Microscopy of Woven and Nonwoven Face Covering Materials: Implications for Particle Filtration

• Published in: Microscopy and microanalysis 2024-03, Vol.30 (1), p.27-40
• Main Authors: Vicenzi, Edward P, Whittaker, Scott, Weaver, Jamie L, Staymates, Matthew E, Radney, James G, Zangmeister, Christopher D
• Format: Article
• Language: English
• ISSN: 1431-9276; 1435-8115
• DOI: 10.1093/micmic/ozad138

A suite of natural, synthetic, and mixed synthetic-natural woven fabrics, along with nonwoven filtration layers from a surgical mask and an N95 respirator, was examined using visible light microscopy, scanning electron microscopy, and micro-X-ray computed tomography (µXCT) to determine the fiber diameter distribution, fabric thickness, and the volume of solid space of the fabrics. Nonwoven materials exhibit a positively skewed distribution of fiber diameters with a mean value of ≈3 μm, whereas woven fabrics exhibit a normal distribution of diameters with mean values roughly five times larger (>15 μm). The mean thickness of the N95 filtration material is 1093 μm and is greater than that of the woven fabrics that span from 420 to 650 μm. A new procedure for measuring the thickness of flannel fabrics is proposed that accounts for raised fibers. µXCT allowed for a quantitative nondestructive approach to measure fabric porosity as well as the surface area/volume. Cotton flannel showed the largest mean isotropy of any fabric, though fiber order within the weave is poorly represented in the surface electron images. Surface fabric isotropy and surface area/volume ratios are proposed as useful microstructural quantities to consider for future particle filtration modeling efforts of woven materials.