Under consideration for publication in J. Fluid Mech. 1 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 re-main separated in a temporary state of noncoalescence, connected through a thin liquid bridge. Here we investigate the transition between the states of instantaneous coales-cence and temporary noncoalescence. 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, Θa and the surface ten-sion differences ∆γ between both liquids. These relevant parameters can be combined into a single system parameter, a specific Marangoni number M ̃ = 3∆γ/(2γΘ 2 a). This M̃ universally characterizes the coalescence respectively transition behavior 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 M̃t and is sharp within the experimental resolution. The experimentally observed threshold value of M̃t ≈ 2 agrees quantitatively with values obtained by simulations assuming authentic real space data. The simulations indicate that the absolute value of M̃t very weakly depends on the molecular diffusivity