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Sharp transition between coalescence and non-coalescence of sessile drops

Published online by Cambridge University Press:  04 March 2014

Stefan Karpitschka  and
Hans Riegler
Stefan Karpitschka*
Affiliation:
Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany
Hans Riegler
Affiliation:
Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany
*
†Email address for correspondence: [email protected]
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Abstract

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, $\overline{\Theta }_{a}$, 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 $\widetilde{M}=3\Delta \gamma /(2\overline{\gamma }\overline{\Theta }_{a}^2)$. This $\widetilde{M}$ 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 $\widetilde{M}_t$ and is sharp within the experimental resolution. The experimentally observed threshold value of $\widetilde{M}_t\approx 2$ agrees quantitatively with values obtained by simulations assuming realistic material parameters. The simulations indicate that the absolute value of $\widetilde{M}_t$ very weakly depends on the molecular diffusivity.

JFM classification

Drops and Bubbles: Breakup/coalescence Convection: Convection Drops and Bubbles: Drops and Bubbles Interfacial Flows (free surface): Interfacial Flows (free surface) Convection: Marangoni convection Interfacial Flows (free surface): Thin films
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 743 , 25 March 2014 , R1
Copyright
© 2014 Cambridge University Press 

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Karpitschka and Riegler supplementary movie

One drop of Tetradecane and one drop of Pentadecane spread on the same substrate. They contact each other at three phase angles above the critical value; Coalescence is (initially) immediate.

Download Karpitschka and Riegler supplementary movie(Video)
Video 140.8 KB

Karpitschka and Riegler supplementary movie

One drop of Tetradecane and one drop of Pentadecane spread on the same substrate. They contact each other at Three phase angles below the critical value; Coalescence is suppressed.

Download Karpitschka and Riegler supplementary movie(Video)
Video 129.8 KB

Karpitschka and Riegler supplementary movie

One drop of Tetradecane and one drop of Hexadecane spread on the same substrate. They contact each other at three phase angles above the critical value; Coalescence is (initially) immediate.

Download Karpitschka and Riegler supplementary movie(Video)
Video 112.5 KB

Karpitschka and Riegler supplementary movie

One drop of Tetradecane and one drop of Hexadecane spread on the same substrate. They contact each other at three phase angles above the critical value; Coalescence is suppressed.

Download Karpitschka and Riegler supplementary movie(Video)
Video 107.1 KB
Supplementary material: PDF

Karpitschka and Riegler supplementary material

Supplementary material

Download Karpitschka and Riegler supplementary material(PDF)
PDF 209.7 KB