Capillary Flow Porometry (CFP) has limitations when it comes to measuring pore sizes in the submicron and nanometer range. The maximum pressure allowed with CFP, 35 bars or 500 psi, is not high enough to displace the wetting liquid out of the small pores (sizes below 15 nm diameter).
An alternative to CFP for the characterisation of micro and nanopores is Liquid Liquid Displacement Porometry (LLDP). The measurement consists on the impregnation of the porous sample with a wetting liquid but, unlike CFP, the displacement of the wetting liquid is carried out by using a second liquid immiscible with the first one (called displacement liquid) at increasing pressure.
Likewise, the pressure is used to calculate the pore size by using the Young-Laplace equation.
P=4*γ*cos θ/D (1)
where (P) is the pressure required to displace the wetting liquid from the pore, (γ) is the interfacial tension (between the two liquids), (θ) the contact angle and (D) is the pore diameter. Because the surface tension at the interface between the two liquids is much lower than the surface tension at the interface gas-liquid, LLDP allows measuring much smaller pores than CFP without the need of applying high pressures. LLDP is also ideal for full characterization of hollow fibres at low pressures, unlike CFP, when normally hollow fibres burst or the collapse due to mechanical damage due to the application of high pressures.