  We isolate a nano-colloidal droplet of surrogate mucosalivary fluid to gain
fundamental insights into the infectivity of air borne nuclei during the
Covid-19 pandemic. Evaporation experiments are performed with salt-water
solutions seeded with a viral load of inactive nanoparticles in an acoustic
levitator. We seek to emulate the drying, flow and precipitation dynamics of
such air borne mucosalivary droplets. Observations with the surrogate fluid are
validated by similar experiments with actual samples from a healthy subject. A
unique feature emerges with regards to the final crystallite dimension; it is
always 20-30% of the initial droplet diameter for different sizes and ambient
conditions. The preserved precipitates from levitated droplets show that 15% of
the total virion population remain dispersed on the outer surface of
air-desiccated air borne nuclei. This fraction increases to ~90% if the
respiratory droplets (of larger initial size) settle on a surface and then
evaporate in the sessile mode.
