Capillary Flow Porometry (also known as Liquid Expulsion Porometry or Gas-Liquid Displacement Porometry)

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Capillary flow porometry (CFP) is a technique used to evaluate the through pore size distribution and permeability characteristics of a variety of materials used as membranes and filtration media, including paper, synthetic polymers, cotton, ceramics, foams, hollow fibers, and more.

Through pores are pores that start at one end of the sample surface and empty out on the opposite end. For a material used as a separation barrier, the size distribution of the through pores can help gauge the size of the particles it can exclude as well as its flow characteristics. The Bubble Point, or the largest limiting through pore size, determines the largest particle size that could pass through the separation barrier. This parameter can be invaluable where 100% rejection of particles above a certain size is required, as well as potential detection of defects. To understand the flow characteristics of the filtration media, permeability could also be determined using an air flow or a liquid of interest.

The instrument utilized at PTL can probe through pores ranging in sizes from <0.02 to 500 µm. As such, this instrument uniquely complements PTL’s existing capabilities for characterizing open pores (any pores with opening to the sample’s surface) by gas physisorption and mercury porosimetry techniques.

For a typical pore size analysis, a sample is wetted with a liquid to fill at least all through pores and sealed into a sample holder. The wetting liquid is emptied from largest to smallest pores as increasing gas pressure (air or nitrogen) is applied to one side of the sample. The resulting flow of gas is measured until all pores are emptied. Analysis is then repeated without wetting. The pore size distribution is subsequently calculated from the wet and dry curves according to the Washburn equation. Optimization of the analysis conditions could include pressure range applied, choice of wetting liquid, and sample size. In addition to thin sheet-like membranes, tubular structures such as hollow fibers could also be analyzed using this technique.

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