There are two main ways to synthesize gels at room temperature. 
The first one consists of a common reaction which occurs in nature where silica chemical species diluted in aqueous solutions condense to lead to the formation of silica network. Such a condensation may occur in various aqueous solutions depending on pH and salt concentration. Different morphologies may be obtained and for silica the most known is the precious "opal". 
The other way to produce silica from solution corresponds to a chemical reaction implying metal alkoxides and water in an alcoholic solvent. The first reaction is an hydrolysis which induces the substitution of OR groups linked to silicon by silanol Si-OH groups. As previously, these chemical species may react together to form Si-O-Si (siloxane) bonds which lead to the silica network formation. This phase establishes a 3D network which invades the whole volume of the container. Of course, for these two syntheses the liquid used as solvent to perform the different chemical reactions remains within the pores of the solid network. A gel is thus obtained. This two phases material consists of shaped solid exhibiting specific properties. 

Drying of GELS

Removing the liquid located within the pores leads to a dried gel named "xerogel", a word issued from the Greek word "xeros" and which means dry. 

The different drying processes are listed below. 

A "cryogel" results from a freeze-drying process. usually a material which is hydrophilic. It may react very quickly with water to lead again a solution identical to that from which  it was obtained.  
An aerogel results from a supercritical drying process. The drying step is performed inside an autoclave which allows to overpass the critical point (P_C, T_C) of the solvent. There are different schedules to reach the critical point of the solvent while the solvent itself may be chosen with respect to the nature of the solid part. Strong inorganic solids are commonly dried using alcohol (or acetone) as solvent. Organic solids which may decompose at temperature above 100°C will be dried using CO₂ as solvent. 

Excepted these two unusual drying ways which are precisely named, other dryings lead to xerogels. Consequently we can say that xerogels refer to gels dried at temperature close to room temperature and under atmospheric pressure. Xerogel is the result of a gentle drying to avoid cracking associated to the very low permeability of the solid network. Such a process is time consuming. 

Colloidal particles and Photonic crystals  

Pure inorganic xerogels are rarely used as they are obtained because of their residual porosity which implies quite low mechanical properties and low durability. 
Most of the xerogels show high specific surface which favours chemical reactions with atmosphere. However a new application is of interest. It is related to the synthesis of sub-micronic dense particles of silica. Particles have about the same size (Stober process). 

Particle sedimentation gives rise to a perfect hexagonal packed array of particles (see work made at Bell-Labs). As natural opal, a great insight has been recently done in the direction of these new synthetic opals. Their optical properties depend on the refractive index difference between the matter constituting the spheres and the inter sphere volume and on the size of spheres (for a given hexagonal compact arrangement). These gels exhibit 3D photonic band gap. They should found applications in the field of optoelectronic devices.

Bulk materials  

Inorganic gel is rarely used as obtained after a simple drying. It is often heat treated to be transformed into a material having a smaller porosity and consequently better mechanical properties. At this stage one can separate inorganic gels in two large families. The first one concerns those for which the densification thermal treatment operates via a viscous flow avoiding the gel crystallization. The second one refers to materials that crystallize during an increasing temperature treatment. A viscous flow sintering is a convenient way to densify silica compound. 
Monoliths obtained from supercritical drying offer the best example of a viscous flow sintering which provides dense silica glass at low temperature (1200°C) and for a short duration of treatment (10 minutes). 
It is worth noticing that the nature and the quantity of impurities and more precisely alkali ions play a very important role on the tendency to crystallize.
The other family of xerogels concerns those that crystallize during heat treatment. When crystallization occurs, sintering is difficult to achieve and generally the shrinkage stops. On the other hand, crystallization induces a sudden structure shrinkage at some location in the amorphous matrix. Consequently most of previously monolithic xerogels are transformed into a fine powder. If the gels are previously seeded with suitable crystals, further crystallization may be oriented to lead to high performance abrasive grains. It is likely the largest industrial application of crystallized xerogels with respect to the volume of matter prepared by year. A similar fast sintering is obtained for xerogels issued from dipping of spinning method on to a variety of substrates.  

Functional coatings 

Antireflection coatings may be prepared from such a process. Sintering at low temperature causes a densification of the xerogel film which becomes dense. Densification by closing the pores hinders the condensation of atmospheric water molecules to condense in smallest pores, a phenomenon which modifies the refractive index and consequently the quality of the antireflection effect.

The composition of coatings may be modified with suitable chemical species :  

	to develop colours
	to prepare planar guiding structures
	to provide optical functions (switching, amplification etc.)
	to strengthen mechanically and chemically the surface of the substrate
	to improve scratch resistance.

Fibers and composites 

Fibers of gels drawn from the solution can be easily converted into xerogels which are further sintered into continuous glass or ceramic fiber.   

Aerospace applications of gel technologies concern both fibers and matrices. However, presently, synthetic mullite, alumina and others are replaced by SiC or C fibres. But matrices made of MAS (MgO-Al₂O₃-SiO₂), AS (Al₂O₃-SiO₂), MASL (MgO-Al₂O₃-SiO₂-Li₂O) are still prepared from gels. 
A pressure aid is often required to achieve fully dense material which can be then subjected to a nucleation - crystallization heat treatment that results in an enhancement of mechanical properties. 

Hybrids 

Full organic resistant gels (a variety far to gelatine and other very compliant organic material) have been investigated. Among them, gels which are dried without cracking and which are pyrolysed to lead to an inorganic structured ultraporous carbon, offer a variety of industrial applications. Thermal insulator and super capacitor carbon electrodes are two examples of such a material which is generally CO₂ supercritically dried. 
Resorcinol - formaldehyde, melamine - formaldehyde and others derived compounds are the starting compounds. 

Organic-Inorganic materials Since 1980, a new family of gels, named hybrids, has been studied. Thanks to the wide potentialities of organic chemistry, it was possible to synthesize suitable chemical precursors which consist of inorganic and organic groups. According to the morphology of the starting molecule (like hyper-branched precursors) or the nature of some bonds (hydrophobic Si-CH₃ or Si-H groups), it is possible to build up a network with controlled pore size. Moreover Si-R groups lead to more compliant xerogels. During drying the shrinkage of the network is of great extent and stresses which act on the solid part are lowered. Finally a nearly fully dense xerogel is obtained. The pore size decreases and reaches low values. The material is transparent and has properties close to those of glass. Accordingly, the diffusion of oxygen molecules is very low. Consequently specific organic molecules may be incorporated to this matrix. 
By changing the nature of R, it is possible to control the interaction between the xerogel matrix which plays the role of a host material and the optically active molecules embedded in such a transparent matrix. The field of optically organic molecules covers a lot of applications. These new hybrid xerogels show very attractive applications (tunable lasers using dye molecules, photochromism using spiropyrane molecules...). 

Conclusions

The behaviour of gels during the different steps of preparation (forming, drying, firing) is now well understood. It is presently possible to select starting molecules to tailor physical and chemical properties of xerogels.
 A such adaptability must merely result in applications in the field of mechanical properties improvements, optics and sensors.