Environmental Engineering Reference
In-Depth Information
(25.5 mN/m). The strongest adhesion occurs between two materials with equal
polarity. Interaction forces of polar, negatively charged molecules are stronger
than non-polar forces acting across hydrophobic interfaces. Initially, this led to
the easy detachment of bacteria from hydrophobic substratum materials with high
surface tension. However, attachment of bacteria and diatoms improved with time.
The attachment remained weaker on surfaces with surface tensions between 20 and
25 mN/m than on surfaces with higher surface tensions.
Muthukumar and co-workers [7] in their study on various polymers and composites
such as, PU, silicone rubber (SR), PET, GFRP, carbon ibre reinforced plastic (CFRP)
and syntactic foams immersed for a year in marine waters found that SR with lowest
surface energy and lexible surface was the least fouled. Fouling load, attachment of
barnacles and polychaetes was correlated with surface energy and surface hardness
with maximum barnacle attachment found on a hard surface, GFRP. This indicates
that hydrophobic surfaces get fouled more than hydrophilic surfaces. Studies
conducted by Artham and co-workers [47] with the polyoleins: HDPE, LDPE and
PP also showed maximum biofouling on the most hydrophobic material.
The main limitation of the Baier curve is that it explains relative adhesion as a
function of the γ
c
of just one surface. However, it is not correct to correlate adhesion
with one surface alone, one also has to consider both the surfaces and the energy at
their interface [45]. From the viewpoint of the surface energy, relative adhesion is
not related to the γ
c
of the adherends. Relative adhesion is lowest when the energy at
the interface is at a minimum. The interfacial energy of the surface can be reduced by
matching the γ
c
of the polymer and the adherend, and by minimising the interactions
of the adhesive with the surface and water [46].
2.4.1.2 Elastic Modulus and Bioadhesion
Elastic modulus (E) is a key factor in the adhesion of biofouling organisms to a
substrate. Bioadhesion correlates directly with E (
Figure 2.10
) [45]. Low bioadhesion
coincides with the lowest value of E, even if it is not the lowest surface energy.
The mobility of the low modulus surface allows the adhesive to slip during interfacial
failure, minimising the energy required to attain the failure [45]. Newby and co-
workers [48], revealed that adhesive slips on a polydimethylsiloxane (PDMS) surface
towards the crack tip, minimising the extension of the polymer and thus, lowering
its tensile stress as well as the energy needed to spread the crack. This was also
demonstrated by Brady, Jr., [45] who showed that PDMS (E = 0.002 GPa) had a
low bioadhesion when compared to luoropolymers or hydrocarbon polymers such
as polyvinylidene luoride (γ
c
= 25.0 mN/m, E = 1.2 GPa) or PE (γ
c
= 33.7 mN/m,
E = 2.1 GPa).
