2.3 Rheometry analysis
In order to determine the viscoelastic parameters for the alginate andP. aeruginosa biofilms, a shear rheometer (Discovery HR-2 Hybrid, TA Instruments, Illinois, USA) was used. Two-step tests were performed in this study: (1) strain sweep tests were used to determine the range of linear viscoelastic behavior; (2) stress relaxation tests were carried out to obtain the mechanical properties of both biofilms. The methods were adapted from Towler et al. (2003). We first performed dynamic strain sweep tests. The dynamic strain sweep provides the information of storage (G′) and loss (G′′) moduli as a function of strain. Second, a strain value was chosen for stress relaxation tests within the range of linear viscoelastic behavior. The stress relaxation tests can be fitted with mathematical models and mechanical properties of biofilms can be obtained. Several mathematical models can be used to analyze the viscoelastic phenomena (Areias & Matouš, 2008), such as Maxwell model, Kelvin-Voigt model, and four-element Burgers model. An Oldroyd-B model is an extension of the upper convected Maxwell model. Therefore, we chose the Maxwell model as the mathematical model for fitting the rheometer data. Fitted data of shear modulus (\(G_{b}\)) and dynamic viscosity (\(\mu_{b}\)) were obtained, then applied to the computational model (see Sec. 2.1.3). All the rheometry analyses were performed with MATLAB (Mathworks, Natick, Massachusetts, USA, www.mathworks.com) using nonlinear least-squares solver. A detailed description of rheometer analysis and mathematical model fitting is provided in the SI.