melt. Due to their low density, aerogels have also been used to capture interstellar dust particles with minimal heat changes in slowing them down (to prevent heat-induced changes in the particles) as part of the Stardust mission. One method to produce a silica aerogel uses a modified Stöber process and supercritical drying. The product appears translucent with a blue tinge as a consequence of Rayleigh scattering; when placed in front of a light source, it becomes yellowish due to Mie scattering. This aerogel has a surface area of 700 m2 g−1 and a density of 0.040 g cm−3; by way of contrast, the density of air is 0.0012 g cm−3 (at 15 °C and 1 atm). Silica aerogels held 15 entries for materials properties in the Guinness World Records in 2011, including for best insulator and lowest-density solid, though aerographite took the latter title in 2012. Aerographene, with a density of just 13% of that of room temperature air and less dense than helium gas, became the lowest-density solid yet developed in 2013. Stöber-like methods have been applied in the preparation of aerogels in non-silica systems. NASA has developed silica aerogels with a polymer coating to reinforce the structure, producing a material roughly two orders of magnitude stronger for the same density, and also polymer aerogels, which are flexible and can be formed into a bendable thin film. Colloidal silica Colloidal silica is widely used in metal casting. Synthetic opals Stöber process may be used to produce spherical particles to grow lustrous opal mineraloids. Notes References Physical chemistry Silicon Condensation reactions Silicon dioxide