Vinaigrettes are temporary emulsions of oil and acidic liquid held together by vigorous whisking; adding egg, mustard, or other emulsifiers creates stable emulsions by introducing compounds that prevent oil and water from separating. Understanding the difference prevents broken dressings.
Make both a temporary vinaigrette and a mustard-stabilized one, observing how each separates or holds together over time. Practice whisking techniques to maintain emulsion stability.
From your work with emulsion basics, you know that oil and water don't mix — their molecules are chemically incompatible, and left alone they will always separate into two distinct layers. An emulsion is a temporary or permanent suspension of one liquid in droplets throughout the other, achieved by mechanical energy and stabilized (or not) by emulsifying agents. Vinaigrette is the kitchen's most visible demonstration of this principle, and understanding why it works and why it fails makes you a far more deliberate cook.
A basic vinaigrette — oil and acid (vinegar or citrus juice) whisked together — is a temporary emulsion. Vigorous whisking or shaking breaks the oil into tiny droplets dispersed through the aqueous acid phase. As long as the droplets are small enough and the liquid is moving, the dressing looks homogeneous. But within minutes of sitting still, the oil droplets begin coalescing and rising, and the dressing separates back into two layers. This is not a failure — it is the expected behavior of an emulsion with no stabilizer. Shake it again, and it re-emulsifies perfectly. A classic French vinaigrette at the 3:1 oil-to-vinegar ratio is designed to be this way.
Emulsifiers change the equation. An emulsifier is a molecule with one end attracted to water (hydrophilic) and the other attracted to fat (hydrophobic). It parks itself at the interface between oil droplets and water, coating each droplet and preventing them from coalescing back into a pool of oil. Mustard contains compounds called mucilage that act as emulsifiers — this is why a teaspoon of Dijon in a vinaigrette keeps it from separating for days. Egg yolk contains lecithin, a powerful phospholipid emulsifier, which is why mayonnaise (also an oil-in-water emulsion) holds its structure indefinitely. Honey, garlic paste, and miso also have mild emulsifying properties and can stabilize a dressing while adding flavor.
The proportion of oil to acid matters for stability: the more oil relative to acid, the more droplets the emulsifier must coat, and the harder the emulsion is to maintain. Classic vinaigrette (3:1) is at the limit of what you can achieve through whisking alone without any emulsifier. Move to 4:1 or 5:1 and you'll need mustard or another agent to prevent immediate separation. When you're seasoning — your other prerequisite here — acidity and salt are in conversation: more vinegar means more sourness and more chemical drive to separate; more oil means richer mouthfeel but also more visual separation if the emulsion breaks. The balance is both chemical and gustatory, and adjusting one variable changes both the flavor and the structure of the dressing.