1 Bubble Point Testing
1 1 Bubble Point Theory
Ideally think the pores in the membrane are spread evenly and with the same size. If the filtration membrane is completely wetted, capillary will happen in membrane pores. When one side of the pore is pushed by air, penetrative liquid will be prevented by capillary pressure and surface tension. With air pressure up to one value, liquid in capillary is just pushed out, and air flow out through the pore. The air pressure value is bubble point pressure. As Figure 1:
Figure1 Imaginary Capillarity in membrane pores
Surface tension of any one point in a circle: fr= kγcosθ
Total surface tension: Fr= KπDγcosθ
At the moment of reaching bubble point, pressures on two sides of liquid are balanced: Pπ( D/ 2)2 = KπDγcosθ
Then bubble point pressure formula: P= 4Kγcosθ/D
P: bubble point pressure K: membrane pores correction coefficient D: membrane pores diameter γ: liquid surface tension θ: contact angle of solid & liquid
1 2 Testing Procedure
( 1) Wet filter cartridges fully by purified water ; ( 2) Open vent valve1, vent valve2; ( 3) Blow compressed air and add the pressure gradually, at the same time watch the outlet and changing pressures; ( 4) When there are continuous or steady bubbles coming out from outlet, record current pressure value, which is bubble point pressure.
1 3 Testing Device as Figure 2
Figure 2 Bubble Point Testing Device Show
Bubble point pressure formula shows that the pressure is in proportion to surface tension, in inverse proportion to pore size. So any factor affecting surface tension γ and θangle——wetting liquid, temperature, membrane character all could affect bubble point value. As well, bubble point pressure could tell whether membrane pores are suitable to remove bacteria.
2 1 Influence of wetting and wetting liquid Since bubble point testing requires membrane to be fully wetted, then membrane wetting directly influence testing results.
2 11 Use clean wetting liquid. Clean wetting liquid could make sure membrane fully wetted. If membrane is used or treated by surfactants, membrane must be well-cleaned first.
2 12 Choice of wetting liquid Up to membrane character, purified water for hydrophilic membranes, 60% IPA for hydrophobic membranes.
2 13 Wetting method: Usually immersed in purified water. Flush long-used filter cartridges with flow water instead of static immersion. Or wet by 65℃ water, then cooled to room temperature.
22 Temperature influence Usually offered bubble point value is tested at room temperature. A number of testing prove that bubble point value at 23℃ is the steadiest. The higher temperature is, the farther bubble point value deviates. Lower temperature won’t affect bubble point value much, but it’s not conducive to practical operation.
23 Membrane influence
231 Using factor influence: Membrane pores may be clogged or changed after multiple using. So for used membranes, bubble point value get larger gradually first due to partly clogged, then get lower rapidly, even couldn’t be tested due to broken. Thus, offer correct prefiltration and proper flow rate of filtrate when using sterile filter cartridges.
232 Filtration area influence: Initial bubbles may accumulate among the center of cartridge without leaving, which leads to bubble point value higher than actual one, easily causing wrong result.
233 Sterilization influence: High temperature sterilization change surfactant of membrane, changing wetting character of membrane, then bubble point value will be changed