Friday, June 14, 2013

1306.3149 (Stefan Karpitschka et al.)

Sharp Transition between Coalescence and Noncoalescence of Sessile
Drops: Interplay between Marangoni Flow and Local Topography

Stefan Karpitschka, Hans Riegler
The first detailed experimental data on the impact of the contact angle on the coalescence behavior of sessile drops from different but completely miscible liquids are presented. For the wide range of investigated surface tensions $\gamma_1$ and $\gamma_2$, and liquid viscosities $\eta$, the arithmetic mean of the contact angles, $\ThetaAdjAv$, and the surface tension difference $\Dg$ are identified as main system control parameters. They govern the transition between the regimes of immediate coalescence and noncoalescence. The theoretical analysis reveals a specific Marangoni number $\M=3\Dg/(2\gAv\ThetaAdjAv^2)$ as the unique key parameter. It universally characterizes the coalescence behavior regarding both, the physicochemical liquid properties and the contact region topography. Experiments and simulations yield a threshold Marangoni number $\Mcrit$ for the rather sharp transition between the coalescence regimes. The simulations reveal a slight dependence of $\Mcrit$ on the molecular diffusivity. The experimentally determined $\Mcrit\approx 2$ is in quantitative agreement with the simulations assuming authentic real space data.
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