Experimental and modeled deposition kinetics of large colloidal particles

The kinetics of the deposition of polymeric microspheres (diameter 3.31–8.74 μm, density 1.5 g cm−3; diameter 4 μm, density 1.055 g cm−3), as well as of polymorphonuclear leukocytes (diameter 7 μm, density 1.11 g cm−3) on smooth, horizontal silica slides was measured using a vertical cylindrical sedimentation cell. The adsorbing surface was observed with an inverted optical microscope connected to a camera and a micro-computer for the image analysis. The experimental deposition kinetics were first compared to the prediction of the ballistic deposition (BD) model aimed at the description of the sedimentation and adhesion under the influence of a strong gravitational field. This model proves effectively to be appropriate in the case of the largest particles used. However, once the gravitational force ceases to strongly dominate the random thermal force, the diffusion of the particles in the liquid becomes an important feature of the process as far as its kinetic aspect is concerned. To take the diffusion into account accurately, especially in the vicinity of the interface, a non-sequential model is developed. Such a model is needed to avoid the problem raised by the treatment of particles trapped by those near the surface, which cannot be solved in a rigorous way in the framework of a sequential model, even though the bulk concentration is small.