Environmental Engineering Reference
In-Depth Information
minimal at 22−24 nM/m [34]. Zhao and co-workers have suggested that an optimal
surface free energy on which bacterial adhesion force is minimal should be between
20−30 nM/m [35].
Fracture mechanics theory states that elastic modulus is a key factor in bioadhesion
and the ability for an organism to release from a surface. Adhesion correlates better
with the square root of critical surface tension multiplied by elastic modulus than
either of the terms independently [30].
The thickness of the bioilm on the surface seems to play an important part in the
adhesion process. Below a 100 μm dry ilm thickness, barnacles can establish a strong
adhesion with the surface. Surface roughness and topography also affects bioadhesion.
The cement lows and spreads over rough surfaces. If the surrounding liquid is viscous
the cement may not low uniformly and ill all the crevices and when it solidiies it
may cause stress concentration regions. Surface roughness may also cause changes in
the surface wettability. Nanotopographies have been shown to exhibit foul repelling
properties. Other properties that play a role in the bioadhesion process are material
surface chemistry as in the case of silicon polymers which exhibit the lowest adhesion,
slippage (peeling off) exhibited by silicon elastomers, friction and lubricity exhibited
by oils. The type and amount of protein that initially form the conditioning layer alters
bioadhesion. The lower the protein adsorption, the lower will be the bioadhesion
[36]. So protein repellent surfaces are good antibiofouling surfaces.
4.8 Conclusions
Studies suggest that the surface properties (roughness, surface energy and hardness)
determine the formation of the initial bioilm and further biofouling does not seem
to depend on the original surface properties. Initial fouling on the surface is directly
proportional (positive correlation) to the surface energy. Surface energy decreases at
the end of one year. Biofouling seem to affect the physical properties of these material
which include gravimetric and thermogravimetric weight loss, tensile strength, surface
energy, and hardness. A seasonal variation of biofouling is observed. However, more
investigations are needed to validate this inding. Biofouling is minimal on silicon
rubber. Attachment of barnacles is minimal on it probably because of its lexible
nature. However, stiff surfaces attract more attachment of barnacles and polycheates.
For barnacles the nature of the material has a distinct role in controlling their adhesion
mechanism. Biochemical differences exist between the various cements (powdery
or rubbery) secreted by the barnacles that attach to different substrates, in terms of
protein content, abundance of individual protein components and the chemistry of
the polymerised cement. The doublet bands of >80 kDa proteins observed in these
cements from different substrates suggest that they may be involved in the activation
