TOPTITECH investigates four basic filtration mechanisms that occur when fluids pass through porous materials:
1. Surface screening: The material's surface acts as a barrier, preventing solid particles larger than the pore size from entering and blocking them, similar to a physical screen.
2. Depth screening: Solid particles larger than the pore channels get trapped at curved or constricted sections within the channels, effectively blocking their passage.
3. Depth filtration: Even solid particles smaller than the pore size of the channels can be captured within the porous material's deep layers. This retention occurs due to the presence of metal fibers or powder particles that constitute the porous material.
4. Cake filtration: Solid particles accumulate on the surface of the porous material, forming a thick layer known as a filter cake. This cake acts as the primary filtration medium, with minimal or no particle leakage. Initially, if the particle size is larger than the pore size of the material, cake filtration resembles surface screening. However, in most cases, the particles are smaller than the pore size, and they form bridges at the pore openings, gradually growing into a filter cake. Subsequently, the filter cake itself becomes the filtering medium, controlling the filtration process and performance parameters. Since the particles being filtered are mostly smaller than the pore size of the material, the "new filter" (the cake) formed by these particles has a much smaller effective pore size than the original porous material. This characteristic allows the use of porous materials with larger pore sizes to filter particles smaller than their own pore sizes.
