Annual Report 2008.pdf - SAMSI

Biofilms are complicated, living conglomerates of microbes (bacteria, archaea, algae, ...) and
self-secreted material (polysaccharides, proteins, DNA, vesicles, ...)found nearly ubiquitously in
wet and damp environments. In fact, almost half of the earth's biomass (and maybe even more)
may exist in the form of biofilms. An estimated 2/3 of human bacterial infections involve
biofilms.
Biofilm structure plays an important role in their function and control. It is thus important to
determine physical properties of biofilms regarded as materials. Measurements indicate that
biofilms can be characterized as viscoelastic fluids. Observations demonstrate the tendency for
material failure via sloughing of large chunks. Qualitative and quantitative study of these
combined and interrelated physical and material phenomena are important for understanding
long-time biofilm behavior. We present a combination of experimental and modeling methods
with the goal of addressing biofilms as physical materials and, in particular, understanding their
response to imposed mechanical stress.
Using the immersed boundary method, a numerical method which allows coupling of elastic and
viscous forces within an interacting fluid-structure system, we have developed a code capable of
simulating the interaction of a viscoelastic, breakable material with a bulk shear flow. The
biofilm model consists of discretized particles connected by generalized, viscoelastic springs.
Spring forces are distributed within the fluid flow, and, in turn, fluid flow advects the discretized
biofilm, all done in such a way as to obey momentum and mass conservation laws. We will
present example simulation results.
Use of these generalized, viscoelastic springs requires calibration from measurements of biofilm
constitutive properties. Available measurements from bulk rheology allow us to conclude that
biofilms behave as viscoelastic fluids, providing significant constraints on choice of spring
model. These measurements also provide some information on material constants which we can
translate into spring parameter values. However, biofilms are heterogeneous materials and it is
plausible to expect that constitutive heterogeneity in particular may have important implications
for mechanical behavior. Since bulk rheology can only provide macroscale information, we have
begun making microscale observations of biofilm material properties using recently developed
brownian motion - Stokes-Einstein relation techniques. We will present preliminary results using
this method.
Harun Kurkcu
University of Minnesota
School of Mathematics
kurkcu@math.umn.edu
“High-Frequency Scattering by Infinite Periodic Rough Surfaces”
Yingjie Liu
Georgia Institute of Technology
Department of Mathematics
yingjie@math.gatech.edu