In contrast to the Granulometry method, Porosimetry is an experimental physical method to determine the size of the pore space.

When measured by porosimetry in laboratory tests (e.g. mercury intrusion porosimetry or liquid extrusion porosimetry), a non-wetting fluid is pressed through the pore space into the structure. Non-wetting means that the invading fluid and the solid materials have a contact angle larger than 90°. The smaller the pore space, which means in the pore throats, the more pressure is needed to pass it. The pore space behind, the pore body, is flooded instantly until smaller passages are reached. There, a higher pressure is needed to permeate the pore throat. Via the Young-Laplace / Washburn equation

the applied pressure is related to the size of the pore throat, where is the radius of the pore throat, is the surface tension and is the contact angle.

In the example used in the Theoretical Basics section, we let a fluid intrude the structure first from left to right (Y- is the intrusion side) and then from right to left (Y+ as intrusion side).

Looking at the results, we can observe that at a lower pressure the red area gets filled. In the volume fraction histogram, this corresponds to the peak at the right with a pore diameter of 1.75 µm. At a larger pressure the yellow throat is passed, and the green area gets filled. The associated diameter (0.75 µm) of the green area is nearly the same as the diameter of the yellow throat and causes the peak at the left side of the histogram.

Thus, the pore bodies are not measured as large pores, because they are hidden behind smaller pore throats. Therefore, the pore bodies are only measured as having the diameter of the smallest pore throat between the intrusion side of the fluid and the corresponding pore body. Furthermore, closed pores are not recognized as they have no connection to the intrusion side of the fluid. Thus, the closed pore at the top has a diameter of 0 µm and colored blue.

Consequently, the computed pore size distribution by porosimetry takes the connectivity of the pores into account. In this case, the definition of what is a pore diameter changes slightly because in fact pore throat diameters are measured:

A voxel is part of a pore with a diameter equal or larger than , if it is included in a sphere of diameter , which is completely included in the pore space and can be moved in through the pores connected with the intrusion side(s) of the fluid.

This definition does not lead to unique pore diameters for the same pore space! The pore diameters are dependent on the chosen intrusion sides.

Looking at the example, where the fluid enters the structure on the right side of the structure, we can see that the histogram and the pore size distribution is mirrored compared to the first porosimetry run. As before, first the red volume is filled with the fluid, but now a bigger fraction of the pore space is occupied. The associated diameter of this red volume is the same as for the red volume in the example before. This is because the structure is periodic in y-direction and the pore throats on both sides of the structure have the same diameter.

The red volume is represented by the peak at the right of the volume fraction histogram. At a higher pressure first the yellow pore throat is passed and then the green volume is filled with the fluid.

We can observe that the pore body on the left side of the structure is assigned to a smaller diameter if the fluid enters from the right side of the structure, compared to the case where the fluid enters from the left. The reason is, that in the first case the fluid has to pass the narrowest point first before entering the left pore body. The diameter of the pore body is then assigned to the diameter of the pore throat.