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Title: Evaporation of binary liquids : planar layers and sessile drops
Author: Williams, Adam Graham Lewis
ISNI:       0000 0004 7969 2440
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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This thesis focuses on the dynamics and stability of liquid pools (layers) and droplets comprising of binary mixtures of miscible components, where surface tension induced (Marangoni) flows play a prominent role. Specifically, evaporation of thin horizontally heated liquid layers and thin sessile droplets spreading on heated surfaces are investigated using both modelling and experimental approaches. Below the capillary length gravitational effects weaken and surface tension becomes the prominent driving force in the ensuing flow dynamics. Surface tension gradients arise over the liquid-vapour (LV) interface due to either a variation in temperature (thermal Marangoni stress) or, in the case of binary liquids, concentration (solutal Marangoni stress). In our case, we consider both. Solutal Marangoni stresses can suppress or enhance thermal Marangoni, leading to interesting behaviour. First, the stability, flow dynamics and evaporation kinetics of bi-component miscible liquid layers subject to a horizontal temperature gradient are investigated by means of two-phase direct numerical simulations (DNS). Both the liquid and gas phases are fully resolved, with the Volume-of-Fluid (VOF) method used to account for the deformable liquid-vapour (LV) interface. Surface tension varies linearly with both temperature and concentration at the interface. In the bulk liquid, thermophoresis (Soret effect) and mixture thermodynamics are accounted for. It is shown that even in absence of evaporation, thermophoresis can drive subtle component separation. Under certain conditions, flow exhibits the so-called hydrothermal wave instabilities with similar concentration fluctuations also propagating at twice their wavelength. Introduction of evaporation over the interface depletes both overall liquid mass and concentration of the more volatile component while the layer remains well mixed due to return flow sustained by thermal Marangoni stress. In the absence of thermal Marangoni, preferential evaporation of the more volatile component from the hot wall combined with solutal Marangoni stress reverses the return flow. Secondly, the dynamics and stability of thin volatile droplets comprising of binary mixtures deposited on heated substrates are investigated using lubrication theory and linear stability analysis under the quasi-steady-state approximation. Solely the liquid phase is focused on and so a novel one-sided model is developed to predict the spreading and evaporation of a binary axisymmetric drop on a heated substrate with high wettability. A thin drop with a moving contact line is considered, taking into account the variation of liquid properties with concentration as well as the effects of inertia. The parameter space is explored and the resultant effects on wetting and evaporation evaluated. Increasing solutal Marangoni stress enhances spreading rates in all cases, approaching those of superspreading liquids. Preliminary results from the stability analysis indicate that the addition of a second component has a strong destabilising effect on the drop. Quantitative and qualitative agreement is found with experiments. Thirdly, experiments are conducted with binary ethanol-water droplets spreading on hydrophilic glass slides heated from below. The spreading rate is quantified, revealing that preferential evaporation of the more volatile component (ethanol) at the contact line drives superspreading, leading in some cases to a contact line instability.
Supervisor: Valluri, Prashant ; Sefiane, Khellil Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: fluid dynamics ; binary liquids ; direct numerical simulations ; linear stability ; sessile droplets ; superspreading ; lubrication approximation ; finite element method ; modelling ; evaporation ; Marangoni flow ; instability