Experimental framework to examine turbulent puffs generated by human coughing

Respiratory infections spread through pathogen-laden droplets and aerosols exhaled by humans as part of turbulent puffs. Understanding the dynamics of these puffs is essential for assessing risks and implementing effective infection control measures. This study introduces an innovative experimental framework that employs neutrally buoyant helium-filled soap bubbles to visualize and quantitatively analyze puffs expelled during respiratory exhalations. This approach allows for the exploration of flow and turbulence statistics in exhalation puffs, an area that has not been previously examined. The experimental setup employs a high-speed camera coupled with light sheet illumination in a controlled environment. This framework can measure puff trajectory and radius directly from the raw images captured by the camera or from the velocity fields obtained through particle image velocimetry. The framework was subsequently applied to 15 coughs from three subjects. Our observations of maximum and mean velocities within the puff align with previous studies on droplet flow statistics. Additionally, based on the vorticity distribution of the exhalation, we observed a two-stage evolution of the puff with an initial jet-like phase where the trajectory scales with t1/2, followed by a puff-like phase with t1/4 scaling. Furthermore, we observed an entrainment coefficient (α) of 0.32 ± 0.06 for the initial jet-like phase and 0.17 ± 0.08 for the puff-like phase. Overall, this framework offers improved insight into the transport mechanisms of respiratory aerosols by enabling the quantification of different flow statistics of turbulent puffs.