W. Richard Bowen and Nidal Hilal 4

2.4 AdHESION FORCES MEASUREd By AFM 69
only the colloid probe method can measure the adhesive forces between
particles and membrane surfaces and hence allow prediction of the membrane
fouling properties of the particles. In addition, the ability to make
measurements in liquid allows the matching of observation conditions to
those that occur in practice.
Studies have also been carried out on the adhesion forces between
calcium carbonate crystals and stainless steel surfaces. The adhesion of
CaCO 3 to steel process equipment surfaces plays an important role in the
formation of scale deposits in desalination plant equipment, as well as in
domestic appliances such as kettles and other household appliances. Al-
Anezi et al. [157] measured the adhesion between CaCO 3 probes and stainless
steel surfaces of different grades of roughness. In Figure 2.7 is an SEM
image showing an example of a CaCO 3 crystal mounted on an AFM cantilever.
The roughest surface had the lowest adhesion than the two smoother
surfaces examined. The effect of liquid environment was also examined.
Adhesive forces measured in sea water to which 2 ppm of an anti-scaling
agent had been added were significantly more reduced than when measured
in sea water alone. In addition, the proportion of measured force
curves that showed no observable adhesion rose from 1% of all obtained
force curves in sea water to 57% with the addition of the anti-scale agent.
FIGuRE 2.7 SEM image of a CaCO 3 crystal mounted upon an AFM cantilever for use in
determining adhesion with stainless steel surfaces.