Special Mathematics Colloquium by Roberto Verzicco (GSSI)

Math Colloquium: "Extended Lifetime of Respiratory Droplets (and its implications on airborne disease transmission)" Abstract: The fate of respiratory droplets under different ambient temperature and relative humidity is investigated by direct numerical simulations of a typical respiratory event. We find that small droplets (with initial diameter of 10  μm) are swept by turbulent eddies in the expelled humid puff and their lifetime gets extended by a factor of more than 30 times as compared to what is suggested by the classical picture by Wells, for 50% relative humidity. With increasing ambient relative humidity the extension of the lifetimes of the small droplets further increases and goes up to around 150 times for 90% relative humidity, implying more than 2 m advection range of the respiratory droplets within 1 sec. Using Lagrangian statistics, we show that the turbulent humid respiratory puff engulfs the small droplets, leading to orders of magnitude increase in their lifetimes, thus implying that they can be transported much further during the respiratory events than the large ones. Ambient relative humidity and temperature interact nonlinearly making possible, within supersaturated respiratory puffs, the droplets growth in cold and humid environments. Our findings provide the starting points for larger parameter studies and may be instructive for developing strategies on optimizing ventilation and indoor humidity control. Such strategies might be useful in mitigating the COVID-19 pandemic.