Bubble point of a membrane is the pressure at which air passes through the pores of an already wetted membrane. After the membrane is wetted, a thin layer of liquid forms on the surface of the membrane, and the membrane doesn’t dry out when we simply remove the liquid in the filter channels without applying high enough pressure to the upstream.
The bubble point is a function of many factors, such as pore size, filter medium wettability, surface tension, etc. When pressure is applied to a filter membrane, bubbles will form from the largest pores first. The bubble point is the gas pressure at this time. Therefore, it only indirectly measures the size of the largest pore on the filter rather than providing information on pore size distribution/variability. However, this information by itself is already a good verification of the pore size and membrane integrity; therefore, the bubble point test can be used for confirmation.
Bubble point test:
- Wet the membrane. For hydrophilic membranes, water is used, and for hydrophobic membranes, isopropyl alcohol can be used.
- Apply compressed air to the upstream side of the membrane filter while submerging the downstream side in water.
- Continuously increase the pressure until bubbles can be seen existing in the submerged downstream tubing. The pressure at this point is defined as the bubble point.