(576f) Influence of Hydrodynamic Interactions on Stratification in Drying Mixtures

Drying films are of great relevance in many technologies, including latex paints, polymer nanocomposites, and inkjet printing. Often, such films contain multiple components, including colloids, polymers, and surfactants, and it is desirable to control the distribution of components in the dried coating. Despite the conceptual simplicity of the process, there is an incomplete theoretical understanding of how evaporation affects structure in drying films. For example, recent experiments [Fortini, Andrea, et al. Phys. Rev. Lett. 116.11, 118301 (2016) ] showed that mixtures of small and large colloids unexpectedly segregate into layers (stratify) with the small colloids surprisingly found on top of the large ones. In the absence of hydrodynamic interactions, such stratification can be reliably modeled using dynamic density functional theory; however, it was recently suggested [Sear, Richard P., and Patrick B. Warren. Phys. Rev. E 96.6, 062602 (2017)] that hydrodynamic interactions play an important and underappreciated role in determining the final film structure.

We investigated the influence of hydrodynamic interactions on stratification in drying polymer mixtures. Nonequilibrium molecular dynamics simulations of both explicit and implicit solvent models were performed. When neglecting hydrodynamic interactions, we find stratification of the short polymers on top of the long polymers, in agreement with previous theoretical modeling and simulations. However, the explicit solvent model does not stratify at the same drying conditions due to hydrodynamic interactions. Our simulations show, that it is critical to incorporate hydrodynamic interactions into models and simulations in order to reliably predict stratification in drying mixtures.