Today’s Rhymes With Orange has Olive Oil and Balsamic Vinegar needing couples therapy, ’cause they just can’t get along — except when they’re together with a salad:
That is, except when they’re mixed together in a salad, except when they’re joined in a vinagrette. From NOAD2 on the noun vinagrette:
(also vinaigrette dressing) salad dressing of oil, wine vinegar, and seasoning. ORIGIN French, diminutive of vinaigre ‘vinegar.’
From a food site with a recipe for Greek vinagrette: red wine vinegar, extra-virgin olive oil, plus lemon juice, garlic, and oregano (and of course salt and pepper):
But how do you get oil and water (vinger is essentially just acetic water with some natural flavoring) to mix? Through vigorous physical action that breaks the oil up into small droplets and distributes these throughout the watery medium — by beating with a fork, or whisking, or a short whirring in a blender, or (as suggested by the Greek vinaigrette site) shaking in a mason jar.
All of this is aimed at producing an emulsion. On the noun emulsion in NOAD2:
a fine dispersion of minute droplets of one liquid in another in which it is not soluble or miscible.; a fine dispersion of one liquid or pureed food substance in another: ravioli with pea and ginger emulsion. ORIGIN early 17th cent. (denoting a milky liquid made by crushing almonds in water): from modern Latin emulsio(n-), from the verb emulgere ‘milk out,’ from e- (variant of ex-)‘out’ + mulgere ‘to milk.’
A bit more detail from Wikipedia (which has a fairly long article):
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). … Examples of emulsions include vinaigrettes, homogenized milk, mayonnaise [and hollandaise sauce], and some cutting fluids for metal working.
Milk is an emulsion in nature: fat droplets suspended in water (though inclined to separate, unless subjected to homogenization). (Butter is the opposite sort of emulsion: tiny droplets of water suspended in fat.)