Gel
A gel (from the  lat. gelufreezing, cold, ice or gelatusfrozen, immobile) is an apparently solid, jelly-like material formed from a colloidal solution.  By weight, gels are mostly liquid, yet they behave like solids due to the addition of a gelling agent.

# Composition
A solid network spans the volume of a liquid medium. Both by weight and volume, gels are mostly liquid in composition and thus exhibit densities similar to liquids. However, they have the structural coherence of a solid. The network can be composed of a wide variety of materials, including particles, polymers and proteins.

## Cationic polymers
Cationic polymers are positively charged polymers. Their positive charges prevent the formation of coiled polymers. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space than a coiled polymer and thus resists the flow of solvent molecules around it. 
Cationic polymers are a main functional component of hair gel, because the positive charged polymers also bind the negatively charged amino acids on the surface of the keratin molecules in the hair. More complicated polymer formulas exist, e.g., a copolymer of vinylpyrrolidone, methacrylamide, and hydrogel N-vinylimidazole.

# Types of gels

## Hydrogels
Hydrogel is a network of polymer chains that are water-insoluble, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are superabsorbent (they can contain over 99% water) natural or synthetic polymers.
Hydrogels possess also a degree of flexibility very similar to natural tissue, due to their significant water content.
Common uses for hydrogels are
- currently used as scaffolds in tissue engineering. When used as scaffolds, hydrogels may contain human cells in order to repair tissue.
- environmentally sensitive hydrogels. These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change.
- as sustained-release delivery system
- provide absorption, desloughing and debriding capacities of necrotics and fibrotic tissue.
- hydrogels that are responsive to specific molecules, such as glucose or antigens can be used as biosensors as well as in DDS.
- In disposable diapers where they "capture" urine, or in sanitary napkins
- contact lenses (silicone hydrogels, polyacrylamides)
- medical electrodes using hydrogels composed of cross linked polymers (polyethylene oxide,polyAMPS and polyvinylpyrrolidone)
- Water gel explosives
Other, less common uses include
- breast implants
- granules for holding soil moisture in arid areas
- dressings for healing of burn or other hard-to-heal wounds. Wound GEL are excellent for helping to create or maintain environment.
- reservoirs in topical drug delivery; particularly ionic drugs, delivered by iontophoresis (see ion exchange resin)
Common ingredients are e.g. polyvinyl alcohol, sodium polyacrylate, acrylate polymers and copolymers with an abundance of hydrophilic groups.
Natural hydrogel materials are being investigated for tissue engineering, these materials include agarose, methylcellulose, hylaronan, and other naturally derived polymers.

## Organogels
An organogel is a non-crystalline, non-glassy thermoreversible solid materials composed of a liquid organic phase entrapped in a structuring network. The liquid can be e.g. an organic solvent, a mineral oil or a vegetable oil. The solubility and particle dimensions of the structurant are important characteristics for the elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules.
Organogels have raised interest for use in a number of applications, such as in pharmaceutics , cosmetics, art conservation, and food. An example of formation of an undesired thermoreversible network is the occurrence of wax crystallisation
in crude oil .

## Xerogels
A xerogel  is a solid formed from a gel by drying with unhindered shrinkage.  Xerogels usually retain high porosity (25%) and enormous surface area (150-900 m2/g), along with very small pore size (1-10 nm). When solvent removal occurs under hypercritical (supercritical) conditions, the network does not shrink and a highly porous, low-density material known as an aerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of the xerogel due to a small amount of viscous flow) and effectively transforms the porous gel into a dense glass.

# Properties
Many gels display thixotropy - they become fluid when agitated, but resolidify when resting.
In general, gels are apparently solid, jelly-like materials. 
By replacing the liquid with gas it is possible to prepare aerogels, materials with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation properties.

## Sound-Induced Gelation
Sound induced gelation is described in 2005  in an organopalladium compound that in solution transforms from a transparent liquid to an opaque gel upon application of a short burst (seconds) of ultrasound. Heating to above the so-called gelation temperature Tgel takes the gel back to the solution. The compound is a dinuclear palladium complex made from palladium acetate and a N,N'-Bis-salicylidene diamine. Both compounds react to form an anti conformer (gelling) and a syn conformer (non-gelling) which are separated by column chromatography. In the solution phase the dimer molecules are bent and self-locked by aromatic stacking interactions whereas in the gel phase the conformation is planar with interlocked aggregates. 
The anti conformer has planar chirality and both enantiomers were separated by chiral column chromatography. The (-) anti conformer has a specific rotation of -375 but is unable to gelate by itself. In the gel phase the dimer molecules form stacks of alternating (+) and (-) components. This process starts at the onset of the sonication and proceeds even without further sonication.

# Applications
Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in manufacture of wide range of products, from foods to paints, adhesives.
In fiber optics communications, a soft gel resembling "hair gel" in viscosity is used to fill the plastic tubes containing the fibers.  The main purpose of the gel is to prevent water intrusion if the buffer tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is bent around corners during installation, or flexed.  Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whist the tube material is extruded around it.

## Hair Gel
Hair gel is a hairstyling product that is used to stiffen hair into a particular hairstyle. The results it produces are usually similar to but stronger than those of hair spray and weaker than those of hair glue or hair wax.

### Types
Many brands of hair gel in North America and the UK come in numbered variants. Higher numbered gels maintain a greater "hold" on hair, while lower numbers do not make the hair as stiff and in some products give the hair a wet look. A category typically referred to as "ethnic" gels are designed and manufactured specifically for sculpting the hair texture common to African Americans.  Ampro Industries is a common example of this category.
Some forms of hair gel include temporary hair colouring for the hair, including variants in unnatural colors.