There are several manifestations of emulsion instability: flocculation, coalescence, stratification (or sedimentation), demulsification, phase inversion and Ostwald ripening.
1.Flocculation
The dispersed droplets in the emulsion aggregate to form clusters of three-dimensional droplets, which are defined as flocs, and this process is called flocculation. Under normal circumstances, there are no significant changes in the size and distribution of droplets within flocs, and droplet coalescence does not occur, with the droplets retaining their original properties. Flocculation is induced by the attractive forces between droplets. Such forces are usually weak, making the flocculation process reversible (known as weak flocculation), and agitation can separate the flocs.
2.Coalescence
If the droplets of flocculates coalesce, the liquid films of the tiny droplets inside rupture and form larger liquid beads. This process is defined as coalescence. Coalescence is an irreversible process, which leads to a reduction in the number of droplets and the complete breakdown of emulsions, namely oil-water separation. Flocculation serves as the prelude to coalescence, while coalescence is the direct cause of emulsion damage.
3.Stratification
Due to the difference in density between the oil phase and the aqueous phase, droplets will float or sink under the action of gravity, establishing a balanced droplet concentration gradient in the emulsion. This process is known as creaming or sedimentation. Although creaming impairs the uniformity of the emulsion, the emulsion itself is not truly destroyed.
An emulsion is a thermodynamically unstable system whose ultimate equilibrium state is oil-water separation and stratification (such as an aqueous layer and an oil layer), and demulsification is its inevitable outcome. In certain industrial fields, demulsification is instead required. For instance, demulsifying crude oil (W/O) to achieve oil-water separation.
5.Variant
An emulsion can transform from the W/O type to the O/W type, or vice versa, due to changes in emulsification conditions, and this process is known as phase inversion.
6.Ostwald ripening
Emulsions with droplets of different sizes (polydisperse emulsions) can remain stable and resist flocculation or coalescence. However, over time, the droplet size distribution shifts toward larger droplets, the droplet size distribution curve becomes more concentrated, and droplet sizes tend to be uniform. This phenomenon, similar to the process of crystal ripening, is called Ostwald ripening.
Post time: Apr-24-2026