1. Evaporation-driven gravitational instability in the liquid layer of a polymer solution: Theoretical and numerical studies.
- Author
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Hong, Joung Sook, Ahn, Kyung Hyun, Fuller, Gerald. G., and Kim, Min Chan
- Subjects
POLYMER solutions ,LIQUID films ,GRAVITATIONAL instability ,POLYMER films ,RAYLEIGH-Taylor instability ,THERMAL instability ,RAYLEIGH number ,CONVECTIVE flow - Abstract
The drying of the thin liquid film of polymer solution is a complex process accompanying instabilities induced by concentration gradients that develop during the evaporation. This study investigates evaporation-driven gravitational instabilities (Rayleigh–Taylor instability) in the thin liquid film of a polymer solution based on theoretical and numerical analyses, specifically focusing on the onset of instability at the early stage of the drying process. Evaporation-driven convection is taken into account by employing a dynamic moving boundary condition and concentration-dependent material functions (dynamic viscosity and diffusivity). A theoretical analysis predicts the onset of gravitational instability ( τ c) , showing good agreement with the results of a numerical simulation analysis ( τ d) , suggesting scaling relationships between the onset of instability and the initial concentration (C
i ), τ c ∼ C i − 1 / 2 , for a slow evaporation rate and dilute concentration range, which rapidly increases with an increase in the concentration due to an instability-retardation effect. A nonlinear two-dimensional numerical simulation visualizes the development of a polymer-dense layer, pluming, and a convection–diffusion flow throughout the liquid film under transient evaporation. The concentration-gradient-driven convection–diffusion flow enhances the gravitational instability and pluming further at higher frequencies. Meanwhile, for a polymer solution with high viscosity, viscosity thickening delays the onset of instability, retarding the development of instability. This study provides a fundamental understanding of the mechanism behind how convective instability develops in a drying polymer solution during evaporation and information on how to develop pluming of a polymer-dense skin layer at early stages of drying of a polymer solution film. The findings suggest that the proper control of hydrodynamic instability induced by a concentration gradient in a thin polymer solution film is important to prevent or enhance the formation of a convection pattern in the thin polymer film. [ABSTRACT FROM AUTHOR] more...- Published
- 2023
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