Fundamental Principle: The process involves using a specific membrane material that is fully wetted with a liquid capable of penetrating its pores. This liquid is held within the membrane's pores due to surface tension. As gas pressure is gradually increased on one side of the membrane, the pressure eventually exceeds the surface tension in certain pores, causing the liquid to be expelled by the gas. Smaller pores require higher gas pressure to push out the liquid because the surface tension is greater at smaller diameters. Therefore, the largest pores are the first to open, allowing gas to pass through. As pressure continues to rise, progressively smaller pores become open, enabling gas to permeate until all pores are fully opened, achieving the same transmittance as a dry film.
The pressure at which the first pore opens is known as the bubble point pressure, and the corresponding pore size is considered the maximum pore size. During this process, both pressure and flow rate are continuously recorded to generate a pressure-flow curve. This curve provides critical information about pore size distribution and the number of pores at different sizes. By testing the pressure-flow curve of the dry membrane, it is possible to calculate key parameters such as the maximum pore size, average pore diameter, minimum pore size, pore size distribution, and overall transmittance of the membrane sample using appropriate formulas.
The relationship between pore size and applied pressure is described by the Washburn equation:
D = 4γCosθ / P
Where:
- D = pore diameter
- γ = surface tension of the liquid
- θ = contact angle
- P = pressure difference across the membrane
Pore size distribution can be calculated using the following formula:
f(D) = -d[(Fw/Fd) × 100]/dD
Where:
- Fw = flow rate of the wet sample
- Fd = flow rate of the dry sample
This method allows for an accurate and detailed characterization of the membrane structure, making it a valuable tool in quality control and performance evaluation of filtration and separation membranes.
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