Sterile Filtration
FDA-mandated bubble point testing for 0.2/0.22 μm sterilizing filters. Pre-use/post-use integrity testing ensures bacterial retention. CFP validates filter ratings and lot consistency.
Pillar III: Through-Pore Analysis
Capillary Flow Porometry
Capillary Flow Porometry (CFP) characterizes only the through-pores that permit fluid transport across membranes and filters. By measuring gas flow through wet and dry samples, CFP determines bubble point (largest pore), mean flow pore size, and complete pore size distribution from 0.013 to 500 μm — providing direct correlation to filtration performance.
0.013 μm
Min Pore Size
500 μm
Max Pore Size
Through
Pore Type
ASTM F316
Key Standard
On This Page
- 1. Physical Principle & Young-Laplace Equation
- 2. Bubble Point Theory & Measurement
- 3. Wet/Dry Curve Method
- 4. Mean Flow Pore Size Determination
- 5. Pore Size Distribution Analysis
- 6. Wetting Fluid Selection
- 7. Liquid-Liquid Porometry
- 8. Darcy's Law & Permeability
- 9. Standards & Test Protocols
- 10. Applications & Case Studies
Fundamental Theory
Physical Principle & Young-Laplace Equation
Capillary flow porometry exploits the Young-Laplace equation to determine through-pore sizes by measuring the pressure required to displace wetting liquid from pores. Unlike porosimetry techniques that measure all accessible pores, CFP exclusively characterizes pores that contribute to fluid transport.
Young-Laplace Equation for CFP
d = 4γ cos θ / ΔP
Where:
d = pore diameter (μm)
γ = surface tension (dynes/cm)
θ = contact angle (typically 0°)
ΔP = differential pressure (psi or Pa)
Key Differences from MIP:
• Wetting fluid (θ ≈ 0°) vs. non-wetting
• Positive pressure displaces liquid
• Measures only through-pores
• Non-destructive testing
Through-Pore vs. Total Porosity
Through-Pores (CFP)
- ✓ Connect both surfaces
- ✓ Permit fluid flow
- ✓ Determine permeability
- ✓ Control filtration rating
- ✓ Relevant for separation
All Pores (MIP/BET)
- • Include blind pores
- • Include closed pores
- • Affect density
- • Influence absorption
- • Impact mechanical properties
Measurement Principle
CFP operates on a simple principle: a completely wetted membrane prevents gas flow until sufficient pressure overcomes the capillary forces holding liquid in the pores. The measurement sequence involves:
- Sample wetting: Membrane immersed in low surface tension liquid that spontaneously fills all pores
- Pressure application: Gradually increase gas pressure on one side of the membrane
- Liquid displacement: At critical pressure, liquid is expelled from largest pores first
- Flow measurement: Monitor gas flow rate as function of pressure
- Comparison with dry: Repeat measurement on dry sample for reference
Critical Measurement
Bubble Point Theory & Measurement
The bubble point represents the pressure at which the first continuous gas flow occurs through a wetted membrane, corresponding to the largest through-pore diameter. This critical parameter serves as the primary quality control metric for membrane manufacturing.
Bubble Point Detection Methods
Visual Method (Manual)
Observe first continuous bubble stream in liquid above membrane. Simple but subjective.
- • Accuracy: ±5-10%
- • Range: 0.1-100 μm pores
- • Standard: ASTM F316 Method A
- • Best for: QC spot checks
Pressure Hold Method
Monitor pressure decay at fixed volume. Sharp increase in decay rate indicates bubble point.
- • Accuracy: ±2-3%
- • Automated detection
- • No visual observation needed
- • Integrity test compatible
Flow Rate Method (Preferred)
Detect onset of gas flow through wetted membrane. Most accurate and reproducible.
- • Accuracy: ±1%
- • Fully automated
- • ASTM F316 Method B
- • Industry standard
Bubble Point Calculations
Integrity Testing Applications
💡 Industrial Application:
Bubble point testing is mandatory for sterile filtration validation in pharmaceutical manufacturing. FDA requires bubble point verification for every sterilizing-grade filter (0.2 μm) before and after use. A decrease in bubble point indicates membrane damage or pore enlargement that could compromise sterility assurance.
Core Methodology
Wet/Dry Curve Method
The wet/dry curve method forms the foundation of modern capillary flow porometry, providing complete pore size distribution from a single measurement sequence. By comparing flow rates through wet and dry samples, the contribution of each pore size to total permeability is quantified.
Measurement Sequence
-
Wet Curve: Gradually increase pressure on wetted sample
- No flow until bubble point reached
- Flow increases as smaller pores empty
- Approaches dry curve at high pressure
-
Dry Curve: Measure flow through dry sample
- Linear relationship (Darcy's law)
- Represents maximum flow capacity
- Reference for pore size calculations
-
Half-Dry Curve: Calculate 50% of dry flow at each pressure
- Intersection with wet curve = mean flow pore
- Key parameter for filter rating
Data Analysis & Interpretation
Pore Size Distribution Calculation
The differential distribution is calculated from the wet and dry curves:
f(d) = -[Fw/Fd] × d[Fw/Fd]/dd
Where Fw = wet flow, Fd = dry flow at same pressure
This gives flow-weighted pore size distribution
Characteristic Parameters
From Wet/Dry Curves
- • Bubble point (first flow)
- • Mean flow pore size
- • Smallest detected pore
- • Pore size distribution
- • Permeability
Quality Metrics
- • Distribution width (σ)
- • Uniformity ratio
- • Flow capacity
- • Tortuosity factor
- • Filter rating correlation
Key Parameter
Mean Flow Pore Size Determination
Mean flow pore size represents the pore diameter at which 50% of total flow passes through larger pores and 50% through smaller pores. This flow-weighted average better predicts filtration performance than number-based or volume-based averages.
Why Mean Flow Pore Size Matters
Flow through a pore is proportional to d⁴ (Hagen-Poiseuille equation), so larger pores dominate flow contribution:
- • A single 10 μm pore carries same flow as 10,000 pores of 1 μm
- • Number-average would underestimate effective pore size
- • Mean flow pore correlates with particle retention rating
Relationship to Filter Ratings
Method Optimization
Wetting Fluid Selection
Proper wetting fluid selection is critical for accurate CFP measurements. The fluid must spontaneously wet the membrane material (θ ≈ 0°), have low volatility, and not interact chemically with the sample.
Commercial Wetting Fluids
Galwick (Standard)
• Surface tension: 15.9 dynes/cm
• Composition: Perfluoroether
• Advantages: Low γ, inert, non-toxic
• Range: 0.02-500 μm
• Most common for general use
Silwick (Silicone)
• Surface tension: 19.1 dynes/cm
• For silicone-based membranes
• Better wetting on hydrophobic
• Range: 0.03-500 μm
• Pharmaceutical applications
Mineral Oil
• Surface tension: 31 dynes/cm
• Low cost alternative
• Good for large pores
• Range: 0.1-500 μm
• Industrial applications
Water/Alcohols
• Surface tension: 22-73 dynes/cm
• For hydrophilic membranes
• Isopropanol common
• Range: 0.5-500 μm
• Evaporation concerns
Selection Criteria
⚠️ Critical Considerations:
- • Complete wetting essential (θ < 10°)
- • No membrane swelling or dissolution
- • Low vapor pressure to prevent evaporation
- • Surface tension matched to pore size range
- • Chemical compatibility verification required
Advanced Technique
Liquid-Liquid Porometry
Liquid-liquid porometry extends measurement capability to smaller pores (down to 2 nm) by using immiscible liquid pairs instead of gas-liquid systems. This technique is essential for ultrafiltration and tight nanofiltration membranes.
Principle & Advantages
Why Liquid-Liquid?
- Lower interfacial tension: Enables measurement at lower pressures
- No compressibility: More accurate flow measurements
- Smaller pores: Can measure down to 2 nm
- No evaporation: Stable during long measurements
Common pairs: Water/alcohol, Water/oil mixtures
Interfacial tension: 1-10 dynes/cm
Pressure range: Up to 1000 psi
Pore range: 2 nm - 2 μm
Applications & Limitations
Ideal Applications
- • Ultrafiltration membranes
- • Tight nanofiltration
- • Virus removal filters
- • Protein separation membranes
- • Dialysis membranes
Limitations
- • Complex fluid preparation
- • Membrane compatibility issues
- • Difficult cleaning
- • Limited fluid choices
- • Higher equipment cost
Industry Applications
CFP Applications Across Industries
Capillary flow porometry is the standard method for quality control and development of filtration membranes, providing direct correlation between pore structure and separation performance.
Water Treatment
Microfiltration (0.1-10 μm) and ultrafiltration (0.01-0.1 μm) membrane QC. Mean flow pore correlates with molecular weight cutoff (MWCO). Monitor fouling via pore size shifts.
Battery Separators
Separator porosity affects ionic conductivity and dendrite prevention. Typical pore size 0.03-1 μm for Li-ion. CFP ensures uniform pore distribution preventing hot spots.
Air Filtration
HEPA/ULPA filter testing per EN 1822. Bubble point indicates maximum penetrating particle size. CFP validates filter class (H13-U17) specifications.
Bioprocessing
Virus removal validation requires log reduction value (LRV) correlation with pore size. CFP determines critical pore size for 20 nm parvovirus retention.
Fuel Filtration
Aviation fuel filters require precise pore control (0.5-5 μm). Water separator coalescer testing. CFP per SAE ARP901 for qualification.
Medical Devices
Venting membranes for IV bags, dialyzers, and wound dressings. CFP ensures proper gas exchange while maintaining bacterial barrier.
Nonwovens & Textiles
Breathability and moisture vapor transmission rate (MVTR) correlation. CFP characterizes pore structure in protective clothing and surgical drapes.
Next Steps
Need Membrane Pore Analysis?
Whether you're qualifying new membranes, troubleshooting filtration issues, or validating manufacturing processes, accurate through-pore characterization is essential. Get expert guidance on test methods and data interpretation.