Aerosol filtration fundamentals: Particles most efficiently captured by Brownian diffusion in filters are typically of what size range?

Introduction / Context:
Air and gas sterilizing filters used in bioprocessing remove particulates and microorganisms by multiple mechanisms: Brownian diffusion, interception, impaction, and sometimes electrostatic attraction. Understanding which particle sizes are dominated by diffusion helps in selecting appropriate media and validating retention for viruses, spores, and fine aerosols.

Given Data / Assumptions:

• Low Reynolds number flow through fibrous media.
• Particle densities near unity, negligible gravitational settling at submicron sizes.
• Filtration velocities typical of HEPA or process vent filters.

Concept / Approach:
Brownian diffusion is strongest for very small particles with high thermal velocity. As particle diameter decreases below ~0.3 μm, random motion increases collisions with fibers, enhancing capture. Larger particles (> ~0.3–0.5 μm) are captured more by interception and inertial impaction, while diffusion becomes less significant relative to inertia.

Step-by-Step Solution:
Identify the diffusion-dominant regime: ultrafine particles.Relate thermally driven motion to diameter: smaller particles diffuse more.Therefore, the most characteristic range for diffusion capture is < 0.25 μm.Select the option matching this range.

Verification / Alternative check:
Most-penetrating particle size (MPPS) for HEPA filters is ~0.1–0.3 μm because it is the crossover between diffusion-dominated and interception/impaction-dominated capture, underscoring the strong role of diffusion below ~0.25 μm.

Why Other Options Are Wrong:
Larger thresholds (<0.5, <0.75, <1 μm) include sizes where interception or impaction contribute substantially; diffusion is not the primary mechanism across those broader ranges.

Common Pitfalls:

• Assuming a single mechanism applies across all sizes; real filters use combined mechanisms.
• Ignoring electrostatic charges that can augment capture outside the diffusion regime.

Final Answer:
< 0.25 μm