Penetration and secondary atomization of droplets impacted on wet facemasks

Face covering, commonly known as a facemask, is considered to be one of the most effective pieces of personal protective equipment to reduce transmissions of pathogens through respiratory droplets—both large droplets and liquid aerosol particles. Facemasks inhibit the expulsion of such respiratory droplets from the user and protect the user from inhaling pathogen-laden potentially harmful droplets or their dried nuclei. While the efficacies of various dry face masks have been explored in the recent past, a comprehensive investigation of a wet mask is lacking. Yet users wear masks for an extended period and, consequently, due to the deposition of respiratory droplets released through multiple respiratory events, the mask matrix becomes wet. We, herein, present an experimental study on the dynamics of sequential impacts of droplets on masks to understand how wetness affects possible penetration and secondary atomization of the impacted droplet. Two different types of masks, hydrophobic and hydrophilic, were used in this study to evaluate the underlying physical mechanisms that control the penetration in each of them.

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Figure 1

Primary components of the experimental setup. The components are not drawn to scale.

Figure 2

High-speed image sequence of droplet impact on Mask A for first (D1), third (D3), and fourth (D4) impacts. Recording rate is 4000 fps. The impact velocity, $U$, is 2.7 m/s.

Figure 3

High-speed image sequence of droplet impact on Mask B for the first (D1), third (D3), and fifth (D5) impacts. Recording rate is 4000 fps. The impact velocity, $U$, is 3.3 m/s.

Figure 4

Regime map of three different masks: (a) Mask A, (b) Mask B, and (c) Mask C. The green diamonds and red dots denote penetration and no-penetration, respectively.

Figure 5

Dynamics of droplet impact on hydrophobic mask (Mask A). (a) Normalized volume of liquid penetrated, $V_p^{*}=V_r/V$, through Mask A. Various impact velocities $U$ (in m/s) are shown as different symbols, as noted in the legend in (c). The impact velocities $U$ (m/s) are reported in the legend with different symbols. (b) Schematic of droplet impact on dry vs. wet hydrophobic masks (not drawn to scale). (c) Normalized volume reflected (volume of “beads”), $V_r^{*}=V_r/V$, after each impact reported in (a). (d) Modified regime map as function of impact velocity $U$ and normalized reflected volume, $V_r^{*}$. Green diamonds: penetration. Red dots: no penetration. The dashed line is scaling, $U_{\mathrm{cr}}=U_0+A_r{\left(V_r^{*}\right)}^{1/2}$. Here $U_0=2.1$ m/s and scaling constant $A_r=1$ m/s.

Figure 6

Dynamics of droplet impact on hydrophilic masks (Masks B and C). [(a) and (b)] Normalized volume of liquid penetrated, $V_p^{*}=V_p/V$, through (a) Mask B and (b) Mask C. Different symbols are for various impact velocity $U(m/s)$, as shown in legends. (c) Schematic of droplet impact on dry vs. wet hydrophilic masks (not drawn to scale). (d) Spread of droplets on hydrophilic mask with increase in the number of droplet impacts.

Figure 7

Modified regime diagram for hydrophilic masks as function of impact velocity ($U$) and normalized cumulative absorbed volume ($\widetilde{V_a^{*}}$). (a) Mask B and (b) Mask C. Green diamonds: penetration; red dots: no penetration. The dashed line shows scaling: $U_{\mathrm{cr}}=U_0+B_a{\left(\widetilde{V_a^{*}}\right)}^{3/2}$.

Figure 8

PDF of diameter of secondary droplets generated during impact on (a) Mask A, (b) Mask B, and (c) Mask C reported for different impact velocities, $U$.