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Sharp Border between Coalescence and Noncoalescence of Sessile Drops from Miscible Liquids

Karpitschka, SA (Max-Planck-Institut für Kolloid- und Grenzflächenforschung)
Monday 19 August 2013, 15:00-15:30



Co-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 [3]. 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.

[1] H. Riegler, P. Lazar, Langmuir 24, 6395 (2008). [2] S. Karpitschka, H. Riegler, Langmuir 26, 11823 (2010). [3] S. Karpitschka, H. Riegler, Phys. Rev. Lett. 109, 066103 (2012).

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