On Secondary Atomization and blockage of surrogate cough droplets in single and multi-layer face masks

TL;DR

This study reveals that single and double-layer masks can cause secondary atomization of cough droplets, increasing aerosolized particles, whereas triple-layer masks effectively prevent penetration and reduce infection risk.

Contribution

It introduces the concept of secondary atomization of droplets in masks and compares the effectiveness of different mask layers in preventing aerosol generation.

Findings

Single-layer masks atomize about 70% of droplets.

Double-layer masks trap 90% of droplet volume.

Triple-layer masks significantly prevent droplet penetration.

Abstract

By now it is well-understood that the usage of facemasks provides protection from transmission of viral loads through exhalation and inhalation of respiratory droplets. Therefore, during the current Covid-19 pandemic the usage of face masks is strongly recommended by health officials. Although three-layer masks are generally advised for usage, many commonly available or homemade masks contain only single and double layers. In this study, we show through detailed physics based analyses and high speed imaging that high momentum cough droplets on impingement on single- and double-layer masks can lead to significant partial penetration and more importantly atomization into numerous much smaller daughter droplets, thereby increasing the total population of the aerosol, which can remain suspended for a longer time. The possibility of secondary atomization of high momentum cough droplets due to impingement, hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the mask has not been explored before. However, this unique mode of aerosol generation poses a finite risk of infection as shown in this work. We also demonstrate that in single layer masks close to 70 % of a given droplet volume is atomized and only 30 % is trapped within the fibers. The entrapped volume is close to 90 % for double layer masks which still allows some atomization into smaller droplets. We however found that a triple-layer surgical mask permits negligible penetration and hence should be effective in preventing disease transmission.