|M.Sc Student||Bar-on Roi|
|Subject||Modeling Hydrodynamic and Physicochemical Effects in|
Micro-Particle and Bacterium Deposition on
|Department||Department of Chemical Engineering||Supervisor||Professor Viatcheslav Freger|
|Full Thesis text|
Aquatic surfaces tend to be fouled over time by both non-living matter (particles, colloids, salts, organics and macromolecules) and microorganisms such as bacteria. Fouling is associated with irreversible particle attachment and harms structures immersed or in contact with water, such as pipes, membranes, vascular grafts etc. This motivates the search for anti-fouling surfaces, which requires a physical understanding and interpretation for bacterium and particle deposition on surfaces and their relation to surface characteristics. For instance, the Groningen group reported that a glass surface coated with polyethylene oxide (PEO), with a thickness of 23.7 nm, largely reduced bacterial accumulation on the surface (Roosjen et al. 2004).
Parallel plate flow chamber (PPFC), is a common experimental setup for studying deposition of bacteria and particles. Its results are commonly analyzed using the classical Smoluchowski-Levich (SL) convection-diffusion model, which neglects gravity and lift forces and assumes a perfectly adsorbing surface. However, recently published experiments as well as our data demonstrate that the SL model doesn’t adequately predict the observed trends. For instance, experiments show that deposition flux increases along the channel, whereas the SL model predicts a decreasing trend. The main goal of this work was to understand the reason for this discrepancy and propose a more adequate model that will cover a variety of particle-surface systems including deposition of bacterial cells.
We addressed the problem by incorporating additional effects, such as gravity, lift and adhesion, into the convection-diffusion model. To address the complex interplay of adhesion forces and near-surface hydrodynamics, we defined a kinetic parameter -deposition coefficient - that is introduced through a boundary condition, and may serve as a simple and practical indicator of the propensity of the surface to be fouled by particles or cells. The numerical solution of the model highlighted that sedimentation (gravity) and adhesion forces are mainly responsible for the observed trends, which explains the poor agreement with the SL model. In particular, we show that reversal of the trend of deposition rate from decreasing to increasing along the channel is associated with the deposition rate coefficient becoming smaller than the terminal sedimentation velocity. In addition, we develop approximate analytical relations that allow a more facile parameter fitting and comparison with experimental results than the full numerical solution. Ultimately, we demonstrate the use of the model for fitting and analyzing real bacterium and particle deposition data. The new model may become a useful tool in analyzing deposition experiments and quantifying propensity of different surfaces to particle fouling and biofouling.