Bags mediated film atomization in a cough machine*

The fragmentation of a liquid mass into smaller droplets is an important physical process for epidemiology as it dictates the generation and transport of pathogen carrying aerosol. In the present study, we combine experiments and numerical computations to examine fluid mechanical processes associated with the generation of minute respiratory droplets (bioaerosol) during violent respiratory manoeuvres, such as coughing or sneezing. Analogous experiments performed in a cough machine allow us to illustrate the changes in liquid topology due to the shearing action of the air stream on a thin film. We identify that aerosol generation due to the shearing of liquid films is mediated by the formation of bag-like structures. It is found that the occurrence of weak spots triggers the eventual demise of these inflated structures by hole expansion. Crucially, increasing viscosity makes the bags puncture when thinner, thus generating smaller droplets. Finally, we show that the breakup of these bags gives rise to a droplet size distribution that is well captured by a log-normal distribution.