Gel
It is the cross-linking within the fluid that gives a gel its structure (hardness) and contributes to the adhesive stick (tack).
The word gel was coined by 19th-century Scottish chemist Thomas Graham by clipping from gelatine.
Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through surface tension effects.
Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids.
A colloidal gel consists of a percolated network of particles in a fluid medium,[5] providing mechanical properties,[6] in particular the emergence of elastic behaviour.
[9][10] The gel is initially formed by the assembly of particles into a space-spanning network, leading to a phase arrest.
In the aging phase, the particles slowly rearrange to form thicker strands, increasing the elasticity of the material.
[13] A hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium.
[clarification needed] Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water.
[14] Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks.
Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content.
As responsive "smart materials," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state.
[16] An organogel is a non-crystalline, non-glassy thermoreversible (thermoplastic) solid material composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network.
The solubility and particle dimensions of the structurant are important characteristics for the elastic properties and firmness of the organogel.
Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m2/g), along with very small pore size (1–10 nm).
When solvent removal occurs under 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) which results in a denser and more robust solid, the density and porosity achieved depend on the sintering conditions.
A wide range of nanoparticles, such as carbon-based, polymeric, ceramic, and metallic nanomaterials can be incorporated within the hydrogel structure to obtain nanocomposites with tailored functionality.
Nanocomposite hydrogels can be engineered to possess superior physical, chemical, electrical, thermal, and biological properties.
For example, a gel could swell to several times its initial volume after being immersed in a solvent after equilibrium is reached.
is most often defined as the free energy difference after and before the swelling normalized by the initial gel volume
Under the affine network approximation, the mean-square end-to-end distance in the gel increases from initial
The modulus of the gel is then this single-strand elastic energy multiplied by strand number density
Consider a hydrogel made of polyelectrolytes decorated with weak acid groups that can ionize according to the reaction is immersed in a salt solution of physiological concentration.
[30] The coupling between the ion partitioning and polyelectrolyte ionization degree is only partially by the classical Donnan theory.
Even though this ionization suppression is qualitatively similar to that of Donnan prediction, it is absent without electrostatic consideration and present irrespective of ion partitioning.
[30] Due to the complexity of the coupled acid-base equilibrium, electrostatics and network elasticity, only recently has such system been correctly recreated in computer simulations.
[33] Hydrogels existing naturally in the body include mucus, the vitreous humor of the eye, cartilage, tendons and blood clots.
[34] Many substances can form gels when a suitable thickener or gelling agent is added to their formula.
Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whilst the tube material is extruded around it.
