Hydrodynamic effects on phase separation morphologies in evaporating thin films of polymer solutions

Abstract

We examine effects of hydrodynamics on phase separation morphologies developed during drying of thin films containing a volatile solvent and two dissolved polymers. Cahn-Hilliard and Flory-Huggins theories are used to describe the free energy of the phase separating systems. The thin films, considered as Newtonian fluids, flow in response to Korteweg stresses arising due to concentration non-uniformities that develop during solvent evaporation. Numerical simulations are employed to investigate the effects of a Peclet number, defined in terms of system physical properties, as well as the effects of parameters characterizing the speed of evaporation and preferential wetting of the solutes at the gas interface. For systems exhibiting preferential wetting, diffusion alone is known to favor lamellar configurations for the separated phases in the dried film. However, a mechanism of hydrodynamic instability of a short length scale is revealed, which beyond a threshold Peclet number may deform and break the lamellae. The critical Peclet number tends to decrease as the evaporation rate increases and to increase with the tendency of the polymers to selectively wet the gas interface. As the Peclet number increases, the instability moves closer to the gas interface and induces the formation of a lateral segregation template that guides the subsequent evolution of the phase separation process. On the other hand, for systems with no preferential wetting or any other property asymmetries between the two polymers, diffusion alone favors the formation of laterally separated configurations. In this case, concentration perturbation modes that lead to enhanced Korteweg stresses may be favored for sufficiently large Peclet numbers. For such modes, a second mechanism is revealed, which is similar to the solutocapillary Marangoni instability observed in evaporating solutions when interfacial tension increases with the concentration of the non-volatile component. This mechanism may lead to multiple length scales in the laterally phase separated configurations.

Dynamic Surface Enrichment in Drying Thin-Film Binary Polymer Solutions

“…The interplay between demixing and stratification may lead to lateral phase separation close to the free surface or may first lead to vertical demixing into phase-separated lammelae that eventually break up due to Marangoni instabilities. − Current three-dimensional phase-field models have included evaporation, multicomponent diffusion, hydrodynamics, evaporation, , thermal fluctuations, and surface roughening , and are capable of qualitatively explaining experimentally encountered dry-layer morphologies. Still, the parameter space is enormous, and quantitative comparison between model and theory remains rare, if not absent.…”

Impact of Particle Arrays on Phase Separation Composition Patterns

“…However, hydrodynamic interactions can be incorporated in our scheme by coupling our phasefield simulations with lattice Boltzmann simulations to address long-time stability of the phase separation structures induced by particles. This method also allows us to incorporate processing related effects encountered in real applications such as fluid flow [28,[82][83][84]. Three-dimensional simulations of the above particle systems will address intricate percolated patterns guided by preferential wetting and geometrical confinement [85].…”

Influence of evaporation on the morphology of a thin film of a partially miscible binary mixture

“…Evaporation induced phase separation in a partially miscible mixture has received rather little attention so far in the literature. Zoumpouli et al [49] performed a numerical analysis of the phase separation morphology developed in a thin film containing a volatile solvent and two dissolved polymers. They modeled the evaporation by a purely empirical model.…”