WHAT ARE GELS AND ITS TYPES.

INTRODUCTION.

A gel is a jelly-like colloidal systemin which a liquid is dispresed in a solid medium.when a warm sol of geiatin is cooled,it set to a semisolid mass which is a gel.The process of gel formation is knownnn as Gelatin.

Gelatin may be thought of as partial coagualation of a sol.The coagulation sol partical first unite to form long thread-like chains.These chans are then interlocked to form a solid framework.The liquid dispersion medium gets trapped in the cavities of frmework.The resulting semisolid porous mass has a gel structure.Asponge soaked in water is an illustration of gel structure.

TWO TYPE OF GEL.

Gels may be classified in two parts:

(a) Elastic gel are those which posses the property of ecesticity. They change their shape on applying force and return to original shape when the force is removed.Gelatin, starch and soaps are example of substance which from elastic gel.

(b) Non-elastic gels are those which are rigid .e g. silica gel.These are prepared by appropriate chemical action.Thus silica gel is produced by adding concentrated hydroelectric acid to sodium silicate solution of the correct concentration.The resulting molecules of silica acid polymerize to from silica gel.It has a network linked by covalent bond which gives a strong and rigid structure.

Properties of Gels

(1) Hydration.A completely dehydrated elastic gel can be regenerated by addition of water.But once a nonelastic gel is freed from moisture,addition of water will not bring about gelatin.

(2) Swelling. Partially dehydrated elastic gel imbibe water.When immersed in the solvent.This cause increase in the volume of the gel and process is called Swelling.

(3) Syneresis. Many inorganic gel on standing undergo shrinkage which is accompanied by exudation of solvent.This process is termed Syneresis.

(4)Thixotropy. Some gels are semisolid when at rest but revert to liquid sol on agitation.This reversible sol-gel transformation is refereed as Thixotropy. Iron oxide and silver oxide gels exhibit this property.The modern thixotropic paint are also example.

Thixotropy is associated with reversible, microstructural and time dependent changes that come about due to local spatial rearrangement of microstructural components in a structured fluid.

This frequently found time-response of a microstructure that is itself changing with time makes thixotropic, viscoelastic behaviour one of the greatest challenges facing rheologists today, in terms of its accurate experimental characterisation and its adequate theoretical description.

In this webinar Professor Howard Barnes will provide an overview of thixotropy including its history, description and importance across various industries and applications. Different examples of thixotropic systems will be given along with a summary of the theories that have been used to describe the phenomenon.

n this article we study thixotropic and time-dependent rheological behavior of commercial acrylic emulsion paint using a framework of soft glassy rheology. We observe that acrylic emulsion paint, which shows thixotropic rheological behavior, demonstrates various characteristic features of a soft glassy materials such as weak but certain increase in elastic modulus as a function of aging time, maximum in viscous modulus as a function of strain at the onset of yielding, and slow increase in elastic modulus as a function of frequency. Stress relaxation measurements, subsequent to step strain, carried out at different aging times demonstrate slowing down of the relaxation dynamics. This behavior is similar to that observed in polymeric glasses and other soft glassy materials undergoing aging dynamics. Relaxation modulus when plotted against appropriately rescaled time yields a superposition irrespective of the aging time. The rescaled time also known as effective time, which adjusts the material clock by accounting for increasing relaxation time, enables estimation of the rate of change of relaxation time with the aging time. We conclude by relating thixotropy in an emulsion paint sample to an interplay between physical aging and shear melting in the same.

A gel is a jelly-like colloidal systemin which a liquid is dispresed in a solid medium.when a warm sol of geiatin is cooled,it set to a semisolid mass which is a gel.The process of gel formation is kn