Abstract
Face masks play a critical role in reducing the transmission risk of COVID-19 and other respiratory diseases. Masks made with nanofibers have drawn increasingly more attention because of their higher filtration efficiency, better comfort, and lower pressure drop. However, the interactions and consequences of the nanofibers and microwater droplets remain unclear. In this work, the evolution of fibers made of polymers with different contact angles, diameters, and mesh sizes under water aerosol exposure is systematically visualized. The images show that capillarity is very strong compared with the elasticity of the nanofiber. The nanofibers coalesce irreversibly during the droplet capture stage as well as the subsequent liquid evaporation stage. The fiber coalescence significantly reduces the effective fiber length for capturing aerosols. The nanofiber mesh that undergoes multiple droplet capture/evaporation cycles exhibits a fiber coalescing fraction of 40%–58%. The hydrophobic and orthogonally woven fibers can reduce the capillary forces and decrease the fiber coalescing fraction. This finding is expected to assist the proper design, fabrication, and use of face masks with nanofibers. It also provides direct visual evidence on the necessity to replace face masks frequently, especially in cold environments.
Highlights
The COVID-19 pandemic has infected over 180 million and claimed the lives of nearly 4 million people since its outbreak in 2019.1 The consensus has been reached by the scientist community2 as well as the World Health Organization1 and the Centers for Disease Control and Prevention (CDC) in the United States3 that aerosol could be a route for the COVID-19 virus transmission
The small fiber diameter renders unique and favorable characteristics of the nanofiber based filtration materials, including: (i) the low flow resistance and low pressure drop due to possible slip effects8,9 as the diameter of the fiber is comparable with the mean free path of the air ($100 nm at 1 atm); (ii) the light weight due to a high surface area to mass ratio; (iii) the semi-transparency owing to small fiber diameters that are less than the visible light wavelength; and (iv) the strong built-in electrostatic field Es near the fiber surface that further enhances the aerosol capturing capability, as Es is inversely proportional to the fiber radius
The crossed fibers make up the majority of the mesh sample and eventually the filter mat used in face masks
Summary
The COVID-19 pandemic has infected over 180 million and claimed the lives of nearly 4 million people (as of June 2021) since its outbreak in 2019.1 The consensus has been reached by the scientist community as well as the World Health Organization and the Centers for Disease Control and Prevention (CDC) in the United States that aerosol could be a route for the COVID-19 virus transmission. The majority of the filtration layers of face masks are made through the melt blown process with polypropylene (PP). The nanofiber has emerged as a high-performance filtration material.. The small fiber diameter renders unique and favorable characteristics of the nanofiber based filtration materials, including: (i) the low flow resistance and low pressure drop due to possible slip effects as the diameter of the fiber is comparable with the mean free path of the air ($100 nm at 1 atm); (ii) the light weight due to a high surface area to mass ratio; (iii) the semi-transparency owing to small fiber diameters that are less than the visible light wavelength; and (iv) the strong built-in electrostatic field Es near the fiber surface that further enhances the aerosol capturing capability, as Es is inversely proportional to the fiber radius
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