The Strangest Solid On Earth Lighter Than Air: A Chemical Perspective

Brett I. Cohen, Ph.D.

2019-07-24 07:41:10

Credit: Brett I. Cohen, Ph.D.

Credit: Brett I. Cohen, Ph.D.

One of the most unique and supernatural materials on Planet Earth is a solid called Aerogels.  This strange synthetic material is straight out of science fiction.  Aerogel is a synthetic porous ultralight solid, which was created by American scientist Samuel Stephens Kistler in 1931.  Kistler created this synthetic solid as a result of a bet with a colleague (Barron & Nellis, 2016).  Chemically, Kistler replaced the liquid in jellies (gels) with gas without causing shrinkage.  A gel is a solid jelly-like soft material.  Gels have properties ranging from soft and weak in structure to hard and tough.  These gels comprise a microporous solid in which the dispersed phase is a gas. 

Aerogels are produced by extracting the liquid component of a gel through supercritical drying.  Supercritical drying (also known as critical point drying) is a chemical process to remove a liquid in a precise and controlled manner (Evangelos and Mujumdar, 2011).  In other words, this chemical process allows the liquid in the gel to slowly dry off without causing the solid matrix in the gel to collapse (due to capillary action) as compared to conventional laboratory chemical evaporation techniques.  As the liquid substance in a liquid body (gel) crosses the boundary from liquid to gas during the supercritical drying process, the liquid changes into a gas at a finite rate, while the amount of liquid in the gel decreases.  When this happens within a heterogeneous environment in the gel, surface tension in the liquid body pulls against any solid structures the liquid might be in contact with.  Delicate structures such as cell walls (the dendrites of the gel), tend to be broken apart by this surface tension as the liquid–gas–solid junction moves by.  At the end of the supercritical drying process, the gel is converted to a porous ultralight solid (producing an aerogel).

 Aerogels are extremely low density solids.  Aerogels in general, have many nicknames such as; solid air, solid cloud, and blue smoke {for silica based aerogels (see below and Figure 1)}.  Aerogels also have the distinction of having low thermal conductivity.  The term aerogel is not a single material with a set chemical formula; instead, the term is used to group all materials with a certain geometric structure.  The first aerogels invented by Kistler were produced from silica gels (with a chemical formula of SiO2).  Later work by Kistler involved aerogels based on alumina (aluminum oxide oxide is an inorganic compound, with the chemical formula of Al2O3), chromia (chromium(III) oxide is an inorganic compound, with the chemical formula Cr2O3) and tin dioxide {tin(IV) oxide (also known as stannic oxide) is an inorganic compound, with the chemical formula SnO2} (Kistler, 1932).   The structure of an aerogel is technically a foam.  Aerogels can be synthesized in many different shapes and forms. The majority of aerogels are composed of silica, but carbon, iron oxide, organic polymers, semiconductor nanostructures, gold and copper can also form aerogel (   It is important to note, that silica aerogel is the most common type of aerogel, and the most extensively studied and used.  After the supercritical drying process, silica aerogels form a three-dimensional, intertwined cluster that make up only approximately 3% of the volume of the aerogel.  The remaining 97% of the volume is composed mainly of air in extremely small nanopores.  Silica aerogels also have a high optical transmission of  approximately 99% and a low refractive index of approximately 1.05 (Gurav, et al. 2010).  In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material.  Refractive index is defined as the ratio of the velocity of light (c) in a vacuum to its velocity in a specified medium.  For example, the refractive index of water is 1.333, meaning that light travels 1.333 times as fast in vacuum as in water.  This makes silica aerogels very poor radiative insulators (see below).  It is interesting to note, that one type of aerogel created by NASA's Jet Propulsion Laboratory in Pasadena, California {derived from pure silicon dioxide (with a chemical formula of SiO2)and sand or silica} holds the Guinness World Record for the lightest solid on Planet Earth (Guinness Records, 2002). 

Aerogels are solid, rigid and dry synthetic materials that do not resemble the original jelly-like gel before the supercritical drying chemical process.  Pressing softly on an aerogel porous ultralight solid typically does not leave a minor mark but pressing more firmly will leave a permanent depression.  Pressing extremely firmly will cause a catastrophic breakdown in the aerogel structure.  This can cause the aerogel to shatter (similar to glass) and this property is called friability.  In other words, aerogels have the tendency to chip, crumble or break following compression.  Despite the fact that aerogels are prone to shattering, they are very strong structurally.  This is due to the dendritic microstructure (see above), in which spherical particles (such as silica or sand) have an average particle size of 2 to 5 nm which are fused together into clusters.  These clusters form a three-dimensional highly porous structure of fractal chains.  It is important to note, that aerogel are a solid material that is 99.8% air.  Therefore, aerogels have a porous solid network that comprises many air pockets, with the air pockets taking up the majority of space within the material and  the lack of solid material allows aerogel to be almost weightless (  Aerogel structures can also form a sol-gel polymerization solid, where monomers (simple unpolymerized molecules) react with other monomers to form a sol.  A sol (colloidal suspension of solid particles) is a substance that consists of bonded, cross-linked macromolecules to give silica sol materials with deposits of liquid solution between them.  This can be done chemically through three different primary reactions (hydrolysis, water condensation and alcohol condensation) to form a silica sol via the alkoxide technique (  During the supercritical drying process, the liquid within the sol gel is evaporated out and a bonded and cross-linked macromolecule frame is left behind.  The result of polymerization with supercritical heating is a material that has a porous strong structure classified as an aerogel.  Variations in synthesis can alter the surface area and pore size of the aerogel.  The smaller the pore size the more susceptible the aerogel is to fracture (

Aerogels are also good thermal insulators because they almost nullify heat transfer (due to chemical processes of conduction and/or convection) because of two factors; 1) they are mostly composed of insulating gas and 2) a microstructure network which prevents gas movement and as a result air cannot circulate through the solid aerogel lattice effectively as compared to normal solid materials.  Conduction is the process by which heat or electricity is directly transmitted through a substance when there is a difference in temperature.  Convection is the movement caused within a fluid by the tendency of a hotter substance and therefore less dense material to rise, which consequently results in transfer of heat.  In other words, aerogels are good conductive insulators because they are composed almost entirely of gases, which are very poor heat conductors.  For this reason silica (with a chemical formula of SiO2) based aerogels are excellent insulators, because silica is a very poor conductor of heat energy.  Aerogels as a result, have a thermal conductivity smaller than that of the gas they contain.  This is caused by the Knudsen effect.  The Knudsen effect results in a reduction of thermal conductivity in gases when the size of the cavity within the aerogel encompassing the gas becomes comparable to the mean free path of the gas.  In physics, the mean free path is the average distance travelled by a moving particle (such as an atom, a molecule, a photon) between successive impacts or collisions.  Essentially, the cavity found within the aerogel restricts the movement of the gas particles, decreasing the thermal conductivity in addition to eliminating convection (see above) as a source of heat energy.

 As an aside, aerogels are very poor radiative insulators due to their low refractive index (see above).  This is because infrared radiation can pass easily through aerogels and transfer of heat is possible.  Because of its hygroscopic (a property of tending to absorb moisture from the air) nature, aerogels feel dry and act as a strong desiccant or a hygroscopic substance used as a drying agent.  Therefore, aerogels are characteristed as hydrophilic, because they absorb moisture.  As a result of water absorption, aerogels usually suffer a structural change, such as contraction, and can deteriorate.  It is interestlng to note, that aerogel degradation due to water absorption can be prevented by making them hydrophobic or tending to repel or fail to mix with water (via chemical treatment, such as silanization).  Aerogels with hydrophobic interiors are less susceptible to water degradation.  For this reason, handling aerogels for an extended period of time results in dry brittle spots on one's skin.  Therefore, it is recommended that handling of aerogels should be performed using gloves for skin protection.  Silica based aerogels are not known to be carcinogenic or toxic.  Silica aerogels are a mechanical irritant to the eyes, skin, respiratory tract, and digestive system (Cryogel_5201_10201_MSDS.pdf).  Silica aerogels can also induce dryness of the skin, eyes, and mucous membranes. Therefore, it is recommended that protective gear including respiratory protection, gloves and eye goggles  be worn whenever handling or processing aerogels (Cryogel_5201_10201_MSDS.pdf).

The slight blue color seen for silica based aerogels (see Figure 1) is due to Rayleigh scattering of the shorter wavelengths of visible light (such as blue color wavelengths) by the nano-sized dendritic structure.  Rayleigh scattering is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation.  This phenomenon causes silica based aerogels to appear smoky blue against a dark background and yellowish against bright backgrounds.  This electromagnetic range is also known as the visible (light) spectrum.  The perception of color that is observed is derived from the stimulation of cone cells in the human eye by electromagnetic radiation.  The human eye is not capable of "seeing" radiation with wavelengths outside the visible spectrum. For example, electromagnetic radiation is characterized by its wavelength (or frequency).  The visible colors of the spectrum from shortest to longest wavelength are: violet, blue (approximately 450 to 490 nm), green (approximately 520 to 560 nm), yellow (approximately 560 to 590 nm), orange, and red (approximately 635 to 700 nm) (Cohen & Cohen 2017).  Interestingly, there are a number of blue holes that are found around the world in the ocean and sea waters.  A blue hole is a large marine sinkhole (a depression and/or hole in the ground caused by some form of collapse of the surface layer) or cavern (a natural void in the ground) (Cohen, 2019).  Blue light is the most enduring part of the spectrum.  The other parts of the visible spectrum; red, yellow, and finally green are absorbed during their path through the ocean and sea water, into the Blue hole, and only blue light manages to reach the white sand containing calcium carbonate and return upon reflection.  Thus, this is the cause of the blue color that emanates or originates from the Blue hole.  This effect is silmilar to that found for the observed blue color for silica aerogels.  The opposite effect occurs when white (pure) light is passed through a prism and a rainbow of different colors (for example, six main bands of colors; red, orange, yellow, green, blue, and violet) are observed (Cohen & Cohen 2017).   A prism is a glass or other transparent object that is triangular with refracting surfaces.  It is interesting to note, that different aerogels have different observed colors associated with them.   For example, iron oxide (or ferric oxide is an inorganic compound with the chemical formula Fe2O3) aerogels have a red (rust) or yellow, opaque color, whereas Chromia (with the chemical formula Cr2O3) aerogels have a deep green or deep blue opaque color (

Applications of Aerogels

 Since aerogels have such diverse chemical and physical properties (see above), it is no surprise that they also have a wide range of useful applications.  Since the 1960’s, aerogel (silica) has been used as the insulating material in the spacesuits of NASA astronauts (due to their superior thermal insulator properties).   Interestingly, NASA has used aerogels to capture space dust.  Aerogel is being used in conjunction with the ‘Stardust’ mission and "Wild 2" (dust collected from comets), which aims to bring back particles from space from beyond the moon for the first time (   Aerogels are able to capture space dust. When the particle hits the aerogel, it will be travelling at speeds of up to 6 times that of a rifle bullet.  Most chemical substances would not be able to slow the dust down without heating, which would result in an alteration to the dust sample collected.   Aerogel however, will allow the dust to bury itself into the porous solid aerogel material and the dust collected is gradually brought to a stop as the dust loses momentum.  Thus, making aerogels the prime material for space dust collection.  Other important applications include; insulations (building structures) and nanotechnology.  Aerogels are also a must-have material in the insulation industry and have been used for several years in cavity injected wall insulation and insulating boards (  Both transparent and opaque silica aerogel based insulation has tremendous potential to displace older technologies such as mineral wool, fiberglass, foams and polyurethanes.  The use of aerogel in this way is extremely energy efficient and environmentally beneficial, as it will significantly cut the use of fossil fuel, used for heating.  From a nanotechnology perspective, silica aerogels will continue to serve as the staple substrate for scientists seeking to develop nanotechnologies which rely on high surface areas or nanoporosity.  Chemical and biological sensors will benefit from silica aerogel based technologies in the coming years and decades (   The future applications for aerogels will result in innovations which will have far reaching benefits for our planet and its inhabitants.



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Cohen, Brett I., Cohen, E. (2017).  Non-Duality: A Prism as a Metaphor or a Representation  for "Ego" Consciousness. Phenomena Magazine Vol. 98: 39-40.

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Evangelos, Tsotsas; Mujumdar, Arun S. (2011). Modern Drying Technology, Volume 3: Product Quality and Formulation. John Wiley & Sons. p. 185.

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