Advances in capillary flow porometry: Turning pore size data into practical insight

Understanding which pores control flow can improve the development and quality control of filtration and separation materials

Pore size is a familiar specification for filters, membranes and many other porous materials. Yet the number on a datasheet does not always tell us how a material will behave in use.

For applications such as filtration, battery separators, protective textiles and technical membranes, the most important pores are those that form continuous pathways through the material. The diameter at their narrowest points influence which particles can pass, which are retained and how easily a fluid can flow through the material. Measuring those pore constrictions requires a technique that reflects the way the material performs.

Capillary flow porometry (CFP) provides that perspective. It characterizes through-pores by combining a wetting liquid, controlled gas pressure and flow measurement. This makes it particularly relevant when researchers and manufacturers need to connect pore structure with filtration, separation or permeability performance.

Why conventional pore size measurements may not be enough

Microscopy, gas adsorption and mercury intrusion porosimetry provide valuable information about porous materials. However, they answer different questions.

Microscopy can show the structure of a sample surface or cross-section, but the observed area may represent only a small portion of the material. A two-dimensional image can also make it difficult to determine whether a pore is connected through the full thickness of the sample.

Gas adsorption and mercury intrusion methods are widely used to investigate pore volume and pore size in powders, pellets and other porous solids. They can assess spaces within a material, including pores that may not contribute to flow through it. For a filtration or separation material, this means that a reported pore diameter does not necessarily represent a functional pathway from one side of the sample to the other.

The distinction matters. A filter can contain many pores, but not every pore contributes equally to flow or particle retention. Performance is influenced by the connected pathways through the material and by the most restrictive point along each pathway.

Capillary flow porometry is designed to examine these functional through-pores.

How capillary flow porometry works

The measurement begins by fully wetting the sample with a liquid that fills its pores. Gas pressure is then applied across the sample. The gas displaces the wetting liquid, beginning with the largest pore throat and progressing to smaller ones as the gas pressure across the porous material is increased.

The relationship between pressure and pore diameter is based on the capillary principle described by the Washburn equation. In practical terms, a larger pore requires less pressure to empty, while a smaller pore requires more pressure. The calculation depends on the surface tension of the wetting liquid and its contact angle with the sample.

The first continuous gas pathway identifies the bubble point, which corresponds to the largest through-pore throat in the sample. Continuing to increase the gas pressure while recording increasing gas flow extends a traditional bubble point test into a full CFP analysis. This can provide several useful parameters:

• Bubble point: the minimum pressure required to force gas through the liquid-wetted largest pore (more specifically, the largest pore throat).
• Mean flow pore diameter: the pore diameter at which half of the flow passes through larger pores and half through smaller pores
• Smallest measured pore size: identified near the point where the wet flow (through wetted pores) and dry flow (through the completely unwetted material) curves meet
• Gas permeability : the ability to allow gas to pass through its pore structure

Together, these results provide a practical view of the pathways that control transport through the material.

Pressure control makes a difference

The principle behind capillary flow porometry is well established, but measurement quality depends heavily on how pressure and flow are controlled.

Traditional scan methods increase pressure continuously and record the resulting flow. They are fast and straightforward, but the selected pressure ramp can influence the result. Individual pores may differ not only in diameter but also in tortuosity and effective length. As a result, the time required for flow through each pore to reach equilibrium can vary significantly. A faster ramp may not allow sufficient time for flow to stabilize in all pores, whereas a slower ramp permits a more complete equilibration of the flow. The same sample may therefore produce different curves when measured at different ramp rates or across different pressure ranges.

Step-based measurements use a different approach. Pressure is increased to a target value and held while the system waits for flow to stabilize. This can provide more detailed information, particularly for thick, complex or previously uncharacterized samples. However, it requires the instrument to reach and maintain each pressure target accurately. Delays, pressure corrections or oscillations can change the time spent at each point and affect the measured response.

The AccuPore has been engineered to support both pressure-ramp and pressure-step methods. Its proprietary sophisticated closed-loop pressure control, with 8 transducers and cutting-edge components, is designed to achieve and maintain target pressures across changing sample conditions, while multiple flow measurement ranges support measurements from low initial flow through to higher bulk flow. This flexibility allows users to select a method that suits the structure and behavior of their material.

Improving first bubble point detection

First bubble point measurement can be challenging because the initial flow signal is close to zero. In a conventional pressure-ramp test, the instrument increases pressure and attempts to detect the first measurable flow. At this point, small changes in the detection threshold can create variability.

The AccuPore SmartFlow approach reverses this relationship. The system controls a low flow and measures the corresponding pressure response. The bubble point is detected from the non-linear change in pressure as the first pore opens. This direct approach is designed to improve the repeatability and accuracy of first bubble point measurements.

Four track-etched membranes with known differences in porosity were evaluated, yielding average first bubble point sizes of 1.166 µm, 0.309 µm, 0.091 µm and 0.054 µm. The reported relative standard deviations ranged from 0.22% to 0.83% across the four membrane samples, indicating excellent measurement repeatability. The consistently low variability observed across membranes spanning a wide range of pore sizes and porosities demonstrates the robustness of the technique and its ability to provide reliable pore size measurements under varying sample characteristics.

This level of repeatability can provide greater confidence when monitoring membrane consistency, qualifying suppliers, or detecting subtle manufacturing changes.

Choosing the right wetting liquid

The wetting liquid is not simply a sample preparation aid. Its physical properties are part of the measurement.

Surface tension, contact angle and viscosity directly affect the relationship between pressure and calculated pore size. The liquid must wet the sample effectively so that pores are filled before the test begins. One factor that is easy to overlook is vapor pressure. If the liquid evaporates during a slow measurement, pores may appear to open before the applied pressure has displaced the liquid, which can shift the apparent pore size distribution. The most appropriate liquid therefore depends on the sample, measurement range and test method.

Wetting Liquid Class	Suitability for CFP
Water	Safe, inexpensive, readily available. High surface tension limits measurements of small pores; unsuitable for hydrophobic membranes
Alcohols (ethanol, IPA, methanol)	Low cost, improved wetting compared with water. Rapid evaporation can affect stability and repeatability; may cause swelling of some polymers
Straight-Chain Fluorocarbons	Low surface tension enables measurement of smaller pores. Higher volatility and environmental concerns; less commonly used today
Perfluorinated Fluids (e.g. Galwick, PoreWick)	Industry standard for CFP. Effective wetting of very fine pores; chemically inert; highly reproducible results
Silicone Oils	Stable, non-volatile, reusable in some applications. Higher viscosity can slow pore filling and flow equilibration; may be difficult to remove completely from samples

Careful fluid selection helps reduce uncertainty and supports more meaningful comparisons between samples.

Capillary flow porometry for today’s materials

Capillary flow porometry provides valuable insight into the through-pore structure of porous materials, but obtaining the most meaningful results requires state-or-the-art understanding of all the factors that influence the measurement. Pressure control, flow detection sensitivity, wetting liquid selection, equilibration behavior and measurement rate can all affect the reported pore size distribution. As a dynamic technique, capillary flow porometry benefits from optimization of test conditions to ensure that measured values accurately reflect the transport pathways within the material.

Versatility is increasingly important for porometry. During method development, high-resolution pressure-step measurements can provide the detailed information needed to understand sample behavior, assess equilibration effects, and optimize test parameters. Once a robust method has been established, pressure-ramp measurements can be used to deliver faster analyses while maintaining sensitivity to the pore characteristics most relevant to product performance. The detailed understanding gained during method development helps ensure that rapid quality-control measurements are both efficient and scientifically justified.

By tailoring measurement conditions and data analysis approaches to the application, researchers and manufacturers can move beyond a single pore size value and develop a more complete understanding of how pore structure influences filtration, separation, permeability, and fluid transport performance. This versatility is particularly valuable when characterizing materials with broad pore size distributions, complex morphologies, or highly tortuous flow pathways.

The AccuPore is uniquely positioned to support this workflow through its combination of both pressure-step and pressure-ramp measurement modes within a single instrument platform. Compliant with ASTM F316, the system covers through-pore sizes from 0.013 µm to 500 µm, with operating pressures up to 500 psi (35 bar) and flow rates up to 200 L/min. Standalone operation via an integrated touchscreen, together with access to the MicroActive software platform, provides flexible data visualization, analysis, and export capabilities.

As laboratory workflows become increasingly digital and interconnected, open access to measurement data is essential for integrating porometry results with broader material characterization and quality-control processes. Through the combination of proven Micromeritics technology and the Malvern Panalytical materials characterization portfolio, the AccuPore provides a versatile platform for the measurement and analysis of through-pore structures across a wide range of applications.

For more information, Explore the AccuPore capillary flow porometer.