Its known to everyone that when a solid is heated up to its melting point it turns into a liquid. What happens when a liquid is heated? Simple, it tends towards becoming gaseous. While making omlettes I found that the yolk is in liquid state and putting it on a pan for heating results in the formation of what we call as the omlette which is a solid. How is this so?
Proteins are long chains of amino acids. They are created in long chains, but interactions between various parts of the chain can result in a folded protein, in what we call its native state, see below:
Interestingly when you apply heat to proteins the reverse happens: leaving the native state and returning to a long chain of amino acids (an un-native state), denaturation is what the process is called. Many of these long chains are created when you are cooking an egg and instead of folding back into the native state, random interactions form between different chains, usually interacting hydprophobic regions avoiding water that is accumulating around the hydrophilic regions of the peptide sequence. As the process continues, water molecules are incorporated into an ever-growing framework and a random three-dimensional structure emerges. Aggregation eventually results in very large masses and we have our gel -- or solid as you say. Same idea to making jello, really.
Proteins that make up the egg are very complex molecules, far away from full thermodynamic equilibrium both when cold and when heated. Thus conclusions of thermodynamics of simple substances do not apply to egg proteins. Solidification of an egg into an omlette involves a process called denaturation.
Emulsions are part of a more generic two-phase systems of matter called colloids. Although the term colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that both the continuous phase is dispersed as liquids.
An emulsion is a mixture of immiscible liquids in a more or less homogeneous. A liquid (dispersed phase) is dispersed in another (the continuous phase or dispersing phase).
An emulsifier (also known as an emulsifier) is a substance which stabilizes an emulsion, frequently a surfactant. Examples of food emulsifiers are egg yolk (where the main chemical emulsifier is lecithin), honey and mustard, where a variety of chemicals in the mucilage around the seed pod act as emulsifiers, proteins and low molecular weight emulsifiers are common.
For example, egg albumin, which is a colloid, has been obtained in crystalline form, which is why the heat is applied to a system which is apparently liquid, you get the texture you can see the omelet.
Indeed, when you heat an egg the proteins denature and become random coils. Now, these random coils blend together and depending on their density can form a viscoelastic fluid or a gel.
Interestingly, cooling the cooked egg does not reverse the process because the coils are entangled but the process can be reversed, e.g., by adding some chemical but it takes 3 hours and the egg will probably be inedible.
To understand a viscoelastic fluid, mix a spoon of starch in a (paper) cup of water. You can easily turn a spoon slowly in the liquid but the resistance increases as you increase the speed by which you turn the spoon. This is because the coils do not have enough time to rearrange.