Reversible drifting versus permanent merging
The most important distinction in stability is reversible versus irreversible. Creaming (droplets drifting upward) and sedimentation (particles drifting down) are just gravity slowly separating the phases — the dispersed phase concentrates at top or bottom but the droplets or particles are still intact. A shake redistributes them. These are cosmetic and recoverable, governed by Stokes’ law.
Flocculation sits in between: droplets cluster loosely but keep their separate surfaces, so it is usually reversible with gentle shaking. The danger is that close-packed flocs are one step from the irreversible failure — coalescence, where droplets actually merge into larger droplets and finally into two clean layers. Once an emulsion has cracked by coalescence, no shaking brings it back; the film around each droplet has failed.
Phase inversion and the other failures
An emulsion can also invert — an o/w flips to w/o or back — if you overload the internal phase, change temperature past the surfactant’s cloud point, or add an ion that switches the emulsifier’s preference. Inversion changes feel and behaviour overnight and is hard to reverse cleanly. Beyond physical failure, the water in any o/w product is a home for microbes, so a preservative is almost always required to keep the product safe over its shelf life.
Naming the failure correctly tells you whether to worry. Creaming on a label that says “shake well” may be perfectly acceptable; cracking is a recall. Always ask first: is the dispersed phase still intact, or have the surfaces merged?
Designing for stability
- Shrink the droplet or particle size — Stokes’ law makes settling/creaming rate fall with the square of size, so finer is dramatically slower.
- Raise the continuous-phase viscosity with a thickener to slow every gravity-driven drift.
- Strengthen the barrier between droplets — enough emulsifier for a robust film, or a high zeta potential for charge repulsion, to stop coalescence and uncontrolled flocculation.
- Protect chemically and microbiologically — add a preservative for aqueous phases, control temperature near the surfactant’s cloud point, and avoid incompatible electrolytes.