Characterization of expiration air jets and droplet size distributions immediately at the mouth opening

Abstract

Size distributions of expiratory droplets expelled during coughing and speaking and the velocities of the expiration air jets of healthy volunteers were measured. Droplet size was measured using the interferometric Mie imaging (IMI) technique while the particle image velocimetry (PIV) technique was used for measuring air velocity. These techniques allowed measurements in close proximity to the mouth and avoided air sampling losses. The average expiration air velocity was 11.7 m/s for coughing and 3.9 m/s for speaking. Under the experimental setting, evaporation and condensation effects had negligible impact on the measured droplet size. The geometric mean diameter of droplets from coughing was 13.5 μm and it was 16.0 μm for speaking (counting 1-100). The estimated total number of droplets expelled ranged from 947 to 2085 per cough and 112-6720 for speaking. The estimated droplet concentrations for coughing ranged from 2.4 to 5.2 cm^-3 per cough and 0.004-0.223 cm^-3 for speaking.

Keywords: Coughing; Expiratory droplets; Interferometric Mie imaging; Particle image velocimetry; Speaking.

Fig. 1

Schematic diagrams of the expiratory droplet investigation setup.

Fig. 2

Measured air velocity fields for coughing and speaking from a male volunteer. The block arrows indicate mouth position.

Fig. 3

(a) Droplet size distribution for coughing and (b) droplet size distribution for speaking.

Fig. 4

(a) Predicted horizontal air velocities at horizontal distances up to 100 mm and (b) predicted distribution of air temperature and RH at horizontal distances up to 100 mm.

Fig. 5

Effect of the shifting of the refractive index and the observation angle on the calculated droplet size.