From: veritasium
Aerogel, often called the world’s lightest solid [00:10:10], can be engineered to exhibit dramatically different interactions with water. Its natural form is highly absorbent (hydrophilic), but it can be chemically modified to repel water (hydrophobic).
Hydrophilic Aerogel: Absorbent by Nature
Normal silica aerogel is naturally hydrophilic [05:06:10]. This characteristic stems from its structure being covered with hydroxyl (OH) groups, which strongly attract water molecules [06:19:00]. When liquid is absorbed, these OH groups cause the aerogel structure to collapse [05:19:00], essentially ruining the material [07:55:00].
Despite this, aerogel is very good at absorbing water due to two key properties:
- High Surface Area An ice-cube-sized piece of aerogel contains half a football field of surface area due to its nanoscale sponge-like structure [05:24:00]. This makes it effective at absorbing a large number of molecules [05:39:00].
- OH Groups The presence of OH groups on its internal structure specifically attracts water molecules [06:19:00]. Aerogel can absorb up to 25 times its weight in water [07:44:00].
Applications of Hydrophilic Aerogel
Scientists have sought to exploit aerogel’s absorbent properties in various applications:
- Physical Insecticide Rather than using neurotoxins, aerogel can act as a physical insecticide by sticking to an insect’s outer skin and drawing out moisture or oils, causing the insect to dry out [05:43:00]. This is analogous to putting salt on a slug [06:11:00].
- Museum Humidity Regulation Aerogel is ideal for use in museum cases, such as those made by the company Opium (which created the case for the Mona Lisa). It acts as a passive moisture regulator, absorbing moisture if humidity increases and releasing it if humidity decreases [06:24:00].
- Mars Quake Detection Aerogel containing zeolite particles was developed for the NASA InSight mission to detect Mars quakes [06:50:00]. These zeolite particles can absorb moisture even at very low pressures [07:00:00], helping to maintain the exceptional vacuum required inside the small seismometers to prevent motion damping [07:10:10]. This moisture absorption acts as a passive vacuum pump, requiring no power or consumption and enabling the mission [07:27:00].
Hydrophobic Aerogel: Waterproofing Capabilities
To counteract the issue of aerogel being damaged by water, it can be made hydrophobic. This is achieved by introducing a reactive chemical (a hydrophobe) that spontaneously rearranges with the internal OH groups, creating large non-polar groups that repel water [08:05:00]. By replacing just 30% of the OH groups lining the inside of the aerogel, a perfectly water-repellent aerogel can be created [08:15:00].
This modified aerogel is totally impervious to water; water bounces off it and does not penetrate [08:25:00]. It can sit on water for months and remain dry [08:31:00].
Applications of Hydrophobic Aerogel
One demonstration of hydrophobic aerogel involves coating the body with tiny particles of the material to become waterproof [04:45:00]. When submerged, a thin layer of air remains next to the skin, causing a silvery appearance due to total internal reflection where light reflects off the water-air interface [03:41:00]. The water is repelled at the molecular level [08:54:00], preventing the skin from feeling wet [04:34:00]. This also provides additional buoyancy [04:18:00].
In summary, aerogel’s properties can be manipulated, allowing it to be either highly absorbent (hydrophilic) or completely water-impervious (hydrophobic) [09:06:00].