Sharp transition between coalescence and noncoalescence 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, , and the surface tension differences between the two liquids. These relevant parameters can be combined into a single system parameter, a specific Marangoni number . This 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 and is sharp within the experimental resolution. The experimentally observed threshold value of agrees quantitatively with values obtained by simulations assuming realistic material parameters. The simulations indicate that the absolute value of very weakly depends on the molecular diffusivity