Elaboration of gels and aerogels

The scattering of light by dust in the atmosphere makes it possible to visualize the path of a light beam, but this cannot be materialized. On the other hand, if a light beam passes through a highly transparent aerogel, its path is perfectly delineated and observed in a similar way to that just described, but the scattering centers are now located in a very light material object.

An image often used to describe aerogel is that of frozen smoke. In fact, the volume fraction of material contained in the aerogel with the lowest density is less than 0.14%. This means that air occupies 99.8% of the aerogel's volume. The lightest aerogel has a density around three times that of air. As a result, this solid has a number of unique properties, which are described in this article. Its texture can be modified by chemical, thermal or even mechanical treatments.

It goes without saying that, to obtain such lightweight yet rigid materials, a highly cross-linked solid network must be synthesized with a minimum of material. From this point of view, organic and inorganic gels are the best candidates. The solid network is made up of fine interconnected links between which the solvent is localized. If the solvent evaporates, the volume of the remaining solid is incomparably smaller than that of the original gel. These gels lose at least 90% of their volume during this drying stage. Drying induces an irreversible collapse of the material network.

An aerogel, on the other hand, is a gel that has been dried in a very special way, preserving the delicate structure of the solid as established in the original gel. Drying is carried out in an autoclave. By raising the temperature and pressure, the critical point of the liquid is exceeded. For this reason, this drying method is called supercritical. In short, aerogel is the product of a singular mode of solid synthesis; it is also the product of an unusual mode of drying.