W. Richard Bowen and Nidal Hilal 4

4.6 ‘vIsUALIsATION’ OF THE REjECTION OF A COLLOId by A MEMbRANE PORE 123
by using membranes with roughness such that only adhesion at peaks is
possible – where the effects of cross-flow are also maximum.
4.6 ‘VISUALISATIOn’ OF ThE REjECTIOn OF A COLLOID
by A MEMbRAnE PORE AnD CRITICAL FLUx
One of the most useful practical operating concepts for membrane processes
is that of a critical filtration flux or critical operating pressure. These
critical parameters are such that below such critical values, rejection will
occur and fouling will be minimum and above these critical values, transmission
and fouling may take place. For colloidal particles, the critical
values may arise as a balance between the hydrodynamic force driving
the solutes towards a membrane pore and an electrostatic (electrical
double layer) force opposing this motion.
Science fiction writers have imagined tiny probes travelling, for example,
through the human body, which would allow us to visualise hidden microscopic
phenomena. Such probes remain figments of the imagination.
However, a colloid probe moving along a membrane surface can allow us to
visualise how a colloidal particle would ‘see’ such a surface, Figure 4.17 [9].
Figure 4.17 shows how a 0.75 �m silica colloid probe ‘sees’ the pores in
a 1.0 �m Cyclopore membrane in solutions of two ionic strengths when
imaged in each case with an applied force of 4.6 mN m –1 . It is only at the
highest ionic strength that the pores are clearly apparent, for at the lowest
ionic strength, such a force is experienced too far from the membrane surface
for the electric field to still show sufficient evidence of membrane
porosity. Cases where the process stream component dimensions are close
to those of the membrane pores should preferably be avoided as rapid
pore blocking may occur if the critical flux or pressure is exceeded. Such
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FIgURE 4.17 Imaging a 1.0 �m Cyclopore membrane with a 0.75 �m colloid probe, in
0.1 M NaCl, pH 8 (left); in 0.0001 M NaCl, pH 8 (right). Normalised imaging force 4.6mN m –1 .
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