Deposition of Particles on Surfaces

Non-equilibrium conditions, i.e. incomplete wetting. This is particularly the case under dynamic conditions. (3) Energy effects associated with polymer morphology. 

The deposition of particles on surfaces is a process that is determined by long-range forces Van der Waals attraction, electrostatic repulsion or attraction and the presence of adsorbed or grafted surfactants, polymers or polyelectrolytes (referred to as steric interaction). 

In this section, I will discuss the role of van der Waals attraction and electrostatic repulsion (or attraction) on particle deposition. 

The above discussion is very important for many fields in personal care applications hair sprays and hair conditioners, foundation, creams and lotions (skin care). 

The essentially keratinous bulk makeup of hair and skin (the stratum comeum, SC) is generally accepted. However, the surfaces of skin and hair are less well defined - it is generally recognised that a sebum layer is frequently present on both of these substrates. A lipid layer can also be intrinsic to the surface - the presence of hydrophobic layers affects the deposition of additives through their influence on van der Waals forces, hydrophobic forces and the like. 

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Turbulent deposition of particles on surfaces becomes an important process in reactor containments only when particle concentrations have become small. Accurate modeling of turbulent deposition rates has been a topic of debate for some time within the aerosol science community. Models currently available are usually assumed to be adequate in light of the relatively crude understanding of turbulent hydraulics within reactor containments that is now available. These models are based on the study of turbulent flows through pipes. The models are based on the hypothesis that turbulent impulses to aerosol particles can thrust particles across laminar boundary layers adjacent to structural surfaces if the layers are thinner than the stopping distance of the particles. 

Condensation Scrubbing The collection efficiency of scrubbing can be increased by the simultaneous condensation of water vapor from the gas stream. Water-vapor condensation assists in particle removal by two entirely different mechanisms. One is the deposition of particles on cold-water droplets or other surfaces as the result of... 

Condensation Scrubbing The collection efficiency of scrubbing can be increased by the simultaneous condensation of water vapor from the gas stream. Water-vapor condensation assists in particle removal by two entirely different mechanisms. One is the deposition of particles on cold-water droplets or other surfaces as the result of Stefan flow. The other is the condensation of water vapor on particles as nuclei, which enlarges the particles and makes them more readily collected by inertial deposition on droplets. Both mechanisms can operate simultaneously. However, for the buildup of particles by condensation to be effective, there must be adequate time for the particles to grow substantially before the principal gas-liquid-contacting operation takes... 

The deposition of particles on macroscopic surface is the primary goal in CVD processes, bnt rednces the efficiency of vapor phase particle synthesis. Particles can deposit by Brownian motion, bnt in high-temperature reactors, thermophoretic deposition often dominates. Thermophoresis is the migration of small aerosol particles as a resnlt of a temperatnre gradient. It causes particles carried in a hot gas to deposit on a cool surface. Eor small particles, Kn 1, a dimensionless group can be created to describe thermophoresis, Th ... 

Fouling. If die gel-polarization layer is not in hydrodynamic equilibrium with die fluid bulk, die membrane may be fouled. Fouling is caused either by adsorption of species on the membrane or on the surface of the pores, or by deposition of particles on die membrane or wiilini the... 

Fig. 11 Template-assisted deposition of particles on wrinkled films. For successful assembly, the surface charge of the particles has to be of like sign as the surface charge of the wrinkled film to avoid adhesion of the particles towards the film surface...

Clough, W.S. (1975) The deposition of particles on moss and grass surfaces. Atmospheric Environment, 9,1113-19. 

Long ago, Langmuir suggested that the rate of deposition of particles on a surface is proportional to the density of particles in the vicinity of the surface and to the available area on the surface [1], However, the calculation of the available area is still an open problem. In a first approximation, one can assume that the available area is the total area of the surface minus the area already occupied by the adsorbed particles [1]. A better approximation can be obtained if the adsorbed particles, assumed to have the shape of a disk, are in thermal equilibrium on the surface, either because of surface diffusion and/or of adsorption/desorption kinetics. In this case, one can use one of the empirical equations available for the compressibility of a 2D gas of hard disks, calculate the chemical potential in excess to that of an ideal gas [2] and then use the Widom relation between the area available to one particle and its excess chemical potential on the surface (the particle insertion method) [3], The method is accurate at low densities of adsorbed particles, where the equations of state are accurate, but, in general, poor at high concentrations. The equations of state for hard disks are based on the virial expansion and only the first few coefficients of this... 

Experimental values of (ihv)J(ihv) are larger than predicted theoretical values at high t, which is attributed to the deposition of particles on the electrode surface. As the electrode is stationary and there exists no convection to sweep the deposited particles away from the electrode surface, this residual current persists after illumination is stopped, distorting the form of the observed light-off current-time transient. Consequently, theoretical analysis of the transient was not attempted. The rate constant k may be obtained from (//, ) and the rotation speed dependence of the photocurrent. From equations (9.101) and (9.76), it can be seen that ... 

Figure 4 shows the total interaction energy between metal surfaces and alumina particles at pH 4 and 11. Particle dip tests showed the heavy deposition of particles on metal surface at both pHs. The magnitude of deposited particles was greater at pH 4. As shown in Figure 4, the strong adhesion of alumina particles on metal surfaces were calculated. 

Zeta potentials of slun particles and wafer surfaces were measured to calculate the DLVO total interaction energy between them at various pHs. Instead of the Debye-Huckel low potential approximation, Overbeek s approximate was applied to the calculation. The repulsive energy was calculated between silica and TEOS wafers. Particle dip test also showed no deposition of particles on TEOS wafer. Due to the low cell constant of conductive W plate, it was not possible to measure the zeta potentials of W. The Hamaker constants of A1 and W were calculated and applied to the calculation of total interaction energy. The theoretical calculation was agreed well with the experimental results. The strong attractive interaction between metal surfaces and alumina particles were observed in both the calculation and experiments. 

Accurate estimates of particle deposition are difficult to obtain because of the complexity of natural surfaces and the diversity of atmospheric particles. The long-term deposition of particles on vegetative surfaces often is estimated using a deposition velocity model in which the flux density of particles is calculated as the product of a modeled deposition velocity and a measured concentration of particles in the air (Hicks et al., 1987). A deposition velocity is used in a manner similar to a piston velocity in air-water gas exchange (Section 2.3). In a deposition velocity model, the flux density of atmospheric particles to the surface is... 

Browne, E.W. B., 1974, Deposition of particles on rough surfaces during turbulent gas flow in a pipe. Atmos. Environment 8, 8, 801. 

In a further study, Chellam and Wiesner (1997) showed that the specific resistance of the deposit increased with shear rate and decreased with initial flux. This implied that the deposit structure is also important. Additionally, Veerapaneni and Wiesner (1994) simulated the deposition of particles on permeable surfaces. Small particles ( 1 pm) and low fluid velocities favoured the formation of loose deposits on the surface, while particles > 1 pm formed dense deposits. These results show the impact of colloid size on particle packing and thus the permeability of the deposit. Particle-particle interactions, however, were neglected. 

In the considered case, the deposition of particles on the surface of the cylinder is represented schematically in Fig. 10.9. 

The deposition of particles on a horizontal cylindrical surface will depend not only on the horizontal component of the particle velocity but also on the ratio of this velocity and the free-fall velocity of the particles Uff. 

The deposition of particles on individual fibers and wires differs from the filling of a filter with dust particles. First, deposition depends not only on adhesion, but also on the conditions of flow around the obstacle and on the elastic properties of the surface (see Section 39) second, in the actual filtration process, the particles fill up the pore space of the filter and block this space. 

The deposition of particles on single filaments and wires is to be distinguished from the filling of a filter with dust particles. Whereas, in the first case, deposition depends on the flow conditions around the obstacle and the elastic properties of the surface as well as adhesion (see 34), in the second (the filtration process) the whole volume of the pores in the filter is filled with particles and clogging takes place. 

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