Lagrangian Particle Methods for Biological Systems - Porous Media: Applications in Biological Systems and Biotechnology - page 264

FIGURE 7.4
DPD simulations of the growth and deformation of a relatively rigid and
strong biofilm. Medium gray represents the substratum, dark gray represents
the flowing fluid, and light gray represents the biofilm. Snapshots are taken
at simulation times t = 500, 550. . . 900.
is controlled by the magnitude of biofilm-biofilm DPD particle-particle inter-
action. For all three investigations, the liquid-biofilm interaction was assumed
to be the same as the liquid-liquid interaction, and the biofilm-biofilm interac-
tion was tuned to simulate soft, medium, and rigid biofilms. In all simulations,
the only length scale of relevance is the cut-off distance r 0 = 1. The DPD par-
ticle density was set to ρ = 4, and the liquid-liquid interaction strength was set
to S u =18 . 75 to match the compressibility of water (Groot and Warren 1997).
A dimensionless force, g =0 . 02, was applied on each DPD particle to sustain
the fluid flow from left to right. The liquid-biofilm interaction was the same
as the liquid-liquid interaction, and the biofilm-biofilm interaction strengths
were set to S bb =0 . 25 S u , S u , and 4 . 0 S u , respectively, for soft, medium, and
rigid biofilms.
For soft biofilm, the biofilm DPD particle-particle interaction strength is
small. The biofilm structure is strongly deformed and stretched in the fluid
flow direction by the shear stress exerted by the flowing liquid, and the biofilm
masses can very easily be detached. The biofilm spreads over the substratum
rather than growing toward the middle of the channel. For medium modu-
lus biofilm, the structure is still strongly influenced by the liquid flow, and it
forms an asymmetric shape. Medium strength biofilm can grow into the liquid,
but not as far as the stiff biofilm. This results in a lower biofilm production
rate compared to stiffer biofilms, because the substrate concentration is lower
close to the substratum and higher deep in the liquid. As expected for rigid
biofilms, the hydrodynamic interactions do not have a significant effect on
the biofilm morphology. The biofilm grows preferentially toward the left (up
stream direction) because more substrate is available on the left-hand side.
For both medium and strong biofilms, detachment did not occur in the simu-
lations, but longer simulations would lead to larger biofilm masses and larger
stresses at the biofilm-substratum interface, which could lead to detachment.