Threads, Sprays, and Charges: An Exploration of Face Mask Filtration Science

Abstract

In the wake of the COVID-19 pandemic, the critical role of face masks in mitigating the spread of airborne pathogens has been clearly underscored. An unprecedented demand for medical-grade masks led to widespread use of alternative materials and highlighted the need for accurate and standardized testing of mask performance metrics. This thesis addresses key challenges in aerosol filtration testing and mask material evaluation through three interconnected studies. First, we investigated the impact of aerosol particle charge on the measurement of particle filtration efficiency (PFE) in face masks. By testing several common mask materials including woven cotton, spunbond polypropylene, and meltblown polypropylene, we demonstrated that unneutralized aerosols can significantly inflate PFE measurements due to electrostatic effects, particularly with polystyrene latex (PSL) particles. Using an aerodynamic aerosol classifier coupled with a scanning mobility particle sizer (AAC-SMPS), we characterized the bipolar charge distributions of both PSL and sodium chloride (NaCl) aerosols. Our findings emphasize the necessity of incorporating aerosol charge neutralizers in testing protocols to ensure accurate and comparable PFE results across different materials and standards. Second, we conducted a systematic evaluation of six different twin-fluid air-assisted atomizers to assess their stability, reproducibility, and the characteristics of the aerosols they generate under varying operational conditions. The study revealed that recirculating atomizers generally produced more stable aerosols with narrower particle size distributions, making them suitable for applications requiring high aerosol uniformity. In contrast, single-pass atomizers offer precise control over liquid feed rates but exhibited greater variability and broader size distributions. Operational parameters such as air pressure, liquid feed rate, and solution concentration were found to significantly influence aerosol properties. These insights provide valuable guidance for selecting and optimizing atomizers for specific applications requiring consistent aerosol generation. Lastly, we analyzed one hundred different commercially available fabrics to identify physical characteristics that contribute to effective cloth mask performance. Using principal component analysis, we determined that fabrics with a combination of higher weight, thickness, and thread diameter, moderate-to-low thread counts, and low porosity tend to achieve a favorable balance between filtration efficiency and breathability. The presence of brushed textures and exposed fibers was also associated with improved performance. These findings challenge some conventional assumptions about fabric selection for cloth masks and highlight the importance of considering multiple structural features simultaneously. Overall, this body of work enhances our understanding of aerosol filtration testing and mask material performance. By addressing the effects of particle charge on filtration measurements, evaluating atomizer designs and operational conditions, and identifying key fabric properties for cloth masks, we contribute to the development of more accurate testing methods and effective personal protective equipment. This research aids public health efforts in mitigating the transmission of airborne pathogens.