When two liquids are immiscible, or not soluble in each other, they can form an emulsion or a colloidal suspension. In these types of mixtures the dispersed liquid (water) forms droplets in the continuous phase (oil or fuel). For instance, in a water-diesel fuel mixture the diesel fuel is the continuous phase and the water will exist as droplets of various sizes.

Traditionally, gravity separators were used in liquid-liquid systems to promote the coalescing of the dispersed phase. In these systems differences in densities of the two liquids cause droplets to rise or fall according to their buoyancy. The greater the differences in densities the easier the separation is, as rising or falling droplets (water) are acted upon by the frictional forces exerted by the viscosity of the continuous phase (oil or diesel fuel). This mechanism is governed by Stoke’s Law.

When the movement of the continuous phase (oil or diesel fuel) is slow as in a gravity settler, the effect of the inertial force is reduced and the buoyant force and the gravity force are equal. This is termed as the terminal velocity of the droplet.

A critical factor in a gravity (static) settler is the residence time. It has to be long enough to allow for the passive process of merging smaller droplets into larger ones.