W. Richard Bowen and Nidal Hilal 4

130 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
Figure 4.25 shows a summary of the adhesion data measured between
the probe and each of the stainless steel surfaces of interest.
As can be seen, sample (c) has the lowest adhesion value measured,
with the probe detaching at a mean force of 65.5 nN. Samples (b) and (a)
showed higher adhesion, with values of 99.3 and 83.2 nN, respectively.
As a general rule, roughness on two interacting surfaces reduces the area
of contact between these surfaces, which would be expected to lead to
a decrease in the measured adhesion. This could explain the increase
in adhesion seen with surfaces (b) and (a) compared with the relatively
rough surface (c). However, the least rough sample (a) has a lower adhesion
than that of sample (b). If there were a simple relationship between
the roughness of the surfaces and adhesion, this would not be expected
to be the case. However, the relationship between roughness and adhesion
is not easily defined and can be dependent upon the length scales
of the roughness on the individual surfaces as well as the geometry of
interaction of the surfaces. In some cases, if the length scales of the probe
or asperities on the probe are similar to the roughness of the surface, it
could be possible for an increased contact area to be achieved [13–16].
4.8.1 Effect of Electropolishing of Steel Surfaces
Electropolishing is a process of dissolving the very top layer of a metal
surface to bring about an increase in the optical brightness or reflectivity
of a surface by the reduction of surface roughness. One advantage of this
over polishing of surfaces by mechanical means is the absence of defects
in the surfaces due to scratches. As the characterisation of the roughness
Mean adhesion (nN)
120
100
80
60
40
20
0
Polished steel Unpolished steel
sample surface
Corroded steel
FIgURE 4.25 Summary of measured adhesion values between a calcium carbonate colloid
probe and three steel sample surfaces.