By porosity one means the percentage of a filter that is its voids or pores; that is, the ratio of space to solid in the membrane matrix.
A given porosity can be constituted of a random combination of various numbers of pores, nonuniform in their lengths and diameters
and of various shapes. The more numerous the pores, the more likely a higher flux. However, the lengthier the pores, the lower
their individual flow rates. Pores of less-restricted diameters would flow faster but would tend to be less retentive. The
filter's sieve retention would be decided by the size of its widest restrictive diameters. It is this dimension that will
decide the largest particle to escape capture by penetrating the pore.
As a foreseeable consequence of random pore arrangements, the same flux rates could characterize two membranes differing from
one another in their mix of longer and shorter pores and of wider or narrower restrictive diameters, as modified by more or
fewer pores. Likewise, the random balancing of numbers, lengths, and widths of pores could produce filters of the same porosity
but differing in their flux and retention. As said, the pore-size ratings are indicative of particle retentions. Flow rates
are not a function of retention ratings. Individual filters of different pore-size ratings could be characterized by similar
rates of flow or even by flow rates opposite from those expected on the basis of the pore-size (retention) rating. Membranes,
if any, based on a randomness in pore formation could be of a same pore-size rating, indicative of retentions, but of higher
porosities and flow rates. This would depend upon the random mix of pore numbers, lengths, and widths. Longer pores, the accompaniment
of thicker membranes produced by filter layering, exhibit reduced flows but tend to greater retentions. Pall and Kirnbauer
showed that layering in increasing the membrane thickness, elevated the bubble point to some leveling value (25).
Figure 4: Modelling a fibrous filter by a system of lines drawn at random (n = 25).
Certain filters, especially depth filters composed of fibrous structures, are manufactured by a technology that produces filters
of randomly arranged pore structures; hence, their broad pore-size distribution (see Figure 4). Membranes fabricated using
the casting technique, however, are characterized by the narrow pore-size distribution that is the result of the physical
laws governing solutions. Within the volume of any solution, an equilibration by diffusion, and hastened by stirring, results
in the solute molecules becoming evenly spaced from one another. The casting formula is such a solution of polymer dissolved
in solvent. Within this solution the polymeric segments are, therefore, essentially equally spaced from one another. It is
this relative regularity of intersegmental spacing that prefigures the similarly sized pores of the finished membranes. The
resulting pore structure is, thus, not the product of random mixes of various pore lengths and diameters but rather the consequence
of influences directed to pore homogeneity. It bespeaks a greater regularity of the pore structures constituting the porosity.
The pores of cast microporous membranes should, therefore, be of a rather uniform size. This is, indeed, the case as shown
by their narrow pore-size distributions (see Figure 5).
Figure 5: Automated flow pore measurements of two different 1.2 μm membranes.
The possibilities are unlikely of pores of such regularity in size exhibiting identical retention ratings but differing in
flow rates. Except for the irregularities introduced by pore-size distributions, organism retention, flux, and porosity should
be parallel expressions of the membrane's pore size. This assumes, however, the use of identical polymers, casting solutions,
and converting techniques, along with standard systems of measurements. None of these conditions are likely met by competitive
filter manufacturers using proprietary practices. Therefore, comparisons, as of flux or of retentions, between filters of
various manufacture may not usefully be extended to speculations concerning their structural features or performances.