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.