Gas Liquid Porometry, also known as Capillary Flow Porometry (CFP), measures pore size and pore size distribution of through pores in materials. The technique is based on the displacement of an inert and nontoxic wetting liquid embedded in a porous network by applying an inert pressurised gas. Therefore, only through pores are measured.
Larger pores become empty first and, as the applied pressure increases, so do the smaller ones until all through pores are empty.
The most challenging part for the gas to displace the liquid along the entire pore path is the most constricted section, also known as pore throat. The diameter measured in CFP is the pore throat, regardless of where it exactly in the pore path is.
The method depends upon the capillary rise created by the surface tension between the liquid and the gas. Therefore, a wetted pore immersed in a liquid draws the liquid up the capillary until reaching equilibrium with the force of gravity. The equilibrium conditions can be expressed as:
2π r γ cos θ = r² π h ρ g …..(1)
r= radius of the capillary (or pore)
D= diameter of the capillary (or pore)
h= height of column of liquid
γ= surface tension of liquid
ρ= density of liquid
θ= contact angle between the liquid and capillary wall
g= acceleration due to gravity
and since pressure (P) = hρg, and D = 2r equation 1 becomes
2π r γ cos θ = r² π P …..(2)
P = 4 γ cos θ / D …..(3)
Therefore, the pressure required to empty pores of a certain diameter is inverse proportional to the pore throat size