Structure–Transport–Efficiency Relationships in Hierarchical Multiscale Fibrous Media for Viable Bioaerosol Filtration

Clean-room and controlled environments require fibrous filtration media capable of retaining viable bioaerosols while maintaining stable airflow resistance under continuous operation. This study investigates structure–transport–efficiency relationships in three polypropylene (PP) fibrous configurations: single-layer melt-blown media (PP1), double-layer stacked media (PP2), and a hierarchical multiscale fibrous medium. Scanning electron microscopy revealed distinct architectural differences. The PP media exhibited uniform micro-scale fibers (mean diameter ≈ 1.21 µm), whereas the hierarchical medium showed a graded bimodal fiber distribution comprising coarse fibers (20–30 µm) integrated with finer submicrometer-to-micrometer fibers. Measured thicknesses were 0.51 mm (PP1), 1.64 mm (PP2), and 0.79 mm for the hierarchical medium. Pressure-drop measurements at 0.2–0.6 L min⁻¹ displayed strong linear dependence (R² > 0.99), confirming viscous-dominated transport consistent with Darcy behavior under creeping-flow conditions (Re < 0.01). Intrinsic permeability values were on the order of 10⁻¹¹ m² and remained independent of flow rate. Viable bioaerosol testing using colony-forming unit (CFU) enumeration yielded bacterial filtration efficiencies of 83.93 ± 0.07% (PP1), 93.71 ± 0.06% (PP2), and 99.83 ± 0.02% for the hierarchical configuration, corresponding to a maximum log reduction of 2.77 (p < 0.05). While geometric stacking improved retention with proportional resistance increase, hierarchical multiscale structuring achieved a superior efficiency–resistance balance by enhancing interception probability and suppressing preferential flow pathways without excessive pressure penalty. These results demonstrate that microstructural organization, rather than thickness augmentation alone, governs the optimization of viable bioaerosol filtration performance under low-Reynolds-number airflow conditions.