Sharp Border between Coalescence and Noncoalescence of Sessile Drops from Miscible Liquids
AbstractCo-author: Hans Riegler (MPIKG)
Recently it has been shown that sessile drops from different but completely miscible liquids do not always coalesce instantaneously upon contact. Quite unexpected it is observed that after contact, the drop bodies remain separated in a temporary state of noncoalescence, connected only through a thin liquid bridge [1,2]. The connected drops move as a twin drop configuration over the surface. The surface energy difference between the liquids causes a Marangoni flow. This stabilizes the bridge and drives the drop motion . Up to now studies regarding the (non)coalescence behavior of sessile drops from different liquids were performed only without a systematic variation of the contact angles. Therefore it is unknown: (I) at which contact angles the transition between temporary noncoalescence and immediate coalescence occurs, (II) whether this transition is sharp or gradual, and (III) whether the behavior is different f or static and dynamic contact angles, respectively. We present quantitative experimental data on the contact angle dependence of the coalescence behavior of sessile drops from completely miscible liquids. We find quantitatively the same coalescence behavior for both static and dynamic contact angles. The border between the coalescence and the noncoalescence regime is sharp and given by a power law relation between contact angle and surface tension contrast. The power laws are explained within a fluid dynamic thin film approach by scaling arguments. The sharp transition is quantitatively reproduced by numerical simulations.
 H. Riegler, P. Lazar, Langmuir 24, 6395 (2008).  S. Karpitschka, H. Riegler, Langmuir 26, 11823 (2010).  S. Karpitschka, H. Riegler, Phys. Rev. Lett. 109, 066103 (2012).