Partial Coalescence of Sessile Drops with Different Miscible Liquids

Rodica Borcia; Michael Bestehorn

doi:10.1021/la3050835

Sharp transition between coalescence and non-coalescence of sessile drops

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.73 ◽

2014 ◽

Vol 743 ◽

Cited By ~ 13

Author(s):

Stefan Karpitschka ◽

Hans Riegler

Keyword(s):

Contact Region ◽

Threshold Value ◽

System Control ◽

Arithmetic Means ◽

Liquid Body ◽

Molecular Diffusivity ◽

Experimental Resolution ◽

Miscible Liquids ◽

Phase Angles ◽

Sessile Drops

AbstractUnexpectedly, 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.

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The shape evolution of liquid droplets in miscible environments

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.535 ◽

2018 ◽

Vol 852 ◽

pp. 422-452 ◽

Cited By ~ 2

Author(s):

Daniel J. Walls ◽

Eckart Meiburg ◽

Gerald G. Fuller

Keyword(s):

Sessile Drop ◽

Liquid Droplet ◽

Leading Edge ◽

Numerical Approach ◽

Shape Evolution ◽

Liquid Droplets ◽

Miscible Liquids ◽

Experimental Findings ◽

Sessile Drops ◽

Pendant Drops

Miscible liquids often come into contact with one another in natural and technological situations, commonly as a drop of one liquid present in a second, miscible liquid. The shape of a liquid droplet present in a miscible environment evolves spontaneously in time, in a distinctly different fashion than drops present in immiscible environments, which have been reported previously. We consider drops of two classical types, pendant and sessile, in building upon our prior work with miscible systems. Here we present experimental findings of the shape evolution of pendant drops along with an expanded study of the spreading of sessile drops in miscible environments. We develop scalings considering the diffusion of mass to group volumetric data of the evolving pendant drops and the diffusion of momentum to group leading-edge radial data of the spreading sessile drops. These treatments are effective in obtaining single responses for the measurements of each type of droplet, where the volume of a pendant drop diminishes exponentially in time and the leading-edge radius of a sessile drop grows following a power law of $t^{1/2}$ at long times. A complementary numerical approach to compute the concentration and velocity fields of these systems using a simplified set of governing equations is paired with our experimental findings.

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