Using elastomeric materials to model biofilm physico-mechanical properties and the associated drag penalty

The physical structure and mechanical properties of a biofilm are known to respond to external stressors, such as hydrodynamic shear, and are expected to play a vital role in determining biofilm-associated drag. Yet, both, how these structural and mechanical properties interact with one another and with fluid flow, and how these interactions influence drag is poorly understood. In part this is due to a lack of standard methods for studying biofilm physical and mechanical (physico-mechanical) properties and relating them to drag. To date, rigid structures, such as sandpaper, have been typically used to model biofilms. Whilst rigid structures can simulate roughness, they neglect features such as viscoelasticity and heterogeneity. To address this, our novel work demonstrates the practical application of new test methods for biofilm research: the use of elastomeric and gel-like materials to better model biofilm physico-mechanics under controlled flow conditions, and the use of tensile and rheological testing to measure the elastic modulus of marine biofilms. Artificial biofilms were cast / made from materials with mechanical properties comparable to natural biofilms. Marine biofilms were grown in-house, within a recirculating system, using a field-sourced, mixed species inoculate. The elastic modulus of marine and artificial biofilms was measured using tensile and rheological testing.  Though elastic modulus has been recorded for biofilms previously, until now, the elastic modulus of marine biofilms had not been recorded; partly due to the complexity of their physical structures and their biological composition (bacterial and microalgal components). Despite biological differences, the elastic modulus of marine biofilms tested sits comfortably within the range recorded for other biofilms studied, at 0.0000098 MPa - 0.0002 MPa. A marine biofilm flow cell was utilised for pressure drop experiments, alongside the use of a non-invasive imaging technique, Optical Coherence Tomography. This experimental set-up enabled real-time visualisation and data collection of the physical response of the elastomers and biofilms grown in the marine environment to different flow rates. We found that for artificial biofilms, elasticity had a greater impact on biofilm-associated drag than roughness (P < 0.05). Biofilms are a unique and complex material, and therefore to better understand their physico-mechanical properties in flow, we first need to understand these properties independent of their complex biology. The use of fully artificial biofilms, with controlled properties, based on mechanical properties of marine biofilms, can help achieve this.