The spreading of silicone oil drops on horizontal surfaces

A study of the spreading of silicone oil drops is presented, with particular attention to the dominant effect of the conditions near the edge and the surface tension forces acting there. It is shown that the drop profiles can be predicted and that this leads to a relation between the edge velocity and the maximum slope, which occurs at an inflection point close to the edge. This relation leads to others governing the variation of drop size and slope with time. The predictions are compared with experiment, using silicone oils, and good agreement obtained. The lubrication approximation theory gives no solution for thicknesses below a value varying from 2 to 9*10^-8 m at the drop edge. At thicknesses of this order, or possibly even larger, other effects become important, and some observations on edge behaviour are included.