Fluid dynamicists have long been occupied by questions about how droplets generated when humans breathe, talk or cough break down, linger and travel in the air. Today, these questions – and their answers – are even more important as the world is confronted by a virus that has evolved to spread really well among humans.
The epidemiological models that researchers use to understand how the novel coronavirus might spread depend on details at the level of a population. However, COVID-19 – and indeed every infectious disease – transmits and infects at the microscopic level. For example, the viral particles, or virions, of the novel coronavirus piggyback on tiny droplets to travel from one (human) host to another.
These droplets are first formed in the human’s respiratory tract – a network of passages through which air moves into and out of the body. These airways have a soft mucus lining, and the flow of air induces forces that partially break down this lining and eject droplets of different sizes. If a person is already infected by the novel coronavirus, the droplets are infested with virions as well.
The bigger the droplet, the more virions it has inside – but bigger droplets are also dangerous in a different way than smaller ones, thanks to the principles of fluid dynamics.
Larger droplets easily overcome the drag forces exerted by the surrounding air, so they move through the air like a thrown ball or rock might – in an arc. They quickly settle down under the influence of gravity and infect nearby surfaces.
On the other hand, smaller droplets tend to evaporate quickly and become even smaller. And eventually, they become so small that they’re too light to overcome the drag, and stay suspended close to where they were breathed, coughed or sneezed out. These ‘droplet nuclei’ are almost completely dry and are called aerosols, and are presumed to be less dangerous because of their inferior viral load.