Fri July 5th 2019
15:00 – 16:30
Z-108
Bachelor defense Wetting of surfactant droplets
Mees Flapper

Details:

Bubbles and droplets are held together by surface tension, which gives them a spherical shape. Even the shape of a droplet on a substrate is largely determined by surface tension, where the droplet typically forms a spherical cap. When adding a surfactant (surfactant active agent) to the droplet, this changes the surface tension and also the shape of the droplet. Generally, adding a surfactant reduces the surface tension and the contact angle, and thereby increases the radius of curvature. Recently however, an interesting experiment was conducted at the Physics of Fluids group by M.A. Hack and co-workers. In this experiment, 1,2-Hexanediol (a surfactant-like molecule) was added to a water droplet on a substrate; the surface tension decreased similarly to other cases, but the contact angle surprisingly increased.
In this thesis, numerous adsorption models are analysed and tested to try to clarify the increasing contact angle when adding surfactant to a sessile drop. Here the model of a thin adsorption layer surrounding the drop proposed by U. Thiele1 continuously plays a vital role. We extend the existing theory by Thiele to soluble surfactants in this report. To clarify the observed behaviour of 1,2-Hexanediol, the Langmuir adsorption model and the Freundlich adsorption model are used to analytically obtain expressions for the surface tension and contact angle. The Langmuir model fits the data nicely for low surfactant concentrations. The Freundlich model fits the data reasonably well, but has significantly larger errors at higher concentrations. The Langmuir model also does not fit the measurements well at higher mass fractions, but still fits significantly better than the Freundlich model in this case.
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The 10th Complex Motion in Fluids 2021
AQUA
Max Planck Gesellschaft
MCEC
Twente
Centre for Scientific Computing
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