colloidal stability
Fresh milk stays creamy and uniform for days, but leave it too long, or add a splash of lemon juice, and it curdles — the once-smooth liquid breaks into lumps that settle out. The difference is colloidal stability: how well a colloid resists having its scattered particles clump together and fall out of suspension.
More precisely, colloidal stability is the tendency of a colloid's dispersed particles to stay separate and suspended rather than aggregating. Left alone, particles that bump into each other would tend to stick and grow into clumps too heavy to float — a process called coagulation or flocculation. Stability comes from forces that keep the particles apart: like electric charges on their surfaces that repel one another, or coatings of surfactant or polymer that physically block them from touching.
This matters because keeping a colloid stable (paint, milk, ink, blood) or deliberately breaking it (clearing muddy water, treating sewage, separating cream) is central to food, medicine, and industry. The honest caveat is that no colloid is stable forever — it is a contest between the gentle repulsions holding particles apart and the ever-present attractions pulling them together, and the balance can be tipped by adding salt, changing acidity, heating, or simply waiting.
Where a muddy river meets the salty sea, the river's tiny clay particles suddenly clump and drop out, building a delta. The salt's ions cancel the charges that kept the clay particles apart, so they stick together and settle.
Stable: particles repel and stay apart. Unstable: they clump and settle.
The classic theory of why charged colloids stay stable is called DLVO theory (for Derjaguin, Landau, Verwey and Overbeek). It treats stability as a balance between electrical repulsion between like-charged particles and the universal van der Waals attraction that pulls all matter together.