I was discussing the least dense solid on earth with someone today, and we agreed that aerogel is one of the least dense solids. When aerogel is measured, it is always a few masses more than the same volume of air. This is because aerogels are made of air, except for the gel that covers them, making them 99.8% air. When the air is removed, aerogel could be less dense than air.
In 1931, Professor Samuel Kistler tried to remove the liquid from jelly without affecting the solid structure. To this, he learned that one could swap out one liquid for another through washing, and he also learned that putting the jelly in a high-pressure volume and getting it to its critical point, then the liquid transforms to a semi-liquid, semi-gas fluid known as a supercritical fluid where there is no difference between a liquid and a gas as the molecules are no longer pulling one another.
After the depressurizing of the vessel, the solid part of the gel is left behind and where there was liquid in the past, it is now gas. The solid skeleton is known as aerogel and it is often referred to as frozen smoke due to its porous appearance and low density. Kistler made aerogel out of anything he could make it from including egg, silica, and many more. Looking opaque in the ultraviolet and transparent in infrared, due to scattered light reaching it according to Rayleigh scattering so it scatters shorter wavelengths than it does longer ones.
Aerogel is are great thermal insulator thanks to its nanoscale pores and the width of its pores makes it a better insulator compared to air even though it is made up of 99.8% air. This effect is known as the Knudsen Effect. The thermal insulating ability of aerogel is why NASA uses it for some things going to space like the Sojourner Rover, Curiosity Rover, and so on, and they still plan to use it for future missions to Mars and other planets.
Aside from using it as an insulator for space missions, NASA has also been able to use it to trap dust in a comet like the Stardust mission. Aerogel can allow dust to enter it due to its low density and porousness, but when they enter, they break apart the network of the aerogel which causes them to lose energy and stop inside the aerogel.
Lots of improvements have been made to aerogel and over time, we might be seeing them in our day-to-day lives. They will be useful in extremely cold regions like Antarctica and other extreme temperature explorations.
You Can Study Further
https://www.aerogel.org/?p=464
https://www.aps.org/apsnews/2021/05/publication-creation-first-aerogel
https://sino-aerogel.com/blogs/about-company/knowledge-of-aerogel
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos
[https://www.sciencedirect.com/topics/mathematics/rayleigh](https://www.sciencedirect.com/topics/mathematics/rayleigh#:~:text=Rayleigh%20scattering%20is%20the%20elastic%20scattering%20of, +%20%E2%84%8F%20%CF%89%20%E2%86%92%20A%20%E2%80%B2%20+)
https://www.sciencedirect.com/topics/physics-and-astronomy/rayleigh-scattering
https://ntrs.nasa.gov/citations/19870026595
https://www.sciencedirect.com/topics/engineering/knudsen-effect
https://ntrs.nasa.gov/citations/20140002089
https://ntrs.nasa.gov/api/citations/20180005488/downloads/20180005488.pdf
https://www.nasa.gov/aeronautics/aerogels-thinner-lighter-stronger/