The motion of respiratory droplets produced by coughing

TL;DR

This study measures and models the motion of respiratory droplets from coughing, revealing how droplet size, evaporation, and weather conditions influence their travel distance, which is crucial for understanding COVID-19 transmission.

Contribution

It combines experimental flow visualization with a physical model to predict droplet behavior under various weather conditions, advancing understanding of respiratory droplet transmission.

Findings

Small droplets evaporate to droplet nuclei.

Large droplets can travel over 2 meters.

Winter conditions promote droplet settling.

Abstract

Coronavirus disease 2019 (COVID-19) has become a global pandemic infectious respiratory disease with high mortality and infectiousness. This paper investigates respiratory droplet transmission, which is critical to understanding, modeling and controlling epidemics. In the present work, we implemented flow visualization, particle image velocimetry (PIV) and particle shadow tracking velocimetry (PSTV) to measure the velocity of the airflow and droplets involved in coughing and then constructed a physical model considering the evaporation effect to predict the motion of droplets under different weather conditions. The experimental results indicate that the convection velocity of cough airflow presents the relationship $t^{-0.7}$ with time; hence, the distance from the cougher increases by $t^{0.3}$ in the range of our measurement domain. Substituting these experimental results into the physical model reveals that the small droplets (initial diameter $D \leq$ 100 $\mu$m) evaporate to droplet nuclei and that the large droplets with $D \geq$ 500 $\mu$m and initial velocity $u_0 \geq$ 5 m/s travel more than 2 m. Winter conditions of low temperature and high relative humidity can cause more droplets to settle to the ground, which may be a possible driver of a second pandemic wave in the autumn and winter seasons.