From: veritasium
Aerogel is known as the world’s lightest, or least dense, solid [00:00:00]. A small piece can have a mass of just 1.22 grams [00:00:05], which is only a few times the mass of the same volume of air [00:00:11]. This is because aerogel is 99.8% air [00:00:18]. Some aerogels are so light that if all the air were removed, they would be less dense than air [00:00:23].
Composition and Creation Process
The invention of aerogel dates back to 1931, when Professor Samuel Kistler made a bet with Charles Learned about removing liquid from a jelly without affecting its solid structure [00:02:01]. Jellies are combinations of liquids and solids, mostly liquid embedded within a 3D solid structure [00:02:12]. A gel, like jello, has a skeleton with nano-sized pores that provide rigidity, making up about 1% of the gel’s mass [00:02:24].
The challenge was that if liquid is simply evaporated, the solid structure shrinks and crumbles [00:02:43]. Kistler solved this in two ways [00:02:56]:
- Liquid Exchange: One liquid can be replaced with another, such as swapping water for alcohol, by thoroughly washing the jelly [00:03:00].
- Supercritical Drying: The gel is then placed in a high-pressure vessel called an autoclave [00:03:11]. By heating it to its critical point (high-temperature, high-pressure), the liquid transforms into a semi-liquid, semi-gas state known as a supercritical fluid [00:03:16]. At this point, there’s no distinction between liquid and gas, and molecules no longer pull on each other [00:03:26]. When the vessel is depressurized, the solid skeleton, about 1% of the original gel’s mass, remains intact, with gas filling the pores where liquid once was [00:03:33]. This nanoporous solid is aerogel [00:03:45]. Kistler published his findings in Nature in 1931 [00:03:50].
Kistler made aerogels from various substances, including eggs, rubber, nitrocellulose, and silica [00:04:42]. A wet silica gel, for example, is rubbery and consists of 97% alcohol within its pores, with the remaining 3% being amorphous silica [00:04:56]. For modern production, the alcohol in the gel is often replaced with liquid carbon dioxide (CO₂), which is non-flammable and has a low critical temperature [00:05:22].
Properties of Aerogel
Physical Characteristics
- Appearance: When viewed against a light background, aerogel is almost transparent [00:07:10]. However, against a darker background, it exhibits a slight bluish color [00:07:18]. This blue hue is due to Rayleigh scattering, similar to why the sky is blue [00:07:28]. Its nanoscale structures scatter shorter wavelengths (like blue) much more intensely than longer wavelengths (yellow or red) [00:07:31]. Consequently, aerogel appears opaque in the ultraviolet spectrum and transparent in the infrared [00:07:49]. When held up to a blue sky, aerogel appears yellow because it scatters out the blue light, allowing longer wavelengths like yellows and oranges to pass through, creating an effect similar to a portable sunset [00:08:00].
- Texture and Durability: Original silica aerogel is not very strong and can be very crumbly and easy to break [00:05:12]. However, scientists are working on increasing its durability and elasticity [00:11:05].
- Hydrophilic and Hydrophobic Characteristics: Original silica aerogel is hydrophilic [00:11:16], meaning it absorbs water and can be ruined by it [00:11:26]. However, methods exist to make it waterproof [00:11:31].
Thermal Insulation Capabilities
The nanoscale pores of aerogel are crucial to its effectiveness as a thermal insulator [00:08:38]. Despite being largely comprised of air (99%), it is a better insulator than air itself [00:09:02]. This is due to the Knudsen effect [00:09:23]: the width of the pores is smaller than the mean free path of air molecules [00:09:05]. This makes it difficult for hot, fast-moving air molecules to diffuse through the aerogel and transfer heat [00:09:14].
For example, in an experiment demonstrating its insulating power, a Bunsen burner was applied to chocolate bunnies placed on a glass petri dish and an aerogel layer [00:00:42].
- Glass: The glass petri dish became very hot quickly [00:01:15]. After just one minute, the chocolate bunny started smoking and melting, eventually undergoing a phase change into a liquid [00:01:20]. The petri dish also cracked due to thermal expansion [00:01:50].
- Aerogel: Even after nearly three minutes, there was no sign of melted chocolate on the aerogel [00:03:54]. A thermocouple showed the flame temperature underneath the aerogel to be 600 degrees Celsius (1250 Fahrenheit) [00:05:45], while the top surface remained only warm to the touch [00:04:37]. The chocolate on top eventually softened and melted only where the flame crawled up and over the sides of the aerogel, not directly through it [00:05:53].
This extreme insulation makes aerogel useful in various applications, including space exploration. NASA has used aerogel insulation on Mars rovers like Sojourner, Spirit, Opportunity, and Curiosity, and plans to use it on future Mars missions to protect electronics from the cold Martian nights [00:09:36].
Future Development
While aerogel has remarkable properties, its high cost and brittleness have limited its widespread everyday use [00:10:40]. Scientists are actively working on reducing manufacturing costs and increasing its durability [00:11:05]. The development of “next-generation aerogels” includes making them waterproof [00:11:31].