Capillary flow porometry

In order to measure the pore size by capillary flow porometry it is necessary to impregnate the samples with a wetting liquid.

[3] The most constricted part of the pore is the most challenging one because it offers the highest resistance to remove the wetting liquid.

This measured pressure permits obtaining the pore diameter, which is calculated by using the Young-Laplace formula P= 4*γ*cos θ*/D in which D is the pore size diameter, P is the pressure measured, γ is the surface tension of the wetting liquid and θ is the contact angle of the wetting liquid with the sample.

This is the traditional approach, in which the pressure increases continuously at a constant rate (figure 1), which can be modified depending the instrument and the user's requirements, and the gas flow through the sample is measured.

It is a fast and reproducible method that is generally recommended for quality control work and for samples with all pores identical.

The acquisition of a data point is only carried out after holding the pressure at a constant value for a user-defined time (see figure 2) and also only after the gas flow through the sample is stable, which is also defined by the user.

This allows enough time for the gas flow to displace the wetting liquid in long and tortuous pores of the same diameter.

Some common wetting liquids used in capillary flow porometry include water, alcohols, silicone oil and perfluoroethers.

The use of water and/or alcohols present the disadvantage that they can evaporate and therefore the samples can partially dry before the actual porometry test begins.

Silicone oil does not present the evaporation drawback but its high viscosity makes the cleaning of the equipment parts in between different measurements not easy.

Figure 1. Continuous increase of pressure in time.
Figure 2. Increase of the pressure with respect of time in steps, holding the actual pressure for a certain time before increasing it to the next value.
Figure 1. Parameters measured in porometry.