Stokes' law

Drop a marble and a pebble into a tall jar of honey and watch which reaches the bottom first. The marble, being larger and denser, sinks faster; the thick honey slows them both. Stokes' law is the equation that captures exactly this — how fast a small particle settles in a liquid under gravity.

The law states that the settling (sedimentation) velocity is proportional to the square of the particle diameter and to the density difference between particle and liquid, and inversely proportional to the viscosity of the liquid. In words: bigger, denser particles in a thin liquid fall fast, while small particles of similar density in a thick liquid fall slowly. The squared dependence on size means that halving the particle diameter slows settling fourfold.

This single relationship guides the design of stable suspensions and emulsions. To slow settling or creaming, a formulator can mill the drug to a smaller particle size, thicken the continuous phase to raise its viscosity, or match the densities of the two phases. The law is strictly valid only for dilute systems of small, rigid, non-interacting spheres under streamline (laminar) flow, so in real concentrated, flocculated, or non-Newtonian products it gives a useful trend rather than an exact number.