A sauce breaks the moment enough oil droplets touch and fuse into ones too large for anything to hold apart — whether that takes ten seconds in a shaken vinaigrette or twenty minutes over a double boiler of hollandaise. Both failures are the same physics. Oil and water were never dissolved into each other, only suspended, and an emulsion survives only as long as something keeps the droplets from finding each other again.
Why oil and water refuse to blend on their own
At any boundary between oil and water, the molecules on each side pull inward, trying to shrink the amount of surface they share. That pull is interfacial tension, and it never turns off. Left alone, it drags neighboring oil droplets into each other one collision at a time, each merger shrinking the total interface a little further, until there's no interface left — just two separate layers sitting one on top of the other. That's the default state. An emulsion is the exception, and it needs something actively working against that pull to exist at all.
What a surfactant is actually doing at the interface
Egg yolk carries lecithin. Mustard carries mucilage. Both are surfactants — molecules with one end that's happy in water and one end that's happy in oil, so they wedge themselves right at the boundary between the two, rather than dissolving into either one. Once a droplet's surface is coated in surfactant, two neighboring droplets that collide bounce off each other instead of merging, because the coating gets in the way. Droplets are colliding constantly, from a whisk or just from drifting into each other, so how much of that coating exists — and how completely it covers every droplet — decides whether an emulsion holds or falls apart within minutes.
Why a shaken vinaigrette collapses and mayonnaise doesn't
A vinaigrette is mostly oil and vinegar with no real surfactant unless you've whisked in mustard. Shake the jar and mechanical force chops the oil into droplets for a moment, but nothing is coating those droplet surfaces, so they start recombining the instant your arm stops. That's a temporary emulsion by definition — not broken, just never stable to begin with.
Mayonnaise starts from the same oil and vinegar, but egg yolk adds enough lecithin and protein to coat every droplet across a much larger volume of oil. Coated droplets in a well-made mayonnaise also run smaller — often just a micron or two across — and droplet size matters on its own: a droplet twice the diameter rises through the mixture roughly four times as fast, since the pull of gravity on a droplet scales with its size squared. Fine droplets barely rise at all. That's why mayonnaise looks and behaves nothing like vinaigrette even though both start from oil suspended in an acidic liquid — one has a permanent coating on tiny droplets, the other has neither.
Why hollandaise breaks from heat and mayonnaise breaks from speed
Hollandaise is warm butterfat emulsified into egg yolk over gentle heat, and the same proteins that help hold it together are also the thing heat destroys. Past roughly 160°F (71°C), egg proteins unfold and start clumping onto each other rather than staying spread across the fat droplets they were coating. The film tears in patches, and the butter pools out — the same protein behavior that squeezes moisture out of a scrambled egg, just happening at the surface of fat droplets instead of inside a curd.
Mayonnaise never sees heat, so it fails a different way. Each addition of oil has to be chopped into droplets and coated before the next addition arrives. Pour the oil in faster than the whisk can keep up, and it recombines into globules too large for the yolk's lecithin to cover in time. The yolk hasn't run out of surfactant — it just can't reach the oil fast enough once the pour outpaces it.
Why a spoonful of water can bring a broken sauce back
Whisking a split sauce into a fresh spoonful of water, mustard, or another yolk doesn't reverse any chemistry. It restarts the same physical process that built the emulsion the first time: the fat that separated out is still fat, still capable of being chopped fine and coated again, as long as there's a fresh batch of surfactant-covered liquid for the whisk to drive it into.