Deposition by diffusion

The deposition of sub-micron aerosols in a hollow cast of human bronchi has recently been measured under realistic conditions (Cohen et al., in press). Typical data are shown in Figure 4. These are inconsistent with convective enhancement of deposition but support the classical treatment of deposition by diffusion (Chamberlain and Dyson, 1956). 

Deposition by diffusion is the main mechanism for particles smaller than 0.5 pm, and is important in bronchioles, alveoli, and bronchial bifurcations. Aerosol particles are displaced by a random collision of gas molecules this results in particle collision with the airway walls [24]. Deposition by diffusion increases with the decrease in particle size, and breath-holding following inhalation was also found to increase this deposition [25]. 

This is of little significance for particles >1 pm. Particles below this size are displaced by a random bombardment of gas molecules, which results in particle collision with the airway walls. The probability of particle deposition by diffusion increases as the particle size decreases. Brownian diffusion is also more prevalent in regions where airflow is very low or absent, e.g. in the alveoli. 

The effectiveness of deposition by diffusion increases as particle size is reduced, which... 

When the particles become sufficiently small, their deposition by diffusion becomes a significant mechanism. The rate of diffusion is proportional to the diffusion coefficient, Dif, that can be calculated from the Stokes-Einstein equation ... 

It IS sometimes possible to predict rates of deposition by diffusion from flowing fluids by analysis of the equation of convective diffusion. This equation is derived by making a material balance on an elemental volume fixed in space with respect to laboratory coordinates (Fig. 2.1). Through this volume flows a gas carrying small particles in Brownian motion. 

In this section and the next, we discuss particle deposition by diffusion from laminar and turbulent flows through a. smooth-walled pii>e. The particle diameter is assumed to be much smaller than the lube diameter (or viscous sublayer thickness for turbulent flow), so the mterception parameter that was important in the previous discussions does not play a role. 

If special precautions are taken to avoid contamination of the bubble surface, particle deposition by diffusion to the water surface is enhtinced by internal circulation. The internal flow can be calculated for very low bubble Reynolds numbers in the creeping flow approximation (Lamb, 1953). A solution ha.s been obtained to the equation of convective... 

Ultrafine particles are particles with real diameters less than or equal to 100 nm (0.1 p,m) in diameter. These particles arc randomly buffeted by gas molecules and deposit by diffusion in the alveoli. Accumulation fractions are the first sizes beyond ultrafine particles (100 to 500 nm), are deposited mostly by diffusion, and have only minimal settling velocity. Both particles with real physical size are small enough to go into the deep Itmg to deposit in the alveoli. 

As the air speed slows, very small particles are also deposited by diffusion - they are knocked against the walls by the air molecules. 

Deposition by sedimentation and impaction is a function of the inertial aerodynamic size characteristics of the aerosol particles. Deposition by diffusion is a function of the diffusional properties of the aerosol. Deposition by interception occurs when one of the edges of a particle touches the surface of the respiratory tract. Interception is an especially important determinant of deposition of fibers. Deposition of particles in the respiratoiy tract by electrostatic precipitation... 

Diffusion. For particles with a diameter less than 0.5 pm, particle displacement is governed mainly by diffusional transport. Collisions between gas molecules and a particle cause numerous very small random displacements of that particle. The distance a particle will travel by diffusion increases with time and with decreasing particle diameter (Table 1). A 0.1-pm particle can cover a distance of 40 pm in 1 s a 0.01-pm particle a distance of 350 pm. Hence, the probability of particles to hit airspace surfaces by diffusional transport is larger the smaller the particles are and the longer they remain in the respiratory system. Consequently, the lung periphery with its small airway dimensions favors deposition by diffusion. Residence time is long, and the distance a particle has to travel before it hits an airspace wall is short. 

Splelman L A and Friedlander S K 1974 Role of the electrical double layer In particle deposition by convective diffusion J. Colloid. Interfaoe. Sol. 46 22-31... 

There are other methods of preparation that iavolve estabhshing an active phase on a support phase, such as ion exchange, chemical reactions, vapor deposition, and diffusion coating (26). For example, of the two primary types of propylene polymerization catalysts containing titanium supported on a magnesium haUde, one is manufactured usiag wet-chemical methods (27) and the other is manufactured by ball milling the components (28). 

A situation which is frequently encountered in tire production of microelectronic devices is when vapour deposition must be made into a re-entrant cavity in an otherwise planar surface. Clearly, the gas velocity of the major transporting gas must be reduced in the gas phase entering the cavity, and transport down tire cavity will be mainly by diffusion. If the mainstream gas velocity is high, there exists the possibility of turbulent flow at tire mouth of tire cavity, but since this is rare in vapour deposition processes, the assumption that the gas widrin dre cavity is stagnant is a good approximation. The appropriate solution of dre diffusion equation for the steady-state transport of material tlrrough the stagnant layer in dre cavity is... 

Alloys are usually made by melting the components and mixing them together while liquid, though you can make them by depositing the components from the vapour, or by diffusing solids into each other. No matter how you make it, a binary alloy can take one of four forms ... 

We have so far assumed that the atoms deposited from the vapor phase or from dilute solution strike randomly and balHstically on the crystal surface. However, the material to be crystallized would normally be transported through another medium. Even if this is achieved by hydrodynamic convection, it must nevertheless overcome the last displacement for incorporation by a random diffusion process. Therefore, diffusion of material (as well as of heat) is the most important transport mechanism during crystal growth. An exception, to some extent, is molecular beam epitaxy (MBE) (see [3,12-14] and [15-19]) where the atoms may arrive non-thermalized at supersonic speeds on the crystal surface. But again, after their deposition, surface diffusion then comes into play. 

Baranowski [680] concluded that the decomposition of nickel hydride was rate-limited by a volume diffusion process the first-order equation [eqn. (15)] was obeyed and E = 56 kJ mole-1. Later, Pielaszek [681], using volumetric and X-ray diffraction measurements, concluded from observations of the effect of copper deposited at dislocations that transportation was not restricted to imperfect zones of the crystal but also occurred by diffusion from non-defective regions. The role of nickel hydride in catalytic processes has been reviewed [663]. 

Anodic shipping voltammetry (ASV) is the most widely used form of stripping analysis, hi this case, the metals are preconcenhated by elechodeposition into a small-volume mercury electrode (a tiiin mercury film or a hanging mercury drop). The preconcenhation is done by catiiodic deposition at a controlled tune and potential. The deposition potential is usually 0.3-0.5 V more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach die mercury electrode by diffusion and convection, where diey are reduced and concentrated as amalgams ... 

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