Experimental and modeled deposition kinetics of large colloidal particles

The kinetics of the deposition of polymeric microspheres (diameter 3.31-8.7 4 mum, density 1.5 g cm(-3); diameter 4 mum, density 1.055 g cm(-3)), as we ll as of polymorphonuclear leukocytes (diameter 7 mum, density 1.11 g cm(-3 )) on smooth, horizontal silica slides was measured using a vertical cylind rical sedimentation cell. The adsorbing surface was observed with an invert ed optical microscope connected to a camera and a micro-computer for the im age analysis. The experimental deposition kinetics were first compared to t he prediction of the ballistic deposition (BD) model aimed at the descripti on of the sedimentation and adhesion under the influence of a strong gravit ational 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 partic les 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 de veloped. Such a model is needed to avoid the problem raised by the treatmen t 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 bul k concentration is small. (C) 2001 Elsevier Science B.V. All rights reserve d.