Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session L03: Heat Transfer, Evaporation and Buoyancy Effects II
8:00 AM–10:36 AM,
Monday, November 20, 2023
Room: Ballroom C
Chair: Etienne Robert, Polytechnique Montréal
Abstract: L03.00004 : Evaporation of a binary mixture droplet with a slightly non-monotonic surface tension*
8:39 AM–8:52 AM
Presenter:
Christian Diddens
(University of Twente)
Authors:
Christian Diddens
(University of Twente)
Pim J Dekker
(University of Twente)
Detlef Lohse
(University of Twente)
In these droplets, water evaporates - preferentially near the contact line - leaving behind the non-volatile 1,2-hexanediol.
With decreasing water concentration, the surface tension of this mixture initially strongly declines, giving rise to an axisymmetric Marangoni flow from the hexanediol-rich contact line along the liquid-gas interface to the water-rich top of the droplet.
However, in the limit of high hexanediol concentrations, the surface tension slightly increases again. This tiny increase of less than half a mN/m triggers the emergence of a reversed Marangoni flow in the rim region of the droplet, which breaks the azimuthal symmetry of the flow and composition field. When the hexanediol concentration near the contact line exceeds the critical composition with minimum surface tension, tiny spots of enhanced hexanediol concentrations appear at the contact line. Subsequently, the Marangoni dynamics leads to a coarsening of these spots until the entire droplet is covered and the flow ceases.
By means of experiments, three-dimensional finite element simulations and numerical azimuthal stability analysis, we investigate this intriguing phenomenon. Furthermore, we predict the wavelength of this remarkable and naively unexpected instability for any locally parabolic surface tension profile by means of a Marangoni number.
*This work was supported by an Industrial Partnership Programme of the Netherlands Organisation for Scientific Research (NWO), co-financed by Canon Production Printing Netherlands B.V., University of Twente, and Eindhoven University of Technology.
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