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Numerical modeling of viscoelastic response of biofilm to fluid flow stress

During the biofilm development process, biofilm detachment may occur in the form of detachment of the biofilm from the substrate and breakdown of the biofilm itself. Currently there is a wholly inadequate mechanistic understanding of the process of biofilm detachment in flowing systems. The key question in relation to the process of biofilm deformation/failure is the mechanical properties of the biofilm. The main aim of this study is to develop a numerical model of the biofilm detachment in a flowing system. A Finite Volume Method (FVM) based Fluid-Structure Interaction (FSI) solver in OpenFOAM package has been developed to model the biofilm response to flow [1]. Dynamic interaction is simulated between an incompressible Newtonian fluid and a bacterial biofilm described as a linear viscoelastic solid. Viscoelastic properties of biofilm was determined by conducting creep experiments on Mixed mode type biofilm and a Generalised Voigt Model (GVM) was found to be well fitted into the experimental data using a non-negative least square method (liu 1999). In this numerical work, constitutive relation of a linear viscoelastic solid is represented by the hereditary integral [2] while the tensile stress relaxation modulus function (E(t)) is expressed by the Generalised Maxwell Model (GMM) which is equevalent to GVM. GMM is obtanied from GVM using a method described by Dooling et. al. [3]. The viscoelastic solver is verified by comparison of the numerical results with corresponding experimental data and a good agreement was demonstrated. Instantaneous elastic shear modulus, obtained from GMM, ranged from 583Pa to 1368Pa which were similar to the previous rheometry studies. In our 2D model, biofilm was considered as semi-hemispherical shape (thickness of 100μm and width of 346μm) attached to the center of the bottom boundary of the square cross-section flow cell. Fluid flow through the flow cell was in laminar regime. Simulation results show that the present method can be successfully used for prediction of the potential site of biofilm deformation in contact with the substrate. In order to develope a full fracture based FSI model to simulate multiple detachmnet paths, Cohesive Zone Model (CZM) will be implemented into the numerical code via Traction-Separation law. Further detachment experiments are ongoing wrok to validate the numerical predictions.

bio-film; viscoelastic; fluid bio-film interaction; CFD; OpenFOAM

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