Impact of Salivary Rheology on Expiratory Aerosol Formation in the Vocal Folds during Phonation

More people die from lower respiratory diseases, like influenza and COVID-19, than any other type of infectious disease. After much early confusion during the COVID-19 pandemic, the scientific consensus now is that many airborne diseases are spread via "aerosols," which are very tiny droplets emitted by humans when they speak or otherwise exhale. These tiny droplets are much too small to see, but they are sufficiently large to carry viruses or other pathogens. Although it is believed that many tiny droplets are formed at the vocal cords during speech, to date no work has directly examined this hypothesis. In this research project, a trained medical doctor will insert a fiber optic camera through the nose of human study participants to provide the first direct video observations of the vocal cords during droplet formation. Simultaneous experiments by engineers will measure the overall rate of droplet emission, as well as the 'viscosity' or thickness of the saliva in each participant. The research will thus directly test the hypothesis that the rate of droplet emission responsible for air-borne disease transmission is directly related to the viscosity of the saliva in infected individuals. The droplet formation rate is hypothesized to be governed by a balance of elastic, capillary, and inertial effects in the saliva that lines the glottis, as characterized by the Deborah and Ohnesorge numbers. Each time the vocal folds move apart (up to hundreds of times per second), thin fluid filaments are stretched and eventually pinch apart, yielding satellite droplets that are caught in the expiratory airflow and ultimately exhaled into the surrounding environment. The research team includes an otolaryngologist with much experience visualizing patients' vocal folds using a laryngoscope (which features a fiber optic cable inserted through the nose). A stroboscopy video system will be used to directly visualize and record the vocal folds in vivo of participants during vocalization at systematically varied loudness, while simultaneous measurements of the expiratory aerosol emission rate will be performed using an aerodynamic particle sizer. Saliva samples from each participant will be collected to measure the storage and loss moduli of the saliva with a double-gap geometry rheometer, and to measure the extensional viscosity and drop formation dynamics using capillary break-up rheometry in a liquid bridge geometry. The combination of these multiple data streams over a statistically significant number of different participants will inform complementary fluid mechanics modeling and provide unprecedented and fundamental insight into expiratory droplet formation, potentially providing a fluid mechanical explanation for why some individuals are super-emitters of expiratory aerosols. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.