Physical Properties of Saliva Change Its Atomization Morphology

Abstract

The fast spread of COVID-19 underscored the need for a better understanding of the transmission mechanism of airborne diseases. Human respiratory activities such as sneezing and coughing expel a huge amount of pathogen-laden liquids that drive the airborne transmission pathway. During drinking and eating, when mask-wearing is impractical, saliva's physical properties are changed. Accordingly, we investigate the atomization morphology of expelled saliva from the perspective of varying fluid physical properties. Using experiments on artificial saliva, on which we apply a step-function pressure profile to mimic the short and violent act of sneezing, we show that the breakup mechanism of saliva is dependent on the fluid physical properties. By analyzing high-speed images, we find that the size of the droplets formed from exhaled respiratory liquid varies with the surface tension and viscosity of saliva. Furthermore, we also show that the viscoelastic behavior of saliva has an effect on the breakup morphology of the ligaments. We observe that the fragmentation of viscoelastic filaments, which form beads-on-a-string structures, takes longer than that of Newtonian fluid filaments. Finally, based on the experimental results, we performed scaling arguments to explicate the observations.