creaming

Leave fresh, unhomogenised milk to stand and a layer of cream rises to the top. That is exactly the phenomenon pharmacists call creaming: in an emulsion, the dispersed droplets drift upward (or sometimes downward) and gather into a concentrated layer, while the rest of the product becomes more dilute.

Creaming happens because the oil droplets and the water phase differ in density. Oil is usually lighter than water, so in an oil-in-water emulsion the oil droplets float upward; if the dispersed phase were denser it would sink instead (downward creaming, or sedimentation). The droplets themselves are still intact and surrounded by their stabilising film — they have only concentrated in one region, not merged.

Because the droplets have not coalesced, creaming is reversible: a gentle shake redistributes them and restores a uniform emulsion. It is therefore a cosmetic and dosing nuisance rather than outright failure, but it is a warning sign, because closely packed creamed droplets are more likely to go on to coalesce. Formulators slow creaming by reducing droplet size, thickening the continuous phase to raise its viscosity, and narrowing the density difference between the phases — all consistent with Stokes' law.