A detailed description of the flow distribution in the radial impinging-jet (RIJ) cell was attained by solving the governing Navier-Stokes equation numerically. It was shown that for tangential distances r/R < 0.25 the flow configuration in the vicinity of the solid interface approached the stagnation point flow with the perpendicular velocity component independent of the radial distance. The intensity of this quasi-stagnation point flow, governed by the α parameter, was calculated numerically as a function of the Reynolds number. It was also found that the flow pattern in the RIJ cell resembled the flow occurring near a sphere immersed in a uniform flow. Knowing the fluid velocity field the convective diffusion equation was formulated. This equation, describing a two-dimensional transport of particles, was solved numerically by using the implicit finite-difference method. In this way the particle deposition rate for the low coverage regime (initial flux) can be determined for various parameters such as particle size, Reynolds number, distance from the stagnation point, etc. The validity of the theoretical predictions was verified experimentally using direct microscope observation of polystyrene latex particles of size 0.87 µm. The initial flux near the stagnation point was measured as a function of Reynolds number and ionic strength of the suspension. The dependence of the local mass transfer rate on the distance from the stagnation point was also determined experimentally. This enabled one to estimate the error associated with indirect (optical) measurements of protein absorption in the RIJ cell. A good agreement between predicted and measured flux values was found, which validates the applicability of the numerical solutions of the flow field and mass transfer in the RIJ cell. It was suggested that by measuring the initial flux for colloid particles microscopically one can determine in a direct way the local mass and heat transfer rates for the impinging-jet configuration used widely in practice. C 2001 Academic Press
Irreversible adsorption of hard spheres at random site surfaces was studied theoretically. In contrast to the previous model of Jin et al. [J. Phys. Chem. 97, 4256 (1993)] the dimension of the sites, having the shape of circular disks, was finite and comparable with the size of adsorbing spheres. Adsorption was assumed to occur if the sphere contacted the disk, i.e., when the projection of the sphere center was located somewhere within the disk only. Numerical simulation of the Monte Carlo type enabled one to determine the available surface function, adsorption kinetics, jamming coverage, and the structure of the particle monolayer as a function of the site density (coverage) and the size ratio particle/site, denoted by λ. It was demonstrated that adsorption kinetics and the jamming coverage increased significantly, at a fixed site density, when the λ parameter increased. It was also proven that the results derived from the Jin et al. model were valid only if λ>10.
Adsorption kinetics of negatively charged polystyrene latex particles (average size 1.38 μm) over a
heterogeneous surface was studied experimentally. The substrate of a controlled heterogeneity was produced
by covering natural mica sheets by smaller (average size 0.55 μm) positively charged latex particles. The
direct microscope observation method combined with the impinging jet technique was used to determine
particle adsorption kinetics. The initial flux of particles (the slope of the kinetic curves in the limit of short
adsorption time) was determined quantitatively as a function of the coverage of smaller particles. It was
demonstrated that the initial flux attains the limiting values for the smaller particle (heterogeneity)
coverage as small as a few percent. The experimental results were interpreted in terms of the generalized
random sequential adsorption model by considering the coupling between the bulk and surface transfer
steps. A good agreement was found which seems to confirm the validity of the theoretical model for predicting
particle adsorption at heterogeneous surfaces.
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