The Principle of Capillary Flow Porometry (CFP)
Capillary flow porometry (CFP) is based on the displacement of a wetting liquid inside a porous network by means of an inert gas flow.
The wetting liquid enters spontaneous the pores in a material as a result of the capillary force until the height of liquid equals with the gravity force.The Young-Laplace equation establishes the relationship between the radius of a capillary and the pressure necessary to displace the liquid of such capillary: Pressure = 4 * γ * cos θ * (shape factor) / diameter
Where γ is the surface tension of the wetting liquid, θ the contact angle of the liquid on the solid. The shape factor is a correction factor parameter that depends on the shape and the path of the pore inside the material (if known).
The surface tension γ is a measurable physical property and is available for many liquids. The contact angle θ depends on the interaction between the material and the wetting liquid. Typical wetting fluids used in porometry are fluorinated hydrocarbons. They have a low surface tension and a contact angle of 0° with nearly all materials.
A typical porometry measurement requires impregnation the sample with a wetting liquid. The pressure builds up inside the pressure chamber starting from ambient pressure and the pores start to open gradually, starting with the largest pore, and the flow of gas through the sample is measured. The pressure increases until the wetting liquid is out of all pores (the sample has completely dried). This measurement is known as wet curve. The instrument deflates and a second run (dry run) is performed, in which the flow rate through the sample is measured. From these measurements it is possible to obtain information on the largest and smallest pores, mean flow pore size, and the distribution of pore sizes in the sample.
There are two measurement methods in porometry, which differ in how the pressure increase is controlled:
In pressure scan, the pressure increases over time at a constant speed linear and data points are continuously recorded. The POROLUX™ 100 and 500 series are based on this procedure.
In the POROLUX™ 1000 series, the pressure is increased in different steps. At each pressure step, both pressure and flow are monitored and a data point is taken when stabilization criteria are met. This pressure step/stability method provides more accurate information, and can also be used at increased pressures. The POROLUX™ 1000 gives the option to determine either the calculated first bubble point or the “true” measured bubble point.