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The mechanism of the development of solutal Marangoni instability in a thin layer of polymer solution, by rapid evaporation of volatile solvent, is studied numerically. By considering the conservation of mass, momentum, and concentration across the evaporating surface, physically reliable kinematic and boundary conditions are derived and implemented in numerical simulations. To simulate the drying of a polymer solution more realistically up to the point where 80% of solvent was evaporated, the concentration-dependent evaporation rate, viscosity, and diffusivity and the movement of the interface are taken into account. Numerical simulations demonstrate that the generation and merging of convective cell motions in a layer during drying lead to surface patterns as the drying process continues. The drying of a polymer film and the development of the surface topography including thickness deviation depend on various physical phenomena such as Marangoni stress, surface tension, vapor recoil pressure, evaporation rate, initial concentration of polymer, and variation of viscosity and diffusivity with concentration. Meanwhile, the vapor recoil force plays little role in the onset of instability motion and the irregularity of the evaporation surface. Furthermore, both the diffusivity reduction and the viscosity thickening due to evaporative concentration play a critical role in the formation of the skin layer, because they suppress the Marangoni instability motion and therefore impede the convective transport of concentrated polymeric solute. [ABSTRACT FROM AUTHOR]

We investigate the onset and growth of viscous fingering (VF) of miscible annulus in a radial Hele-Shaw cell. Systematic numerical study on a finite annulus domain is performed by employing finite element method solver in COMSOL Multiphysics software. We justify that concentration field analysis is not a good choice for dynamic study in radial flows. Instead, velocity magnitude is a better tool to understand the dynamics. Therefore, we propose velocity field analysis to better differentiate the stable and unstable states and present a new stability criterion using the velocity field method. Most interestingly, using the velocity field analysis and the new stability criterion, we show a restabilization of the VF at a critical time when the system becomes diffusion dominant and able to provide both the onset time, τon (time at which instability develops), and the time at which the interface returns to the stable state, τd. Furthermore, the study successfully suggests the critical values for several dimensionless parameters, the Péclet number (Pe), log-viscosity ratio (R), and volumetric ratio (ra) and time (τ), to induce instability. When Pe is higher than 103, the evolution of VF instability is no longer enhanced by Pe, and Rc converges to a certain value. In particular, for the transiently unstable system of low Pe, the restabilization of VF instability is identified even though R is higher than Rc. The unstable system (τ > τ o n) returns to the stable state as injection time increases further. Moreover, we obtained a critical value of the volumetric ratio ( r c , a ). [ABSTRACT FROM AUTHOR]

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 (Ci), 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]

The dissolution of a horizontal salt body is theoretically and numerically investigated by considering the coupled dynamics of the dissolution reaction and the diffusive and convective mass transfer at the salt surface. To take into account the nature of the recessing salt surface and the dissolution-driven convection, a dynamic moving boundary condition coupled with a dissolution reaction are employed By employing a newly derived moving interface condition and parameters, a theoretical analysis predicts the onset of gravitational instability, suggesting scaling relationships between parameters to describe the onset of instability (in transport-dominant (Damkholer number $Da \to \infty $ and Rayleigh number $Ra > {10^5}$), onset time ${\tau _d}\sim R{a^{ - 2/3}}$ and ${\tau _d}\sim (RaDa)^{1/4}$ for reaction-dominant regimes $(Da \to 0)$). The scaling relationships on dissolution (the average height change, $\Delta {h_{avg}}\sim \sqrt \tau $ for the diffusion-dominant regime and $\Delta {h_{avg}}\sim R{a^{0.79/3}}{\tau ^{0.86}}$ for the convection-dominant regime) are also suggested to explain the pattern formation. Under a convective mass transfer condition, the concentration gradient of the solute developed during the recession of the salt surface results in an adverse density gradient, which causes gravitational instability motion and finally determines the formation of a dissolution pattern. In addition, two-dimensional and three-dimensional numerical simulations visualize dissolution-driven convective flow fields, the recession of the liquid-solid interface and pattern formation on the dissolving interface. Theoretical and numerical analyses of the dissolution process suggest that the control of gravitational instability is important to prevent or enhance dissolution pattern formation. This systematic parameter study provides a deep understanding of the effect of gravitational instability on convection-driven dissolution in a horizontal geometry. [ABSTRACT FROM AUTHOR]

In this study, the poly(lactic acid) (PLA)/poly(butylene adipate‐co‐terephthalate)(PBAT) blend is investigated to improve rheological and mechanical performances of PBAT based on rheological, mechanical, and thermal behavior analyses. The multi‐step mixing method is developed to fabricate the blend with non‐spherical morphology. In the multi‐step mixing method, blends with a wide composition range (25/75–75/25) are mixed with additional PBAT at a mixing temperature between the melting temperatures of PBAT and PLA to produce the PBAT blend embedded with non‐spherical PLA particles (10 wt%). The embedding of non‐spherical PLA particles in PBAT increases the resistance against deformation, resulting in strain hardening behavior and an increase in the yield strength as well as the tear resistance of the PBAT. The presence of stiff PLA particles enhances the crystallization behavior of PBAT, meaning that polymer chains may interpenetrate. The findings of this study suggest that the multi‐step mixing method is beneficial for embedding non‐spherical PLA particles into a PBAT matrix, which in turn facilitates the maintenance of good interfacial adhesion to increase the melt strength, yield strength, and tear resistance. [ABSTRACT FROM AUTHOR]