Sharp transition between coalescence and non-coalescence of sessile drops

Unexpectedly, under certain conditions, sessile drops from different but completely miscible liquids do not always coalesce instantaneously upon contact: the drop bodies remain separated in a temporary state of non-coalescence, connected through a thin liquid bridge. Here we investigate the transition between the states of instantaneous coalescence and temporary non-coalescence. Experiments reveal that it is barely influenced by viscosities and absolute surface tensions. The main system control parameters for the transition are the arithmetic means of the three-phase angles, (Theta) over bar (a), and the surface tension differences Delta gamma between the two liquids. These relevant parameters can be combined into a single system parameter, a specific Marangoni number (M) over tilde = 3 Delta gamma/(2 (gamma) over bar(Theta) over bar (2)(a)). This (M) over tilde universally characterizes the coalescence transition behaviour as a function of both the physicochemical liquid properties and the shape of the liquid body in the contact region. The transition occurs at a certain threshold value (M) over tilde (t) and is sharp within the experimental resolution. The experimentally observed threshold value of (M) over tilde (t) approximate to 2 agrees quantitatively with values obtained by simulations assuming realistic material parameters. The simulations indicate that the absolute value of (M) over tilde (t) very weakly depends on the molecular diffusivity.