IB-FT GmbH

Measurement principle of capillary flow porometry


A capillary flow Porometer uses the existing physical interaction between liquids and a capillary space: the liquid is sucked into this capillary opening. Different parameters determine the height obtained by the liquid inside this capillary opening: the diameter of the opening, air pressure, the interphase interaction between the liquid and solid, the density of the liquid, temperature, viscosity, etc. Already in 1921, Washburn considered this existing relationship between pressure and pore size.

He established a simplified rule for this effect, whether a liquid is pushed into the pores as in mercury porosimetry or a liquid is pushed out of the pores as is the case in capillary flow porometry. The following relationship is valid for these methods:

Pressure = 4 * γ * cos θ * (shape factor) / diameter

 

The instrument will build up pressure inside the pressure chamber (yellow zone) starting from ambient pressure, over a wetted sample (in red). Gradually the pores will start to open starting with the largest pores and a gas flow will be measured.

The pressure build up is performed until all pores are opened and the sample has completely dried during this so-called wet curve. The instrument deflates and a second run is performed, called the dry curve. This curve is necessary for the calculation of all measured parameters: largest and smallest pores, mean flow pore size, and so on. The pressure may be increased by the pressure control in two differentiated ways. A first one is the linear increase of pressure over time with immediate data sampling for pressure and flow. We provide this pressure scan methodology in our POROLUX™ 100 series. In the POROLUX™ 1000 series, the pressure is increased in different steps.

With the pressure in bar, the diameter in μm, γ the surface tension of the wetting liquid, θ the contact angle of the liquid on the solid. The shape factor is a parameter depending on the shape and the path of the pore inside the material. The surface tension γ is a measurable physical property and is available for many liquids.

The contact angle θ however depends on the interaction between the material and the wetting liquid. Typical wetting fluids used in porometry are perfluoroethers. They have a low surface tension and a contact angle of 0° with nearly all materials. A capillary flow porometer consists of one or more pressure and flow sensors with a separate pressure control.

 At each pressure step, both pressure and flow are monitored and a datapoint is taken when stabilization criteria are met. This pressure step/stability method provides more accurate information, and can also be used at increased pressures.
Porometer.com also allows you to determine the first bubble point with the calculated bubble point and/or the “true” measured bubble point method.