Droplets infused with particles — like coffee — can leave complex stains once they evaporate. Here researchers show the complex cracking pattern that develops as a droplet with nanoparticles evaporates. The central image in the poster actually shows the drop’s pattern changing in time. The initial drop is shown at 9 o’clock, and as you move clockwise around the drop, time passes and the crack structure becomes more complex. What a neat way to visualize the changes! (Image and research credit: P. Lilin and I. Bischofberger)
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Wet Masks Block Droplets Better
As wearing face masks for long periods has become more typical, you may have wondered whether a soggy mask offers less protection. All masks — cloth, surgical, and N-95s — get moist from their wearer’s breath. A recent study indicates this isn’t a cause for alarm, though.
Researchers looked at how relatively high-speed droplets (like those from a cough or sneeze) impact dry and wet masks. These high-speed droplets can break into smaller droplets upon impact with a mask layer. The more layers a mask has, the fewer droplets make it through. But even for single-layer masks*, a moistened mask layer lets fewer droplets through. So you don’t have to worry if it’s a little humid in there. Your mask is still working! (Image credit: top – V. Davidova, other – S. Bagchi et al.; research credit: S. Bagchi et al.; via APS Physics)
* To be clear, you should be wearing masks that are more than a single layer thick. Personally, I’m still only going into indoor public spaces in an N-95 at this point.
Droplet penetration through a mask. Top row: dry, single layer mask. Middle row: wet, single layer mask. Bottom row: wet, triple layer mask. When wet, masks permit fewer droplets through. 
Gathering Droplets
In deserts around the world, plants have adapted to collect as much moisture as they can. Geometry aids them in this endeavor because droplets on the tip of a cone will move toward its thicker base. The motion takes place due to a imbalance in surface tension forces on either end of the droplet.
As the droplet moves up a cone, it changes shape from a barrel-like drop that fully covers the conical surface to a clamshell-shaped droplet that hangs only from the bottom of the cone. (Image and research credit: J. Van Hulle et al.)
Droplets From Speaking
Illnesses like COVID-19 can spread through droplets and aerosols produced by coughing, sneezing, or even speaking. New research looks at how regular speech patterns produce a spray of droplets. Researchers found that pronouncing many consonants causes a sheet of saliva to form between the speaker’s lips. That sheet stretches into filaments that then break into a spray of droplets.
Strong, plosive consonants like /p/ and /b/ create the most droplets (Images 2 and 3), but even milder consonants like /m/ create some (Image 1). Interestingly, the researchers found that wearing lip balm drastically decreased droplet production by altering the saliva sheet formation. Even so, there’s no substitute for wearing a properly fitted mask! (Image credits: masks – K. Grabowska, droplets – M. Abkarian and H. Stone; research credit: M. Abkarian and H. Stone; via APS Physics)
