Stretching and break-up of saliva filaments during speech: A route for pathogen aerosolization and its potential mitigation

Speech is a potent route for viral transmission in the COVID-19 pandemic. Informed mitigation strategies are difficult to develop since no aerosolization mechanism has been visualized yet in the oral cavity. Here we show with high-speed imaging how phonation of common stop consonants, found in most of the world's spoken languages, form and extend salivary filaments in a few milliseconds as moist lips open or when the tongue separates from the teeth. Both saliva viscoelasticity and airflow associated with the plosion of stop consonants are essential for stabilizing and subsequently forming centimeter-scale thin filaments, tens of microns in diameter, that break into speech droplets. Moreover, these plosive consonants induce vortex rings that drive meter-long transport of exhaled air, tying this mechanism to transport associated with speech. We demonstrate that a similar mechanism of aerosolization occurs during the vibration of reeds in wind instruments and may occur during the flapping of the glottis folds. Finally, our research suggests a mitigation of droplet production during speech by using a lip balm.

Figure 1

Plosives, flow, and droplets. (a) Velocity field and (b) vorticity field of the airflow induced by a speaker on the right saying “Pa” adjacent to a laser sheet. (c) Two images of the laser sheet when the speaker says “Pa” or “Ta” directed perpendicular to and at 15 cm from the sheet. The maximum intensity has been accumulated over 0.1 s for visualization purposes. (D) Temporal cumulative number of the light flashes representative of the passage of droplets through the laser sheet as a function of time, from repetitively saying “$X\mathrm{a}-\mathrm{a}X\mathrm{a}-\mathrm{a}X$,” with $X=/\mathrm{p}/,/\mathrm{b}/,/\mathrm{k}/,/\mathrm{d}/,/\mathrm{t}/$, and /f/, with a rapid inspiration in between (approximately 10 to 12 times). We also show the results of saying ten times the vowels /a/,//,/i/,/o/.

Figure 2

Film-to-filament-to-droplet. Sequence of images while saying “Pa” in “PaPa” with (a) the initial film formation and destabilization to form filaments, (b) closer view of the junction of the lips as the filaments form, and (c) subsequent sequence of the aerodynamic extension and snapping of the filaments. (d) Measurements of the projected length as a function of time for three filaments (continuous lines) and the associated separation distance between the lips (dashed lines). (e) A flying filament exhibiting the beads-on-a-string instability.

Figure 3

Filaments and plosive types. (a) Filament stretching and wiggling while saying the second “Ba” in “BaBa” with the associated intensity kymograph. (b) Sequence of one filament stretching while saying the first “Ma” in “MaMa” without visible destabilization from the airflow. (c) Sequence of film-to-filament destabilization between the tongue and the front teeth while saying the second “Ta” in “TaTa.”

Figure 4

A lip balm mitigates formation of small droplets. (a) Sequence of images showing the opening of the lips covered with a lip balm while saying the first “Pa” in “PaPa.” (b) Saliva droplet spread between two fingers (left) without and (right) with the balm. (c) Temporal average cumulative number of the light flashes representative of the passage of droplets while saying ten times “Pa” over four runs with and without the balm. (d) Droplets production from the mouthpiece (Meyer, Ebonite, Medium Chamber) of a saxophone alto mounted with a wood reed (D'Addario Select Jazz 2H): (i) side view, (ii) inside view, (iii) laser sheet image projected over 0.1 s when blowing through the mouthpiece constantly, and (iv) high-speed image sequence of filaments and droplets produced between the reed and the tip of the beak, viewed looking towards the beak.