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Item Rheology of biofilms(2003) Winston, Matthew T.; Rupp, Cory J.; Vinogradov, A. M.; Towler, Brett William; Adams, Heather; Stoodley, PaulThe paper describes an experimental study concerning the mechanical properties of bacterial biofilms formed from the early dental plaque colonizer Streptoccocus mutans and pond water biofilms. Experiments reported in this paper demonstrate that both types of biofilms exhibit mechanical behavior similar to that of rheological fluids. The time-dependent properties of both biofilms have been modeled using the principles of viscoelasticity theory. The Burger model has been found to accurately represent the response of both biofilms for the duration of the experiments. On this basis, the creep compliances of both biofilms have been characterized, and the respective relaxation functions have been determined analytically.Item Viscoelastic properties of a mixed culture biofilm from rheometer creep analysis(2003-10) Towler, Brett William; Rupp, Cory J.; Cunningham, Alfred B.; Stoodley, PaulThe mechanical properties of mixed culture biofilms were determined by creep analysis using an AR1000 rotating disk rheometer. The biofilms were grown directly on the rheometer disks which were rotated in a chemostat for 12 d. The resulting biofilms were heterogeneous and ranged from 35 microns to 50 microns in thickness. The creep curves were all viscoelastic in nature. The close agreement between stress and strain ratio of a sample tested at 0.1 and 0.5 Pa suggested that the biofilms were tested in the linear viscoelastic range and supported the use of linear viscoelastic theory in the development of a constitutive law. The experimental data was fit to a 4-element Burger spring and dashpot model. The shear modulus (G) ranged from 0.2 to 24 Pa and the viscous coefficient (eta) from 10 to 3000 Pa. These values were in the same range as those previously estimated from fluid shear deformation of biofilms in flow cells. A viscoelastic biofilm model will help to predict shear related biofilm phenomena such as elevated pressure drop, detachment, and the flow of biofilms over solid surfaces.Item Evapotranspiration crop coefficients for cattail and bulrush(2004-05) Towler, Brett William; Cahoon, Joel; Stein, Otto R.Accurate estimates of evapotranspiration from constructed wetlands are required to establish design flow estimates and to assess the effectiveness of wetland water quality amelioration. Water consumption by two wetland plant species, Typha latifoilia (broadleaf cattail) and Scoenoplectus acutus (hardstem bulrush), was measured in a greenhouse for eight months. Measurements of actual evapotranspiration (ETC) from replicates of both plant treatments were related to potential evaporation (ET0) as approximated by evaporation from saturated gravel beds. Ratios of ETC to ETO used to develop crop coefficients (KC) for each plant species. The relationship between cattail ETC/ET0 and the ratio of vegetative to open water surface area (SV/S0) agreed with previous investigations. A linear relationship was used to account for advective energy fluxes due to peripheral canopy area. Cattail crop coefficients were scaled according to this relationship. The resulting scaled crop coefficient curve may be transferable to constructed wetlands of a known SV/S0.Item A model of fluid-biofilm interaction using a burger material law(2007-02) Towler, Brett William; Cunningham, Alfred B.; Stoodley, Paul; McKittrick, Ladean RobertA two-dimensional finite element model of the biofilm response to flow was developed. The numerical code sequentially coupled the fluid dynamics of turbulent, incompressible flow with the mechanical response of a single hemispherical biofilm cluster (100 µm) attached to the flow boundary. A non-linear Burger material law was used to represent the viscoelastic response of a representative microbial biofilm. This constitutive law was incorporated into the numerical model as a Prony series representation of the biofilm's relaxation modulus. Model simulations illuminated interesting details of this fluid-structure interaction. Simulations revealed that softer biofilms (characterized by lower elastic moduli) were highly susceptible to lift forces and consequently were subject to even greater drag forces found higher in the velocity field. A bimodal deformation path due to the two Burger relaxation times was also observed in several simulations. This suggested that interfacial biofilm may be most susceptible to hydrodynamically induced detachment during the initial relaxation time. This result may prove useful in developing removal strategies. Additionally, plots of lift versus drag suggested that the deformation paths taken by viscoelastic biofilms are largely insensitive to specific material coefficients. Softer biofilms merely seem to follow the same path (as a stiffer biofilm) at a faster rate. These relationships may be useful in estimating the hydrodynamic forces acting on an attached biofilm based on changes in scale and cataloged material properties.Item Evapotranspiration crop coefficients for two constructed wetland macrophytes : cattail and bulrush(Montana State University - Bozeman, College of Engineering, 1999) Towler, Brett WilliamItem Modeling the non-linear response of mixed culture biofilm structures to turbulent flow(Montana State University - Bozeman, College of Engineering, 2004) Towler, Brett William; Chairperson, Graduate Committee: Ladean McKittrick.Microbial biofouling of wetted interfaces can negatively impact the hydrodynamic performance of pressurized conduits. These impacts are due, in part, to the material properties of biofilm, yet few studies have examined this polymeric substance in the context of a constitutive relation. The goal of this research was two-fold; 1) to determine a suitable constitutive model for a mixed-culture biofilm and 2) use this material model in a numerical simulation to evaluate biofilm mechanical behavior in response to varying hydrodynamic conditions. Creep tests revealed that these biofilms may be classified as viscoelastic fluids. Furthermore, results indicated the presence of viscous, time-dependent and instantaneous components to the biofilm compliance functions. A regression analysis (r2 = 0.8819) supported the treatment of these samples as linear viscoelastic fluids within the stress range of 0.1 Pa to 0.5 Pa. A specific linear viscoelastic constitutive equation was then determined by fitting experimental results to analytical solutions using an optimization algorithm. It was found that the Burger material model closely approximated the behavior of all samples. A numerical fluid-structure interface model was then developed and employed in a parametric study to investigate biofilm behavior. The effect of the Burger material parameters, mean flow velocity and biofilm size were examined. Simulations showed that weaker or softer biofilms (characterized by lower elastic moduli) were highly susceptible to lift forces. Additionally, polar diagrams were generated by plotting the coefficients of drag versus lift. The plots suggested that in the first few seconds after loading, the deformation paths taken by hemispherical biofilms are largely insensitive to specific material coefficients. Moreover, the diagrams illustrated that the effects of biofilm strength, size and channel velocity on displacement were predictable. These relationships may lead to the development of a simple, yet accurate method for predicting the hydrodynamic forces acting on an attached biofilm.