Hydrodynamic and Surface-Wetting Effects on Phase Separation Dynamics in Thin-Film Melts

Understanding the phase separation dynamics in thin films is crucial for the performance of many optoelectronic and photovoltaic device because their functioning relies on a well-defined morphology. The morphology forms in the fluid stages of the production process, and to control and improve the performance of thin-film composites it is vital to predict it quantitatively. Typically, the dynamics of fluid-fluid demixing is modeled using relaxational phase field-type theories that ignore the role of hydrodynamics. We study the impact of hydrodynamics on the demixing of binary fluid mixtures in contact with a wetting substrate by comparing lattice Boltzmann simulations with a diffusion-dominated phase field theory. Special focus is on the impact of a short-range surface interaction that favours one of the two fluids, particularly important in the context of photovoltaics. We find that incorporating hydrodynamics is crucial to quantitatively predict the relevant length scales both in the early and late stages of demixing. Indeed, we find that hydrodynamic processes suppress any dependence of the associated growth exponents on the strength of the substrate interaction predicted by phase field theory. We attribute this to flow-induced transport that significantly speeds up coarsening.