Frequently asked questions

How does gas liquid porometry work?

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.

How does liquid-liquid porometry work?

Capillary Flow Porometry (CFP) has limitations when it comes to measuring pore sizes in the submicron and nanometre 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 of 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=4γcos 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) is 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 to apply high pressures. LLDP is also ideal for the full characterisation of hollow fibres at low pressures, unlike CFP, when normally hollow fibres burst or collapse due to mechanical damage due to the application of high pressures.

What is measured with a Porometer?

The equilibrium conditions can be expressed as: 2π r γ cos θ = r² π h ρ g …..(1) Where: 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.

What is the difference between measured and calculated first bubble Point?

One of the most important parameters measured by a porometer is the first bubble point or FBP. This point corresponds to the largest pore(s) present inside the material.

The ASTM F-316 standard defines the FBP at the pressure at which the first continuous bubbles are detected. This is based on the traditional approach of placing the sample in a housing with liquid placed on the top side. Then a pressurised gas is applied under the sample and the pressure is gradually increased over time and when a constant flow of rising bubbles is observed on the top side of the sample, it is assumed that the gas pressure has reached bubble point.

However, this is a visual and, therefore, subjective approach. When do we consider that the first continuous bubbles are detected? It differs significantly depending on the person.

For that reason the FBP can be defined at different flow rates, e.g. at 30, 50, 100 ml/min. So for a certain target flow the pressure required to achieve it is used to calculate the pore size, using the Young-Laplace equation as previously explained. Because with this approach there is already flow at the FBP, by definition, this calculated FBP is always smaller than the real bubble point and thus the calculated FBP never represents the real opening of the largest pores.

There are multiple criteria to select the pressure to calculate the FBP. A POROLUX™ user has the choice to select among different calculation methods.

There is another, more accurate, approach for detecting the largest pore. It is the so-called measured bubble point.

The fully wetted sample inside the sample chamber forms a closed system. If we increase the pressure on the sample using a small, constant flow of gas towards the sample chamber, as the volume is fixed, this constant flow will result in a linear rise of the pressure above the sample. At the moment the first and largest pore is opened, there will be a change in the linear pressure increase. This change can be regarded as the true first bubble point of the material and this pressure is used to obtain the pore size. This method to measure the FBP shows an excellent accuracy and reproducibility.

What is the difference between pressure scan and pressure step stability?

Based on many years of experience in capillary flow porometry we have developed POROLUX™, a range of instruments for testing membranes, filters, nonwovens, papers, hollow fibres and ceramics, amongst other. They are widely used to measure pore size distribution and gas permeability with improved accuracy and reproducibility compared to other porometers in the market.

PRESSURE SCAN METHOD: POROLUX™ Cito (fka 100 - 200 - 500) SERIES PRESSURE STEP/STABILITY METHOD: POROLUX™ 1000 SERIES Control of the pressure increase With a single valve, which is continuously being opened during the measurement With a cascaded pressure control set up, using a specially designed needle valve Stability algorithms No Yes, for pressure and gas flow Measurements Immediate, continuous measurement of both pressure and gas flow.

The pressure increases at a constant rate, which can be selected by the user. A data point is only recorded when the stability algorithms (defined by the user) are met for both pressure and flow.

The porometer detects when a pore empties at a certain pressure and waits until all pores of the same diameter have been completely emptied before accepting a data point. Advantages Very suited for quality control work The most suitable for research and development work. Essential for samples with complex pore structures Disadvantage Pressure regulation is sometimes not linear over the entire pressure range (high P). Slower measurements Key words Speed and reproducibility Precision and accuracy

What wetting liquid should I choose?

A good wetting liquid for capillary flow porometry should have the following properties:

• Zero contact angle
• Low surface tension
• Low vapor pressure

Also, it has to be chemically inert and should not cause swelling of the sample.

In many cases, the best results are obtained by using fluorinated hydrocarbons. These liquids have low surface tension (around 16 dyn/cm) and low vapour pressures, which minimises any potential evaporation during the measuement. Sometimes silicone oils are used. They have a high viscosity, a low surface tension and a low vapor pressure. The disadvantage of silicone oils is that it is hard to clean them out of inserts. the sample holder and supports. The residual silicone oil can contaminate subsequent tests, which can prevent the porometer from getting good dry curves.

The use of water is not recommended because it tends to evaporate during measurements, which may lead of an overestimation of the pore sizes (the pores are detected as open too early). Moreover, the surface tension of water is rather high (72 dyn/cm), which means that for a given pore size a much higher pressure than when measuring with Porefil or Galpore is required. Furthermore, water, like some alcohols, often cause swelling of samples.

We recommend using as much as possible the same liquid, because, in theory, as long as you apply the correct vapor pressure, different liquids should give the same results, but in practice it is not always the case.

Porometer offers different types of wetting fluids in different quantities. All of our wetting fluids come with a certificate showing the measured surface tension.