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.