechanisms whereas particles on the other areas of the nose are removed with mucus. The cilia move the mucus toward the glottis where the mucus and the particles are swallowed. In the tracheobronchial area, the mucus covering the tracheobronchial tree is moved upward by the cilia beating under the mucus. This mucociliary escalator transports deposited particles and particle-filled macrophages to the pharynx, where they are also swallowed. Mucociliary clearance is rapid in healthy adults and is complete within one to two days for particles in the lower airways. Infection and inflammation due to irritation or allergic reaction can markedly impair this form of clearance. 

The principal mechanisms of disposition in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. During the initial operating period, particle deposition takes place mainly by inertial and flow-line interception, diffusion, and gravity. Once the dust layer has been fully established, sieving is probably the dominant deposition mechanism. 

Figure 2. Direct deposition mechanism for particle growth compared with aggregation of adsorbed species.

Figure 3. Direct deposition mechanism leading to highly dispersed and nanosized particle formation, compared with reaction between adsorbed species.

The previous section described active samplers where the air is swept of particles using mechanical mechanisms. This section describes passive samplers that do not move, but collect material that deposits by impaction or sedimentation deposition. These types of collector are the most common type for field studies aimed at assessing exposure of aquatic and terrestrial organisms to pesticides. 

Fig. 2.1. Plausible mechanisms of influence of metal particles deposited on adsorbent on adsorption-caused response of its electrophysical characteristics a - chemical sensitization the dope particles cause the activation and spill-over of adsorbate b - electronic sensitization the dope particles become donors or acceptors of electrons in dependence on the conditions in gas phase.

The ICRP deposition model estimates the fraction of inhaled material initially retained in each compartment (see Figure 3-2). The model was developed with five compartments (1) the anterior nasal passages (ET,) (2) all other extrathoracic airways (ET2) (posterior nasal passages, the naso- and oropharynx, and the larynx) (3) the bronchi (BB) (4) the bronchioles (bb) and (5) the alveolar interstitium (AI). Particles deposited in each of the regions may be removed and redistributed either upward into the respiratory tree or to the lymphatic system and blood by different particle removal mechanisms. 

Particulate diffusion does not play a significant role in the deposition of pharmaceutical aerosols. However, it is worth noting the mechanism by which diffusion of particles occurs in the lungs. The principle of Brownian motion is responsible for particle deposition under the influence of impaction with gas molecules in the airways. The amplitude of particle displacement is given by the following equation ... 

The radiation dose will depend critically on the efficiency with which the particles are deposited on the airway surfaces. In addition the pattern of deposition is important because substantial radioactive decay of the short lived radon daughters will take place before the initial particle deposit can be removed by normal clearance mechanisms (Cohen, et al., 1985). 

While the development of codeposition theories was essentially dormant, the understanding of the kinetics of particle deposition from suspensions was rapidly evolving. The omnipresence of the interaction of particles with surfaces [70] and the importance to deep-bed granular filtration, deposition of paints, fouling of coolant circuits, chemical reactors and membranes, led to careful theoretical and experimental investigations of the mechanism of deposition. The theory is most advanced in the area of filtration and a number of comprehensive reviews exist [71-73]. It is striking that of... 

Thermal deposition is only a minor factor in practical dust-collection equipment because the thermophoretic force is small. Table 17-2 lists these six mechanisms and presents the characteristic parameters of their operation [Lundeand Lapple, Chem. Eng. Prog., 53,385 (1957)]. The actions of the inertial-deposition, flow-line-interception, and diffu-sional-deposition mechanisms are illustrated in Fig. 17-35 for the case of a collecting body immersed in a particle-laden gas stream. 

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