On secondary atomization and blockage of surrogate cough droplets in single- and multilayer face masks

Abstract

Face masks prevent transmission of infectious respiratory diseases by blocking large droplets and aerosols during exhalation or inhalation. While three-layer masks are generally advised, many commonly available or makeshift masks contain single or double layers. Using carefully designed experiments involving high-speed imaging along with physics-based analysis, we show that high-momentum, large-sized (>250 micrometer) surrogate cough droplets can penetrate single- or double-layer mask material to a significant extent. The penetrated droplets can atomize into numerous much smaller (<100 micrometer) droplets, which could remain airborne for a significant time. The possibility of secondary atomization of high-momentum cough droplets by hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the single/double-layer mask material needs to be considered in determining mask efficacy. Three-layer masks can effectively block these droplets and thus could be ubiquitously used as a key tool against COVID-19 or similar respiratory diseases.

Efficacy of homemade face masks against human coughs: Insights on penetration, atomization, and aerosolization of cough droplets

“…The movies were made using Kdenlive software. Experimental methodology is similar to Sharma et al 32 …”

“…Fluid dynamically droplet impact on a facemask is similar to impingement on porous networks like metallic wire meshes, 19–25 fibers, 26–30 and textiles. 31 Sharma et al 32 focused on the effect of the number of layers present in a surgical mask and showed that the number of cough droplets penetrating through the mask is significantly reduced with each additional mask layer. The experiments were performed for droplet sizes of ∼300–600 μ m and velocity range of 2–10 m/s (Ref.…”

“…The experiments were performed for droplet sizes of ∼300–600 μ m and velocity range of 2–10 m/s (Ref. 32 ) mimicking real cough events. The penetration of the impacting droplet depends on the surface wettability conditions, 20,23,33,34 mesh sizes, 23,24 fluid properties, 28 and impact velocities 23,27 of the droplets.…”

“…A single-layered mask efficacy is lower than a multi-layer mask and has been investigated in the existing literature. 32 However, this remains a secondary focus of our work. We primarily focus on identifying the most suitable and easily accessible fabric for making a homemade face mask, giving a framework for manufacturer/user to select a suitable material.…”

“… 40,41 However, if the mask is worn by an infected person, though it blocks a significant fraction of the droplet volume, it causes secondary atomization through the small pores of the mask fabric. 32 The daughter droplets, thus, formed (typically of size 100 m) fall within the aerosol range. Therefore, it may not be accurate to conjecture that any face mask is beneficial to curb the viral spread.…”