W. Richard Bowen and Nidal Hilal 4

122 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
The adhesion of the colloid probe is markedly lower at the peaks on
the membrane surface than in the valleys, with the difference increasing
with decreasing salt concentration, and reaching a factor of more than 20
in 10 �3 M solution. The wide variation in interactions shows that theoretical
descriptions of membrane fouling need to take surface morphology
into explicit account. Further, the results show that the selection of membranes
for the filtration of specific process streams would benefit from an
assessment of the size of likely foulants and the membrane roughness,
especially the periodicity of the roughness. Fouling could be minimised
F/R / mN/m
6
4
2
0
–2
–4
–6
Peak
Valley
–8
0 50 100 150
Separation distance / nm
FIgURE 4.16 Adhesion of a silica colloid probe at a peak and a valley on an AFC99
membrane in 10 �3 M NaCl solution (pull-off force).
TAbLE 4.4 Normalised Adhesion Forces for silica Colloid Probes and AFC99
Membrane.
Surface type [NaCl] (M) F/R (mN/m)
Membrane peak 10 �3 �0.3 (�0.17)
10 �2 �1.4 (�0.43)
10 �1 �2.3 (�0.48)
Membrane valley 10 �3 �6.5 (�2.2)
10 �2 �8.1 (�3.5)