Material modeling of biofilm mechanical properties.

A biofilm material model and a procedure for numerical integration are developed in this article. They enable calculation of a composite Young's modulus that varies in the biofilm and evolves with deformation. The biofilm-material model makes it possible to introduce a modeling example, produced by the Unified Multi-Component Cellular Automaton model, into the general-purpose finite-element code ABAQUS. Compressive, tensile, and shear loads are imposed, and the way the biofilm mechanical properties evolve is assessed. Results show that the local values of Young's modulus increase under compressive loading, since compression results in the voids "closing," thus making the material stiffer. For the opposite reason, biofilm stiffness decreases when tensile loads are imposed. Furthermore, the biofilm is more compliant in shear than in compression or tension due to the how the elastic shear modulus relates to Young's modulus.

On the calculation of the elastic modulus of a biofilm streamer.

Biofilm mechanical properties are essential in quantifying the rate of microbial detachment, a key process in determining the function and structure of biofilm systems. Although properties such as biofilm elastic moduli, yield stress and cohesive strength have been studied before, a wide range of values for the biofilm Young's modulus that differ by several orders of magnitude are reported in the literature. In this article, we use experimental data reported in Stoodley et al. [Stoodley et al., Biotechnol Bioeng (1999): 65(1):83-92] and present a methodology for the calculation of Young's modulus, which partially explains the large difference between the values reported in the literature.

Variation in the mechanical properties of a porous multi-phase biofilm under compression due to void closure.

Biofilm properties change drastically from one point to another inside the matrix, and from one minute to the next, bringing about similar variations in biofilm mechanical properties, both in time and space. In this article, we present a theory that quantifies deformation-dependent changes in the mechanical properties of a composite porous material that undergoes compression. Such changes are a result of the pores either closing (when the biofilm is under compression) or opening (when under tension). The theory borrows well-established principles of continuum mechanics and is modified to represent a biofilm composed of four different phases, three different solid biomass materials (active biomass, extracellular polymers and inert biomass) and pores. We see that, when the evolution of the volume fractions of the different phases in a uniaxial compression test is taken into account, the material "hardens" or becomes stiffer as the deformation increases, due to void closure. Once complete void closure is achieved, the material reaches its maximum stiffness. Different homogenisation schemes are presented and comparisons are performed with stress-strain calculations for all of them.