This study is by no means conclusive, but it helped visualize some of the things talked about above. They tested a bandana style folded handkerchief, a homemade cloth cloth mask consisting of an old shirt or whatever folded several times over, and the widely available cone-shape mask.
End result was surgical>cotton>>>>>handkerchief. Fit and number of layers seemed to be important, which makes sense. But my neck gaiter for fishing and outlaw-style bandana coverings may not be doing much good based on this.
https://www.cnn.com/2020/06/30/health/masks-homemade-design-trnd-wellness-scn/index.html
The paper is here:
https://aip.scitation.org/doi/full/10.1063/5.0016018
Physics of Fluids (PoF) is a top journal paper for fluid dynamics and related problems. The other top journal publication is JFM (Journal of Fluid Mechanics) and Experiments in Fluids. In my view, for PoF papers, it can take several months to get approved, even a year, so to see such a fast turn around in submission to publication shows the urgency of studying these issues.
This is the first paper anyone has posted that explains the evaporation effects:
After being expelled into the ambient environment, the respiratory droplets experience varying degrees of evaporation depending on their size, ambient humidity, and temperature. The smallest droplets may undergo complete evaporation, leaving behind a dried-out spherical mass consisting of the particulate contents (e.g., pathogens), which are referred to as “droplet nuclei.”
It really beggars belief that they are called "droplet nuclei" when all the liquid content has evaporated. A really poor choice of terminology but it is what it is I guess. Where they say " sheet-like layers of respiratory fluids " you can just call these " liquid ligaments".
All in all I strongly encourage anyone interested to read that intro. They explain a lot of the concepts well and in my view an accessible manner.
We use a recreational fog/smoke machine to generate tracer particles for visualizing the expelled respiratory jets, using a liquid mixture of distilled water (4 parts) and glycerin (1 part).
They probably add glycerin to actually reduce the evaporation rate of the droplets, allowing the visualization technique to last for a slightly longer period of time. Fog machines/generators are also used in wind tunnels (with PIV) and other experimental set ups to measure gas velocities.
The resulting “fog” or “smoke” is visible in the right panel of Fig. 1 and is composed of microscopic droplets of the vaporized liquid mixture. These are comparable in size to the smallest droplets expelled in a cough jet (∼1 μm–10 μm). We estimate that the fog droplets are less than 10 μm in diameter, based on Stokes’ law and our observation that they could remain suspended for up to 3 min in completely still air with no perceptible settling.
An educated guess, they've probably had to guess the diameters because the fog machine was probably bought from a company that usually sells them to theatres - so the droplet size would not have been reported by the manufacturer.
The laser source used to generate the visualization sheet is an off-the-shelf 5 mW green laser pointer with 532 nm wavelength.
This explains why they were not able to get the velocities of the turbulent jet. With a more capable laser connected to the appropriate equipment, they can get this with little problem. Something for a future study.
We observed high variability in droplet dispersal patterns from one experimental run to another, which was caused by otherwise imperceptible changes in the ambient airflow
This is also because the jet itself is turbulent and one instance of an experiment will never match another instance. The way engineers overcome this issue is to use averaging techniques. A common method may be ensemble averaging, but the problem with averaging is that the equations of motion of a gas, when averaged, aren't "closed", so you have to close them with "made up" models, and this is, in essence, what turbulence modelling is about.
The periphery of the jet (its funky shaped edges) shown in Fig. 2 are caused by ambient air being "drawn in" or "entrained" into the jet core by turbulent vorticies/eddies located inside the jet. A side note - they could have painted the body in matte black to reduce laser reflections and considerably improve the quality of the images. This is something they can improve in a future study. Its a very simple but effective trick.
All in all, a pretty neat study.