Step 1: Understanding the Concept:
Filtration can occur via different mechanisms: sieving (size exclusion), adsorption, or entrapment. Membrane filters are thin, polymeric films with a highly uniform pore structure.
Step 2: Detailed Explanation:
1. Size Exclusion: Membrane filters act as a sieve. Any particle larger than the designated pore size (e.g., 0.22 \(\mu\)m) is physically blocked from passing through. This is the primary mechanism used for cold sterilization of heat-sensitive liquids.
2. Adsorption: This occurs when particles stick to the filter medium due to electrostatic or surface forces. While some membrane filters may have minor adsorptive properties, it is not their main mechanism.
3. Sedimentation: This is the settling of particles due to gravity, which is not a filtration mechanism.
Step 3: Final Answer:
The primary characteristic of a membrane filter is its "absolute" pore size, which operates through size exclusion.