Digital in-line holography to explore saliva aerosolization mechanisms in speech

Phonation produces aerosols, and speech has proven to be a pernicious, invisible, yet potent viral transmission route among asymptomatic individuals in the COVID-19 pandemic. To develop comprehensive mitigation strategies and mechanisms to control transmission, it is crucial to measure both the amount of aerosols produced by a speaking individual and the associated atomization processes. However, these biological emissions are hard to characterize since they are intermittent, faint, range drastically in size between 1 and 1000 mu m, and form and travel rapidly, with speeds of a few to tens of meters per second. Here, we present digital in-line holography (DIH) as a lens-less and powerful tool to overcome many of these experimental challenges. Focusing on speech-induced aerosolization, we demonstrate the capacity of DIH to image the rapid formation, elongation, and deformation of saliva filaments in three dimensions and directly at the mouth of a speaking individual. The size of both the filaments and the subsequent aerosolized droplets are accessible over a field of view spanning several centimeters cubed. We quantify the droplets produced by distinct plosive consonants (/p/ and/t/) and we observe how filaments destabilize with a characteristic whipping dynamic, emitting rotational dumbells with liquid lobes linked by thinning saliva threads. We also image high-speed air fronts produced during plosion, with speeds as fast as tens of meters per second, classifying plosive consonant phonation as a violent atomization event like sneezing.