Fluid dynamics of speech: Mechanisms underlying pathogen transmission

Howard A. Stone,

Princeton University

Abstract

Speech is a potent route for asymptomatic 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 nor has the relationship of speech to the spatial and temporal characteristics of the exhaled flow been documented. Here we show with high-speed imaging how phonation of common stop-consonants 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 small droplets. These plosive consonants induce starting jets and vortex rings that drive meter-long transport of exhaled air, tying this drop-formation mechanism to the major transport process associated with speech; the transport features, including quasi-steady conical jets and phonetics, are demonstrated using order-of-magnitude estimates, numerical simulations, and laboratory experiments. These ideas also make possible a “space-time” social distancing diagram. We hope that this work will inform thinking about the role of ventilation, aerosol transport in disease transmission for humans and other animals, and yield a better understanding of “aerophonetics''.

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