spreading coefficient

Drop a little oil on water and sometimes it races out into a thin film, while at other times it stays as a lens-shaped blob. The spreading coefficient is a simple number that predicts in advance which of these will happen.

It is defined by comparing how strongly the spreading liquid wants to cover the underlying one against how strongly it wants to hold itself together. In terms of tensions, the spreading coefficient of liquid B over liquid A equals the surface tension of A minus the sum of the surface tension of B and the A-B interfacial tension. A positive value means spreading is favourable and the liquid spreads spontaneously; a negative value means it stays as a non-spreading drop or lens.

In pharmaceutics this predicts whether a topical product, an oily vehicle, or a lung surfactant will spread evenly over skin, mucosa, or the watery lining of the airways, which affects coverage, feel, and drug delivery. The same idea guides whether an emulsifier film will spread at an oil-water interface. A practical caveat: the spreading coefficient often becomes positive only after the two liquids have had time to mutually saturate, so initial and final spreading behaviour can differ.