Ninety years of chasing a clear ghost

1931Samuel Stephens Kistler invents the aerogel

At the College of the Pacific, chemical engineer Samuel S. Kistler publishes "Coherent Expanded Aerogels and Jellies" in Nature — the first gels dried without collapse. His trick: take the pore liquid supercritical, so liquid and gas never form a meniscus and capillary force cannot crush the network. Silica first, then alumina and others. The "bet with Charles Learned" story told around it is popular lore — the record does not preserve the circumstances.

Kistler S.S., Nature 127, 741 (1931) · J. Phys. Chem. 36, 52 (1932) · biography

1940s–70sMonsanto's Santocel — aerogel goes industrial (opaque)

Kistler's own employer, Monsanto, sells granulated silica aerogel (Santocel) for decades — as thickener, insulator, and the original "frozen smoke" of advertisements. Industrial volume arrives; transparency does not. Granules scatter light regardless of how clear a monolith would be.

1985Tewari, Hunt & Lofftus — the CO₂ route (Lawrence Berkeley Lab)

P.H. Tewari, A.J. Hunt and K.D. Lofftus introduce ambient-temperature supercritical drying: swap the gel's water for liquid CO₂, then dry supercritically near room temperature instead of ≥270 °C with alcohol. This is the process behind most transparent silica aerogels made since.

Tewari P.H., Hunt A.J., Lofftus K.D., Materials Letters 3, 363 (1985) — abstract

1989Robert Pekala — organic aerogels (LLNL)

At Lawrence Livermore, R.W. Pekala polycondenses resorcinol with formaldehyde into the first organic aerogels — pyrolyzable into carbon aerogels, and the start of aerogels beyond silica.

Pekala R.W., J. Materials Science 24, 3221 (1989)

1991Tom Tillotson — the two-step sol-gel (LLNL)

Thomas Tillotson's two-step sol-gel process produces ultralow-density silica aerogels described as highly transparent and almost completely non-blue, down to 0.0031 g/cm³ — the density of smoke itself.

aerogel.org historical record (secondary)

1990sArlon Hunt hunts the haze — in orbit

LBNL's Arlon Hunt targets the last enemy: Rayleigh scattering from non-uniform framework particles. His team gels aerogel in microgravity — KC-135A parabolic flights, then aboard STS-95 in 1998, the shuttle mission that also carried John Glenn back to space — testing whether gravity-driven convection is what seeds the blue haze.

aerogel.org — The 1990s

1999–2006Stardust — aerogel goes to a comet

NASA's Stardust probe launches in 1999 carrying ~1,000 cm² of gradient-density silica aerogel developed under Peter Tsou at JPL. It catches dust from comet Wild 2 in 2004 and returns the grains to Utah in January 2006 — the fastest atmospheric reentry ever flown at the time. Principal investigator: Donald Brownlee, University of Washington.

2001–12Aspen Aerogels; Leventis & Meador make it strong

Aspen Aerogels (founded 2001, Massachusetts) commercializes flexible fiber-reinforced aerogel blankets — aerogel leaves the lab for the pipeline and the building site. Nicholas Leventis (Missouri S&T) pioneers polymer-crosslinked silica; Mary Ann Meador's team at NASA Glenn delivers mechanically strong, flexible polyimide aerogels (2012).

Meador M.A.B. et al., ACS Appl. Mater. Interfaces (2012) · Capadona/Meador et al., Polymer 47, 5754 (2006)

2004–14Cellulose joins in

Jin, Nishiyama, Wada & Kuga report nanofibrillar cellulose aerogels (2004); Kobayashi, Saito & Isogai at the University of Tokyo produce transparent, tough cellulose aerogels (2014). The ghost learns to grow from plants.

Colloids Surf. A 240, 63 (2004) · Angew. Chem. Int. Ed. 53, 10394 (2014)

2019MIT — clarity that survives fire

Strobach, Bhatia, Yang, Zhao and Evelyn Wang at MIT demonstrate transparent silica aerogels with >95% visible transmission at 4 mm, stable for months at 400–800 °C — built for solar-thermal receivers that need light in and heat kept.

APL Materials 7, 081104 (2019)

2023–24Smalyukh's SiCellA — the Guinness record

Ivan Smalyukh's group at the University of Colorado Boulder creates SiCellA — silanized cellulose aerogel at 97–99% visible transmission, clearer than ordinary glass. Nature Energy publishes it in 2023; Guinness World Records certifies it the most transparent material on Earth in 2024.

Abraham/Senyuk et al., Nature Energy 8, 381 (2023) · CU Boulder, 2024