NANODROP

The propagation of respiratory diseases involves the expectoration of small droplets, in various manners such as speach, sneeze or cough. A single sneeeze may send droplets up to six meters. Droplets as small as 100nm may contain the SARS Cov 2 virus which has a diameter of approximately 60-140 nm. The natural history and quantitative analysis of these tiny droplets is very poorly known. While droplets of sizes larger than a few microns can in principle be observed experimentally by optical methods, the observation is not easy and the number and physical fate of droplets of small sizes is thus unknown. This motivates the study which is organized on three axes - how many droplets of each size are created in a cough or sneeze in connection with an extensive experience and an intense ongoing effort on the topic of droplet size distribution; - how do small particles diffuse/disperse in the environment (This may seem a well studied topic, but the interaction with turbulence, rheology, multicompositional systems and evaporation dynamics make it much more complex); - how do small particles of mucus liquid dry in the environment both when suspended in air and when deposited on a surface (This topic is related to that of heat and mass transfer from droplets) . The local environment of a micron size droplet in even very moderately turbulent air may be fluctuating so the rate of drying may be a characteristic of air turbulence (just as a hot wire signal) or of local fluctuations in the humidity and temperature. Starting on this basis, the project will be performed in collaboration between the Paris-SU group specialising on theoretical and numerical aspects of atomization and the Cambridge-MIT group. The groups are already involved in collaborative work on the physics of drops, and the MIT group on mathematical-statistical studies of covid transmission. The most typical mechanisms for the production of the droplet size distribution are the hole formation and the ligament breakup mechanism. Both will be reviewed together with an analysis of the ligament size distribution and a re-analysis of the experiments. For a given expectorated mass, the likely ranges of minimum and maximum droplet sizes and the corresponding numbers will be predicted from the literature and previous studies. We stress that the objective of the project is not to perform new experiments or computer simulations of the processes, but organize what is already known in a manner that allows useful quantitative predictions for epidemiologists. In particular, attention will be focused on the aerosol (diameter less than 100 microns) droplet formation mechanisms. These are - the satellite droplet mechanism which leads to the formation of approximately ten times smaller droplets than the main droplets in an atomization process. - the thin sheet/hole perforation mechanism for saliva or mucus, currently unexplored. The diffusion and dispersion of droplets in the environment is a critical process. Micron-size droplets settle to the floor in thirty minutes to eight hours in a perfectly quiet atmosphere, but the situation is much more complex whenever air turbulence, always present to some degree, is taken into account. Turbulent dispersion will considerably increase the range of droplet sizes that remain suspended in air for a long time. The humidity of the air influences the survival of the virus, by affecting its bilipidic layer envelope, thus the dynamics of vapor exchange on small droplets have relevance. As the research will considerably improve our knowledge of the aerosol and large droplet properties, it will have enormous impact on recommendations for transmission reduction. In particular, it is extremely important to exclude or confirm the existence of long distance disease transmission by areosol particles.