Particle Deposition Studies in a Laminar Wall Jet - The Hydrodynamic Influence on the Kinetics of Colloidal Deposition

University dissertation from Food Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

Abstract: In the research area of cleaning and process hygiene it is important to understand the underlying mechanisms behind the deposition of particles and macromolecules onto surfaces. The deposition process is described as the transport of particles to the surface followed by attachment. If the hydrodynamic forces are strong enough, the particles can be re-entrained. In this work particle deposition was studied in situ. In order to obtain a well-defined mass transfer to a glass surface covered with indium-tin oxide onto which deposition was to occur, an experimental setup consisting of a wall-jet cell was employed. Model solutions consisting of polystyrene latex particles of two particle radii, 0.23 um and 0.38 um, were used in the deposition experiments. The deposited particles were visualized by total internal reflection microscopy. The construction of the cell made it possible to radially scan the surface and thus follow the deposition of particles at positions exposed to different wall shear rates in one single experiment. To be able to better understand the different phenomena which are thought to affect deposition a model was employed. By using a transport term describing the particle flux to the surface and another term expressing the detachment of particles, transport to and from the collector surface could be included in the model. The flux term to the surface included a so-called shielding function. The phenomenon of surface shielding was used to explain the decrease in deposition rate. The study of shielding was expanded by applying a computer algorithm based on the concept of random sequential adsorption. It was found that the size of the shielded area increased with increasing wall shear rate, as well as with decreasing bulk particle concentration.

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